FOR VIDEO AND IMAGE CODING AND DECODING

BACKGROUND

[001] Engineers use compression (also called source coding or source encoding) to reduce the bit rate of digital video. Compression decreases the cost of storing and transmitting video information by converting the information into a lower bit rate form. Decompression (also called decoding) reconstructs a version of the original information from the compressed form. A "codec" is an encoder/decoder system.

[002] Over the last two decades, various video codec standards have been adopted, including the ITU-T H.261 , H.262 (MPEG-2 or ISO/TEC 13818-2), H.263 and H.264 (MPEG-4 AVC or ISO/IEC 14496-10) standards, the MPEG-1 (ISO/IEC 1 1 172-2) and MPEG-4 Visual (ISO/IEC 14496-2) standards, and the SMPTE 421M standard. More recently, the HEVC standard (ITU-T H.265 or ISO/IEC 23008-2) has been approved.

Extensions to the HEVC standard (e.g., for scalable video coding/decoding, for

coding/decoding of video with higher fidelity in terms of sample bit depth or chroma sampling rate, or for multi-view coding/decoding) are currently under development. A video codec standard typically defines options for the syntax of an encoded video bitstream, detailing parameters in the bitstream when particular features are used in encoding and decoding. In many cases, a video codec standard also provides details about the decoding operations a decoder should perform to achieve conforming results in decoding. Aside from codec standards, various proprietary codec formats define other options for the syntax of an encoded video bitstream and corresponding decoding operations.

[003] In base color index map ("BCIM") mode, a video encoder encodes sample values using index values that represent base colors. Each of the index values is associated with a different value ("base color") among the sample values. For example, if an 8x8 block has four different values among the 64 sample values of the block, the encoder assigns four different index values for those four base colors. During encoding, the sample values are replaced with corresponding index values. The encoder encodes and signals a mapping of index values to base colors ("base color table") as well as data for the arrangement of index values representing the sample values ("index map"). A decoder receives and decodes the mapping of index values to base colors. Using that mapping, the decoder replaces index values in the index map with base colors for the original sample values.

[004] BCIM mode can be helpful when encoding certain "artificially" created video content such as screen-capture content. Screen-capture content typically includes repeated structures (e.g., graphics, text characters) that use few distinct sample values, compared to natural video. This provides opportunities for BCIM to improve performance.

[005] The current design of BCIM mode for the HEVC standard and implementation in some reference software for the HEVC standard has several problems. For example, BCIM requires many new syntax elements and syntax structures for the base color mapping and index map, which complicates implementation. Also, the approach to encoding/decoding the base color mapping and index map uses tools unlike those already included in the HEVC standard, again complicating implementation. Also, encoding of the base color table and index map is inefficient.

SUMMARY

[006] In summary, the detailed description presents innovations in base color index map ("BCIM") mode. For example, some of the innovations relate to reusing a syntax structure for transform coefficients in non-BCIM mode and for elements of an index map in BCIM mode. This can reduce the number of modifications made to support BCIM mode. Other innovations relate to strategies for mapping of index values or prediction of elements of an index map, which can improve the coding efficiency of BCIM mode. Still other innovations relate to handling of exception values in BCIM mode.

[007] The innovations for base color index map mode can be implemented as part of a method, as part of a computing device adapted to perform the method or as part of a tangible computer-readable media storing computer-executable instructions for causing a computing device to perform the method. The various innovations can be used in combination or separately. [008] The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[009] Figure 1 is a diagram of an example computing system in which some described embodiments can be implemented.

[010] Figures 2a and 2b are diagrams of example network environments in which some described embodiments can be implemented.

[011] Figure 3 is a diagram of an example encoder system in conjunction with which some described embodiments can be implemented.

[012] Figure 4 is a diagram of an example decoder system in conjunction with which some described embodiments can be implemented.

[013] Figures 5a and 5b are diagrams illustrating an example video encoder in conjunction with which some described embodiments can be implemented.

[014] Figure 6 is a diagram illustrating an example video decoder in conjunction with which some described embodiments can be implemented.

[015] Figure 7 is chart illustrating base color index map mode for a block of a picture.

[016] Figures 8 and 9 are diagrams illustrating example process flow during encoding and decoding, respectively, when the same coefficient coding syntax structure can be used to signal elements of an index map or transform coefficients.

[017] Figures 10 and 11 are flowcharts illustrating generalized techniques for encoding and decoding, respectively, that use a coefficient coding syntax structure selectively representing elements of an index map or transform coefficients. [018] Figure 12 is a flowchart of an example technique for decoding that uses a coefficient coding syntax structure that selectively represents elements of an index map or transform coefficients.

[019] Figure 13 is chart illustrating mapping of index values to packed index values and mapping of packed index values back to index values.

[020] Figures 14 and 15 are flowcharts illustrating generalized techniques for mapping of index values for base colors during encoding and decoding, respectively.

[021] Figures 16 and 17 are charts illustrating prediction of elements of an index map using subtraction operations during encoding and addition operations during decoding.

[022] Figure 18 is a chart illustrating prediction of an element of an index map using an XOR operation during encoding, and corresponding reconstruction of the element using an XOR operation during decoding.

[023] Figures 19 and 20 are flowcharts illustrating generalized techniques for prediction of elements of an index map during encoding and decoding, respectively.

[024] Figures 21a and 21b are charts illustrating processing of exception values and index values for BCIM mode during encoding and decoding, respectively.

[025] Figures 22 and 23 are flowcharts illustrating generalized techniques for encoding and decoding, respectively, using index values and exception values in a BCIM mode.

[026] Figure 24 is a flowchart illustrating an example technique for using index values and exception values in a BCIM mode.

DETAILED DESCRIPTION

[027] The detailed description presents innovations in the use of base color index map ("BCIM") mode during encoding and/or decoding. In particular, the detailed description presents innovations for reusing a syntax structure that is adapted for transform coefficients to instead signal data for elements of an index map in BCIM mode, mapping of index values in BCIM mode, prediction of elements of an index map and handling of exception values in BCIM mode. Some of these innovations simplify implementation by reducing the number of modifications made to support BCIM mode. Other innovations improve coding efficiency of BCIM mode.

[028] Although operations described herein are in places described as being performed by a video encoder or video decoder, in many cases the operations can be performed by another type of media processing tool (e.g., image encoder, image decoder).

[029] Some of the innovations described herein are illustrated with reference to syntax elements and operations specific to the HEVC standard. For example, reference is made to the draft version JCTVC-N1005 of the HEVC standard - "High Efficiency Video Coding (HEVC) Range Extensions Text Specification: Draft 4," JCTVC-N1005, July 2013. The innovations described herein can also be implemented for other standards or formats.

[030] More generally, various alternatives to the examples described herein are possible. For example, some of the methods described herein can be altered by changing the ordering of the method acts described, by splitting, repeating, or omitting certain method acts, etc. The various aspects of the disclosed technology can be used in combination or separately. Different embodiments use one or more of the described innovations. Some of the

innovations described herein address one or more of the problems noted in the background. Typically, a given technique/tool does not solve all such problems.

I. Example Computing Systems.

[031] Figure 1 illustrates a generalized example of a suitable computing system (100) in which several of the described innovations may be implemented. The computing system (100) is not intended to suggest any limitation as to scope of use or functionality, as the innovations may be implemented in diverse general-purpose or special-purpose computing systems.

[032] With reference to Figure 1 , the computing system (100) includes one or more processing units (1 10, 1 15) and memory (120, 125). The processing units (1 10, 1 15) execute computer-executable instructions. A processing unit can be a general-purpose central processing unit ("CPU"), processor in an application-specific integrated circuit ("ASIC") or any other type of processor. In a multi-processing system, multiple processing units execute computer-executable instructions to increase processing power. For example, Figure 1 shows a central processing unit (1 10) as well as a graphics processing unit or co-processing unit (1 15). The tangible memory (120, 125) may be volatile memory (e.g., registers, cache, RAM), non- volatile memory (e.g., ROM, EEPROM, flash memory, etc.), or some

combination of the two, accessible by the processing unit(s). The memory (120, 125) stores software (180) implementing one or more innovations for BCIM mode, in the form of computer-executable instructions suitable for execution by the processing unit(s).

[033] A computing system may have additional features. For example, the computing system (100) includes storage (140), one or more input devices (150), one or more output devices (160), and one or more communication connections (170). An interconnection mechanism (not shown) such as a bus, controller, or network interconnects the components of the computing system (100). Typically, operating system software (not shown) provides an operating environment for other software executing in the computing system (100), and coordinates activities of the components of the computing system (100).

[034] The tangible storage (140) may be removable or non-removable, and includes magnetic disks, magnetic tapes or cassettes, CD-ROMs, DVDs, or any other medium which can be used to store information and which can be accessed within the computing system (100). The storage (140) stores instructions for the software (180) implementing one or more innovations for BCIM mode.

[035] The input device(s) (150) may be a touch input device such as a keyboard, mouse, pen, or trackball, a voice input device, a scanning device, or another device that provides input to the computing system (100). For video, the input device(s) (150) may be a camera, video card, TV tuner card, or similar device that accepts video input in analog or digital form, or a CD-ROM or CD-RW that reads video samples into the computing system (100). The output device(s) (160) may be a display, printer, speaker, CD-writer, or another device that provides output from the computing system (100). [036] The communication connection(s) (170) enable communication over a communication medium to another computing entity. The communication medium conveys information such as computer-executable instructions, audio or video input or output, or other data in a modulated data signal. A modulated data signal is a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media can use an electrical, optical, RF, or other carrier.

[037] The innovations can be described in the general context of computer-readable media. Computer-readable media are any available tangible media that can be accessed within a computing environment. By way of example, and not limitation, with the computing system (100), computer-readable media include memory (120, 125), storage (140), and combinations of any of the above.

[038] The innovations can be described in the general context of computer-executable instructions, such as those included in program modules, being executed in a computing system on a target real or virtual processor. Generally, program modules include routines, programs, libraries, objects, classes, components, data structures, etc. that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or split between program modules as desired in various embodiments. Computer-executable instructions for program modules may be executed within a local or distributed computing system.

[039] The terms "system" and "device" are used interchangeably herein. Unless the context clearly indicates otherwise, neither term implies any limitation on a type of computing system or computing device. In general, a computing system or computing device can be local or distributed, and can include any combination of special-purpose hardware and/or general-purpose hardware with software implementing the functionality described herein.

[040] The disclosed methods can also be implemented using specialized computing hardware configured to perform any of the disclosed methods. For example, the disclosed methods can be implemented by an integrated circuit (e.g. , an ASIC (such as an ASIC digital signal process unit ("DSP"), a graphics processing unit ("GPU"), or a programmable logic device ("PLD"), such as a field programmable gate array ("FPGA")) specially designed or configured to implement any of the disclosed methods.

[041] For the sake of presentation, the detailed description uses terms like "determine" and "use" to describe computer operations in a computing system. These terms are high- level abstractions for operations performed by a computer, and should not be confused with acts performed by a human being. The actual computer operations corresponding to these terms vary depending on implementation.

II. Example Network Environments.

[042] Figures 2a and 2b show example network environments (201, 202) that include video encoders (220) and video decoders (270). The encoders (220) and decoders (270) are connected over a network (250) using an appropriate communication protocol. The network (250) can include the Internet or another computer network.

[043] In the network environment (201) shown in Figure 2a, each real-time

communication (" TC") tool (210) includes both an encoder (220) and a decoder (270) for bidirectional communication. A given encoder (220) can produce output compliant with a variation or extension of the HEVC standard, SMPTE 421M standard, ISO-IEC 14496-10 standard (also known as H.264 or AVC), another standard, or a proprietary format, with a corresponding decoder (270) accepting encoded data from the encoder (220). The bidirectional communication can be part of a video conference, video telephone call, or other two-party communication scenario. Although the network environment (201) in Figure 2a includes two real-time communication tools (210), the network environment (201) can instead include three or more real-time communication tools (210) that participate in multiparty communication.

[044] A real-time communication tool (210) manages encoding by an encoder (220). Figure 3 shows an example encoder system (300) that can be included in the real-time communication tool (210). Alternatively, the real-time communication tool (210) uses another encoder system. A real-time communication tool (210) also manages decoding by a decoder (270). Figure 4 shows an example decoder system (400), which can be included in the real-time communication tool (210). Alternatively, the real-time communication tool (210) uses another decoder system.

[045] In the network environment (202) shown in Figure 2b, an encoding tool (212) includes an encoder (220) that encodes video for delivery to multiple playback tools (214), which include decoders (270). The unidirectional communication can be provided for a video surveillance system, web camera monitoring system, remote desktop conferencing presentation or other scenario in which video is encoded and sent from one location to one or more other locations. Although the network environment (202) in Figure 2b includes two playback tools (214), the network environment (202) can include more or fewer playback tools (214). In general, a playback tool (214) communicates with the encoding tool (212) to determine a stream of video for the playback tool (214) to receive. The playback tool (214) receives the stream, buffers the received encoded data for an appropriate period, and begins decoding and playback.

[046] Figure 3 shows an example encoder system (300) that can be included in the encoding tool (212). Alternatively, the encoding tool (212) uses another encoder system. The encoding tool (212) can also include server-side controller logic for managing connections with one or more playback tools (214). Figure 4 shows an example decoder system (400), which can be included in the playback tool (214). Alternatively, the playback tool (214) uses another decoder system. A playback tool (214) can also include client-side controller logic for managing connections with the encoding tool (212).

III. Example Encoder Systems.

[047] Figure 3 is a block diagram of an example encoder system (300) in conjunction with which some described embodiments may be implemented. The encoder system (300) can be a general-purpose encoding tool capable of operating in any of multiple encoding modes such as a low-latency encoding mode for real-time communication, transcoding mode, and regular encoding mode for media playback from a file or stream, or it can be a special- purpose encoding tool adapted for one such encoding mode. The encoder system (300) can be implemented as an operating system module, as part of an application library or as a standalone application. Overall, the encoder system (300) receives a sequence of source video frames (311) from a video source (310) and produces encoded data as output to a channel (390). The encoded data output to the channel can include content encoded using BCIM mode.

[048] The video source (310) can be a camera, tuner card, storage media, or other digital video source. The video source (310) produces a sequence of video frames at a frame rate of, for example, 30 frames per second. As used herein, the term "frame" generally refers to source, coded or reconstructed image data. For progressive video, a frame is a progressive video frame. For interlaced video, in example embodiments, an interlaced video frame is de-interlaced prior to encoding. Alternatively, two complementary interlaced video fields are encoded as an interlaced video frame or separate fields. Aside from indicating a progressive video frame, the term "frame" or "picture" can indicate a single non-paired video field, a complementary pair of video fields, a video object plane that represents a video object at a given time, or a region of interest in a larger image. The video object plane or region can be part of a larger image that includes multiple objects or regions of a scene.

[049] An arriving source frame (311) is stored in a source frame temporary memory storage area (320) that includes multiple frame buffer storage areas (321, 322, ... , 32n). A frame buffer (321, 322, etc.) holds one source frame in the source frame storage area (320). After one or more of the source frames (311) have been stored in frame buffers (321, 322, etc.), a frame selector (330) periodically selects an individual source frame from the source frame storage area (320). The order in which frames are selected by the frame selector (330) for input to the encoder (340) may differ from the order in which the frames are produced by the video source (310), e.g., a frame may be ahead in order, to facilitate temporally backward prediction. Before the encoder (340), the encoder system (300) can include a pre-processor (not shown) that performs pre-processing {e.g., filtering) of the selected frame (331) before encoding. The pre-processing can also include color space conversion into primary and secondary components for encoding. Typically, before encoding, video has been converted to a color space such as YUV, in which sample values of a luma (Y) component represent brightness or intensity values, and sample values of chroma (U, V) components represent color-difference values. The chroma sample values may be sub-sampled to a lower chroma sampling rate (e.g., for YUV 4:2:0 format), or the chroma sample values may have the same resolution as the luma sample values (e.g., for YUV 4:4:4 format). Or, the video can be encoded in another format (e.g., RGB 4:4:4 format).

[050] The encoder (340) encodes the selected frame (331) to produce a coded frame (341) and also produces memory management control operation ("MMCO") signals (342) or reference picture set ("RPS") information. If the current frame is not the first frame that has been encoded, when performing its encoding process, the encoder (340) may use one or more previously encoded/decoded frames (369) that have been stored in a decoded frame temporary memory storage area (360). Such stored decoded frames (369) are used as reference frames for inter- frame prediction of the content of the current source frame (331). Generally, the encoder (340) includes multiple encoding modules that perform encoding tasks such as partitioning into tiles, intra prediction estimation and prediction, motion estimation and compensation, frequency transforms, quantization and entropy coding. The exact operations performed by the encoder (340) can vary depending on compression format. The format of the output encoded data can be a variation or extension of HEVC format, Windows Media Video format, VC-1 format, MPEG-x format (e.g., MPEG-1 , MPEG-2, or MPEG-4), H.26x format (e.g., H.261 , H.262, H.263, H.264), or another format.

[051] The encoder (340) can partition a frame into multiple tiles of the same size or different sizes. For example, the encoder (340) splits the frame along tile rows and tile columns that, with frame boundaries, define horizontal and vertical boundaries of tiles within the frame, where each tile is a rectangular region. Tiles are often used to improve options for parallel processing. A frame can also be organized as one or more slices, where a slice can be an entire frame or region of the frame. A slice can be decoded independently of other slices in a frame, which improves error resilience. The content of a slice or tile is further partitioned into blocks or other sets of samples for purposes of encoding and decoding.

[052] For syntax according to the HEVC standard, the encoder splits the content of a frame (or slice or tile) into coding tree units. A coding tree unit ("CTU") includes luma sample values organized as a luma coding tree block ("CTB") and corresponding chroma sample values organized as two chroma CTBs. The size of a CTU (and its CTBs) is selected by the encoder, and can be, for example, 64x64, 32x32 or 16x16 sample values. A CTU includes one or more coding units. A coding unit ("CU") has a luma coding block ("CB") and two corresponding chroma CBs. For example, a CTU with a 64x64 luma CTB and two 64x64 chroma CTBs (YUV 4:4:4 format) can be split into four CUs, with each CU including a 32x32 luma CB and two 32x32 chroma CBs, and with each CU possibly being split further into smaller CUs. Or, as another example, a CTU with a 64x64 luma CTB and two 32x32 chroma CTBs (YUV 4:2:0 format) can be split into four CUs, with each CU including a 32x32 luma CB and two 16x16 chroma CBs, and with each CU possibly being split further into smaller CUs. The smallest allowable size of CU (e.g., 8x8, 16x16) can be signaled in the bitstream.

[053] Generally, a CU has a prediction mode such as inter or intra. A CU includes one or more prediction units for purposes of signaling of prediction information (such as prediction mode details, etc.) and/or prediction processing. A prediction unit ("PU") has a luma prediction block ("PB") and two chroma PBs. For an intra-predicted CU, the PU has the same size as the CU, unless the CU has the smallest size (e.g., 8x8). In that case, the CU can be split into four smaller PUs (e.g., each 4x4 if the smallest CU size is 8x8) or the PU can have the smallest CU size, as indicated by a syntax element for the CU. A CU also has one or more transform units for purposes of residual coding/decoding, where a transform unit ("TU") has a transform block ("TB") and two chroma TBs. A PU in an intra-predicted CU may contain a single TU (equal in size to the PU) or multiple TUs. As used herein, the term "block" can indicate a CB, PB, TB or other set of sample values, depending on context. The encoder decides how to partition video into CTUs, CUs, PUs, TUs, etc.

[054] Returning to Figure 3, the encoder represents an intra-coded block of a source frame (331) in terms of prediction from other, previously reconstructed sample values in the frame (331). For intra spatial prediction for a block, the intra-picture estimator estimates extrapolation of the neighboring reconstructed sample values into the block. The intra- prediction estimator outputs prediction information (such as prediction mode (direction) for intra spatial prediction), which is entropy coded. An intra-prediction predictor applies the prediction information to determine intra prediction values. For BCIM mode, the encoder represents an intra-coded block with index values for base colors among the sample values of the block, using a base color table for the index values and using elements of an index map. The encoder can also represent exception values in the BCIM-mode block without using index values, as described below.

[055] The encoder (340) represents an inter-coded, predicted block of a source frame (331) in terms of prediction from reference frames. A motion estimator estimates motion of the block with respect to one or more reference frames (369). When multiple reference frames are used, the multiple reference frames can be from different temporal directions or the same temporal direction. A motion-compensated prediction reference region is a region of samples in the reference frame(s) that are used to generate motion-compensated prediction values for a block of samples of a current frame. The motion estimator outputs motion information such as motion vector information, which is entropy coded. A motion compensator applies motion vectors to reference frames (369) to determine motion- compensated prediction values.

[056] In non-BCIM modes, the encoder determines the differences (if any) between a block's prediction values (intra or inter) and corresponding original values. These prediction residual values are further encoded using a frequency transform, quantization and entropy encoding. For example, the encoder (340) sets values for quantization parameter ("QP") for a picture, tile, slice and/or other portion of video, and quantizes transform coefficients accordingly. In BCIM mode, the encoder skips the transform and quantization operations, except for certain quantization operations when handling exception values.

[057] The entropy coder of the encoder (340) compresses quantized transform coefficient values as well as certain side information (e.g., motion vector information, QP values, mode decisions, parameter choices). In particular, the entropy coder can compress data for elements of an index map using a coefficient coding syntax structure. Typical entropy coding techniques include Exp-Golomb coding, arithmetic coding, differential coding, Huffman coding, run length coding, variable-length-to-variable-length ("V2V") coding, variable-length-to-fixed-length ("V2F") coding, LZ coding, dictionary coding, probability interval partitioning entropy coding ("PIPE"), and combinations of the above. The entropy coder can use different coding techniques for different kinds of information, and can choose from among multiple code tables within a particular coding technique.

[058] The coded frames (341) and MMCO/RPS information (342) are processed by a decoding process emulator (350). The decoding process emulator (350) implements some of the functionality of a decoder, for example, decoding tasks to reconstruct reference frames. The decoding process emulator (350) uses the MMCO/RPS information (342) to determine whether a given coded frame (341) needs to be reconstructed and stored for use as a reference frame in inter-frame prediction of subsequent frames to be encoded. If the MMCO/RPS information (342) indicates that a coded frame (341) needs to be stored, the decoding process emulator (350) models the decoding process that would be conducted by a decoder that receives the coded frame (341) and produces a corresponding decoded frame (351). In doing so, when the encoder (340) has used decoded frame(s) (369) that have been stored in the decoded frame storage area (360), the decoding process emulator (350) also uses the decoded frame(s) (369) from the storage area (360) as part of the decoding process.

[059] The decoded frame temporary memory storage area (360) includes multiple frame buffer storage areas (361 , 362, ... , 36n). The decoding process emulator (350) uses the MMCO/RPS information (342) to manage the contents of the storage area (360) in order to identify any frame buffers (361 , 362, etc.) with frames that are no longer needed by the encoder (340) for use as reference frames. After modeling the decoding process, the decoding process emulator (350) stores a newly decoded frame (351) in a frame buffer (361 , 362, etc.) that has been identified in this manner.

[060] The coded frames (341) and MMCO/RPS information (342) are buffered in a temporary coded data area (370). The coded data that is aggregated in the coded data area (370) contains, as part of the syntax of an elementary coded video bitstream, encoded data for one or more pictures. The coded data that is aggregated in the coded data area (370) can also include media metadata relating to the coded video data (e.g., as one or more parameters in one or more supplemental enhancement information ("SEI") messages or video usability information ("VUI") messages). [061] The aggregated data (371) from the temporary coded data area (370) are processed by a channel encoder (380). The channel encoder (380) can packetize the aggregated data for transmission as a media stream (e.g., according to a media stream multiplexing format such as ISO/IEC 13818-1), in which case the channel encoder (380) can add syntax elements as part of the syntax of the media transmission stream. Or, the channel encoder (380) can organize the aggregated data for storage as a file (e.g., according to a media container format such as ISO/IEC 14496-12), in which case the channel encoder (380) can add syntax elements as part of the syntax of the media storage file. Or, more generally, the channel encoder (380) can implement one or more media system multiplexing protocols or transport protocols, in which case the channel encoder (380) can add syntax elements as part of the syntax of the protocol(s). The channel encoder (380) provides output to a channel (390), which represents storage, a communications connection, or another channel for the output.

IV. Example Decoder Systems.

[062] Figure 4 is a block diagram of an example decoder system (400) in conjunction with which some described embodiments may be implemented. The decoder system (400) can be a general-purpose decoding tool capable of operating in any of multiple decoding modes such as a low-latency decoding mode for real-time communication and regular decoding mode for media playback from a file or stream, or it can be a special-purpose decoding tool adapted for one such decoding mode. The decoder system (400) can be implemented as an operating system module, as part of an application library or as a standalone application. Overall, the decoder system (400) receives coded data from a channel (410) and produces reconstructed frames as output for an output destination (490). The coded data can include content encoded using BCIM mode.

[063] The decoder system (400) includes a channel (410), which can represent storage, a communications connection, or another channel for coded data as input. The channel (410) produces coded data that has been channel coded. A channel decoder (420) can process the coded data. For example, the channel decoder (420) de-packetizes data that has been aggregated for transmission as a media stream (e.g. , according to a media stream multiplexing format such as ISO/IEC 13818-1), in which case the channel decoder (420) can parse syntax elements added as part of the syntax of the media transmission stream. Or, the channel decoder (420) separates coded video data that has been aggregated for storage as a file (e.g., according to a media container format such as ISO/IEC 14496-12), in which case the channel decoder (420) can parse syntax elements added as part of the syntax of the media storage file. Or, more generally, the channel decoder (420) can implement one or more media system demultiplexing protocols or transport protocols, in which case the channel decoder (420) can parse syntax elements added as part of the syntax of the protocol(s).

[064] The coded data (421) that is output from the channel decoder (420) is stored in a temporary coded data area (430) until a sufficient quantity of such data has been received. The coded data (421) includes coded frames (431) and MMCO/RPS information (432). The coded data (421) in the coded data area (430) contain, as part of the syntax of an elementary coded video bitstream, coded data for one or more pictures. The coded data (421) in the coded data area (430) can also include media metadata relating to the encoded video data (e.g., as one or more parameters in one or more SEI messages or VUI messages).

[065] In general, the coded data area (430) temporarily stores coded data (421) until such coded data (421) is used by the decoder (450). At that point, coded data for a coded frame (431) and MMCO/RPS information (432) are transferred from the coded data area (430) to the decoder (450). As decoding continues, new coded data is added to the coded data area (430) and the oldest coded data remaining in the coded data area (430) is transferred to the decoder (450).

[066] The decoder (450) periodically decodes a coded frame (431) to produce a corresponding decoded frame (451). As appropriate, when performing its decoding process, the decoder (450) may use one or more previously decoded frames (469) as reference frames for inter-frame prediction. The decoder (450) reads such previously decoded frames (469) from a decoded frame temporary memory storage area (460). Generally, the decoder (450) includes multiple decoding modules that perform decoding tasks such as entropy decoding, inverse quantization, inverse frequency transforms, intra prediction, motion compensation and merging of tiles. The exact operations performed by the decoder (450) can vary depending on compression format.

[067] For example, the decoder (450) receives encoded data for a compressed frame or sequence of frames and produces output including decoded frame (451). In the decoder (450), a buffer receives encoded data for a compressed frame and, at an appropriate time, makes the received encoded data available to an entropy decoder. The entropy decoder entropy decodes entropy-coded quantized data as well as entropy-coded side information, typically applying the inverse of entropy encoding performed in the encoder. A motion compensator applies motion information to one or more reference frames to form motion-compensated prediction values for any inter-coded blocks of the frame being reconstructed. An intra prediction module can spatially predict sample values of a current block from neighboring, previously reconstructed sample values. For BCIM mode, the decoder reconstructs an intra-coded block with index values for base colors among the sample values of the block, using a base color table for the index values and using elements of an index map.

[068] In non-BCIM modes, the decoder (450) also reconstructs prediction residuals. An inverse quantizer inverse quantizes entropy-decoded data. For example, the decoder (450) sets values for QP for a picture, tile, slice and/or other portion of video based on syntax elements in the bitstream, and inverse quantizes transform coefficients accordingly. An inverse frequency transformer converts the quantized, frequency domain data into spatial domain information. In BCIM mode, the decoder skips the inverse quantization and inverse frequency transform operations, except for certain inverse quantization operations when decoding exception values. For an inter-predicted block, the decoder (450) combines reconstructed prediction residuals with motion-compensated predictions. The decoder (450) can similarly combine prediction residuals with predictions from intra prediction. A motion compensation loop in the video decoder (450) includes an adaptive de-blocking filter to smooth discontinuities across block boundary rows and/or columns in the decoded frame (451).

[069] The decoded frame temporary memory storage area (460) includes multiple frame buffer storage areas (461 , 462, ... , 46n). The decoded frame storage area (460) is an example of a decoded picture buffer. The decoder (450) uses the MMCO/RPS information (432) to identify a frame buffer (461, 462, etc.) in which it can store a decoded frame (451). The decoder (450) stores the decoded frame (451) in that frame buffer.

[070] An output sequencer (480) uses the MMCO/RPS information (432) to identify when the next frame to be produced in output order is available in the decoded frame storage area (460). When the next frame (481) to be produced in output order is available in the decoded frame storage area (460), it is read by the output sequencer (480) and output to the output destination (490) (e.g., display). In general, the order in which frames are output from the decoded frame storage area (460) by the output sequencer (480) may differ from the order in which the frames are decoded by the decoder (450).

V. Example Video Encoders.

[071] Figures 5a and 5b are a block diagram of a generalized video encoder (500) in conjunction with which some described embodiments may be implemented. The encoder (500) receives a sequence of video pictures including a current picture as an input video signal (505) and produces encoded data in a coded video bitstream (595) as output.

[072] The encoder (500) is block-based and uses a block format that depends on implementation. Blocks may be further sub-divided at different stages, e.g., at the prediction, frequency transform and/or entropy encoding stages. For example, a picture can be divided into 64x64 blocks, 32x32 blocks or 16x16 blocks, which can in turn be divided into smaller blocks of sample values for coding and decoding. In implementations of encoding for the HEVC standard, the encoder partitions a picture into CTUs (CTBs), CUs (CBs), PUs (PBs) and TU (TBs).

[073] The encoder (500) compresses pictures using intra-picture coding and/or inter- picture coding. Many of the components of the encoder (500) are used for both intra-picture coding and inter-picture coding. The exact operations performed by those components can vary depending on the type of information being compressed.

[074] A tiling module (510) optionally partitions a picture into multiple tiles of the same size or different sizes. For example, the tiling module (510) splits the picture along tile rows and tile columns that, with picture boundaries, define horizontal and vertical boundaries of tiles within the picture, where each tile is a rectangular region. The tiling module (510) can then group the tiles into one or more tile sets, where a tile set is a group of one or more of the tiles.

[075] The general encoding control (520) receives pictures for the input video signal (505) as well as feedback (not shown) from various modules of the encoder (500). Overall, the general encoding control (520) provides control signals (not shown) to other modules (such as the tiling module (510), trans former/scaler/quantizer (530), sealer/inverse

transformer (535), intra-picture estimator (540), motion estimator (550) and intra/inter switch) to set and change coding parameters during encoding. In particular, the general encoding control (520) can decide whether and how to use BCIM mode during encoding. The general encoding control (520) can also evaluate intermediate results during encoding, for example, performing rate-distortion analysis. The general encoding control (520) produces general control data (522) that indicates decisions made during encoding, so that a corresponding decoder can make consistent decisions. The general control data (522) is provided to the header formatter/entropy coder (590).

[076] If the current picture is predicted using inter-picture prediction, a motion estimator (550) estimates motion of blocks of sample values of the current picture of the input video signal (505) with respect to one or more reference pictures. The decoded picture buffer (570) buffers one or more reconstructed previously coded pictures for use as reference pictures. When multiple reference pictures are used, the multiple reference pictures can be from different temporal directions or the same temporal direction. The motion estimator (550) produces as side information motion data (552) such as motion vector data and reference picture selection data. The motion data (552) is provided to the header formatter/entropy coder (590) as well as the motion compensator (555).

[077] The motion compensator (555) applies motion vectors to the reconstructed reference picture(s) from the decoded picture buffer (570). The motion compensator (555) produces motion-compensated predictions for the current picture. [078] In a separate path within the encoder (500), an intra-picture estimator (540) determines how to perform intra-picture prediction for blocks of sample values of a current picture of the input video signal (505). The current picture can be entirely or partially coded using intra-picture coding. Using values of a reconstruction (538) of the current picture, for intra spatial prediction, the intra-picture estimator (540) determines how to spatially predict sample values of a current block of the current picture from neighboring, previously reconstructed sample values of the current picture.

[079] For BCIM mode, the encoder (500) represents an intra-coded block with index values for base colors among the sample values of the block, using an index map for the index values and using elements of an index map.

[080] The intra-prediction estimator (540) produces as side information intra prediction data (542), such as information indicating whether intra prediction uses spatial prediction or BCIM mode (e.g. , a flag value per intra block or per intra block of certain prediction mode directions), prediction mode direction (for intra spatial prediction). The intra prediction data (542) is provided to the header formatter/entropy coder (590) as well as the intra-picture predictor (545). According to the intra prediction data (542), the intra-picture predictor (545) spatially predicts sample values of a current block of the current picture from neighboring, previously reconstructed sample values of the current picture, or reconstructs sample values of BCM-mode blocks.

[081] In non-BCM modes, the intra/inter switch selects values of a motion- compensated prediction or intra-picture prediction for use as the prediction (558) for a given block. In non-BCIM modes, the difference (if any) between a block of the prediction (558) and corresponding part of the original current picture of the input video signal (505) provides values of the residual (518). During reconstruction of the current picture, reconstructed residual values are combined with the prediction (558) to produce a reconstruction (538) of the original content from the video signal (505). In lossy compression, however, some information is still lost from the video signal (505).

[082] In the trans former/scaler/quantizer (530), for non-BCIM modes, a frequency transformer converts spatial domain video information into frequency domain (i.e., spectral, transform) data. For block-based video coding, the frequency transformer applies a discrete cosine transform ("DCT"), an integer approximation thereof, or another type of forward block transform to blocks of prediction residual data (or sample value data if the prediction (558) is null), producing blocks of frequency transform coefficients. The encoder (500) may also be able to indicate that such transform step is skipped. The sealer/quantizer scales and quantizes the transform coefficients. For example, the quantizer applies non-uniform, scalar quantization to the frequency domain data with a step size that varies on a frame-by- frame basis, tile-by-tile basis, slice-by-slice basis, block-by-block basis or other basis. The quantized transform coefficient data (532) is provided to the header formatter/entropy coder (590).

[083] In the sealer/inverse transformer (535), for non-BCIM modes, a sealer/inverse quantizer performs inverse scaling and inverse quantization on the quantized transform coefficients. An inverse frequency transformer performs an inverse frequency transform, producing blocks of reconstructed prediction residuals or sample values. The encoder (500) combines reconstructed residuals with values of the prediction (558) (e.g., motion- compensated prediction values, intra-picture prediction values) to form the reconstruction (538).

[084] For intra-picture prediction, the values of the reconstruction (538) can be fed back to the intra-picture estimator (540) and intra-picture predictor (545). Also, the values of the reconstruction (538) can be used for motion-compensated prediction of subsequent pictures. The values of the reconstruction (538) can be further filtered. A filtering control (560) determines how to perform deblock filtering and sample adaptive offset ("SAO") filtering on values of the reconstruction (538), for a given picture of the video signal (505). The filtering control (560) produces filter control data (562), which is provided to the header formatter/entropy coder (590) and merger/filter(s) (565).

[085] In the merger/filter(s) (565), the encoder (500) merges content from different tiles into a reconstructed version of the picture. The encoder (500) selectively performs deblock filtering and SAO filtering according to the filter control data (562), so as to adaptive ly smooth discontinuities across boundaries in the frames. Tile boundaries can be selectively filtered or not filtered at all, depending on settings of the encoder (500), and the encoder (500) may provide syntax within the coded bitstream to indicate whether or not such filtering was applied. The decoded picture buffer (570) buffers the reconstructed current picture for use in subsequent motion-compensated prediction.

[086] The header formatter/entropy coder (590) formats and/or entropy codes the general control data (522), quantized transform coefficient data (532), intra prediction data (542) and packed index values, motion data (552) and filter control data (562). For example, the header formatter/entropy coder (590) uses context-adaptive binary arithmetic coding ("CABAC") for entropy coding of various syntax elements of a coefficient coding syntax structure.

[087] The header formatter/entropy coder (590) provides the encoded data in the coded video bitstream (595). The format of the coded video bitstream (595) can be a variation or extension of HEVC format, Windows Media Video format, VC-1 format, MPEG-x format (e.g., MPEG-1 , MPEG-2, or MPEG-4), H.26x format (e.g., H.261 , H.262, H.263, H.264), or another format.

[088] Depending on implementation and the type of compression desired, modules of the encoder can be added, omitted, split into multiple modules, combined with other modules, and/or replaced with like modules. In alternative embodiments, encoders with different modules and/or other configurations of modules perform one or more of the described techniques. Specific embodiments of encoders typically use a variation or supplemented version of the encoder (500). The relationships shown between modules within the encoder (500) indicate general flows of information in the encoder; other relationships are not shown for the sake of simplicity.

VI. Example Video Decoders.

[089] Figure 6 is a block diagram of a generalized decoder (600) in conjunction with which several described embodiments may be implemented. The decoder (600) receives encoded data in a coded video bitstream (605) and produces output including pictures for reconstructed video (695). The format of the coded video bitstream (605) can be a variation or extension of HEVC format, Windows Media Video format, VC-1 format, MPEG-x format (e.g., MPEG-1 , MPEG-2, or MPEG-4), H.26x format (e.g., H.261 , H.262, H.263, H.264), or another format.

[090] The decoder (600) is block-based and uses a block format that depends on implementation. Blocks may be further sub-divided at different stages. For example, a picture can be divided into 64x64 blocks, 32x32 blocks or 16x16 blocks, which can in turn be divided into smaller blocks of sample values. In implementations of decoding for the HEVC standard, a picture is partitioned into CTUs (CTBs), CUs (CBs), PUs (PBs) and TU (TBs).

[091] The decoder (600) decompresses pictures using intra-picture decoding and/or inter-picture decoding. Many of the components of the decoder (600) are used for both intra- picture decoding and inter-picture decoding. The exact operations performed by those components can vary depending on the type of information being decompressed.

[092] A buffer receives encoded data in the coded video bitstream (605) and makes the received encoded data available to the parser/entropy decoder (610). The parser/entropy decoder (610) entropy decodes entropy-coded data, typically applying the inverse of entropy coding performed in the encoder (500) (e.g., context-adaptive binary arithmetic decoding). For example, the parser/entropy decoder (610) uses context-adaptive binary arithmetic decoding for entropy decoding of various syntax elements of a coefficient coding syntax structure. As a result of parsing and entropy decoding, the parser/entropy decoder (610) produces general control data (622), quantized transform coefficient data (632), intra prediction data (642) and packed index values, motion data (652) and filter control data (662).

[093] The general decoding control (620) receives the general control data (622) and provides control signals (not shown) to other modules (such as the sealer/inverse transformer (635), intra-picture predictor (645), motion compensator (655) and intra/inter switch) to set and change decoding parameters during decoding.

[094] If the current picture is predicted using inter-picture prediction, a motion compensator (655) receives the motion data (652), such as motion vector data and reference picture selection data. The motion compensator (655) applies motion vectors to the reconstructed reference picture(s) from the decoded picture buffer (670). The motion compensator (655) produces motion-compensated predictions for inter-coded blocks of the current picture. The decoded picture buffer (670) stores one or more previously

reconstructed pictures for use as reference pictures.

[095] In a separate path within the decoder (600), the intra-prediction predictor (645) receives the intra prediction data (642), such as information indicating whether intra prediction uses spatial prediction or BCIM mode (e.g., a flag value per intra block or per intra block of certain prediction mode directions), prediction mode direction (for intra spatial prediction). For intra spatial prediction, using values of a reconstruction (638) of the current picture, according to prediction mode data, the intra-picture predictor (645) spatially predicts sample values of a current block of the current picture from neighboring, previously reconstructed sample values of the current picture. For BCIM mode, the decoder reconstructs an intra-coded block with index values for base colors among the sample values of the block, using a base color table for the index values and using elements of an index map.

[096] In non-BCIM modes, the intra/inter switch selects values of a motion- compensated prediction or intra-picture prediction for use as the prediction (658) for a given block. For example, when HEVC syntax is followed, the intra/inter switch can be controlled based on a syntax element encoded for a CU of a picture that can contain intra-predicted CUs and inter-predicted CUs. The decoder (600) combines the prediction (658) with

reconstructed residual values to produce the reconstruction (638) of the content from the video signal.

[097] To reconstruct the residual, for non-BCIM modes, the sealer/inverse transformer (635) receives and processes the quantized transform coefficient data (632). In the sealer/inverse transformer (635), a sealer/inverse quantizer performs inverse scaling and inverse quantization on the quantized transform coefficients. An inverse frequency transformer performs an inverse frequency transform, producing blocks of reconstructed prediction residuals or sample values. For example, the inverse frequency transformer applies an inverse block transform to frequency transform coefficients, producing sample value data or prediction residual data. The inverse frequency transform can be an inverse DCT, an integer approximation thereof, or another type of inverse frequency transform. [098] For intra-picture prediction, the values of the reconstruction (638) can be fed back to the intra-picture predictor (645). For inter-picture prediction, the values of the reconstruction (638) can be further filtered. In the merger/filter(s) (665), the decoder (600) merges content from different tiles into a reconstructed version of the picture. The decoder (600) selectively performs deblock filtering and SAO filtering according to the filter control data (662) and rules for filter adaptation, so as to adaptively smooth discontinuities across boundaries in the frames. Tile boundaries can be selectively filtered or not filtered at all, depending on settings of the decoder (600) or a syntax indication within the encoded bitstream data. The decoded picture buffer (670) buffers the reconstructed current picture for use in subsequent motion-compensated prediction.

[099] The decoder (600) can also include a post-processing deblock filter. The postprocessing deblock filter optionally smoothes discontinuities in reconstructed pictures. Other filtering (such as de-ring filtering) can also be applied as part of the post-processing filtering.

[0100] Depending on implementation and the type of decompression desired, modules of the decoder can be added, omitted, split into multiple modules, combined with other modules, and/or replaced with like modules. In alternative embodiments, decoders with different modules and/or other configurations of modules perform one or more of the described techniques. Specific embodiments of decoders typically use a variation or supplemented version of the decoder (600). The relationships shown between modules within the decoder (600) indicate general flows of information in the decoder; other relationships are not shown for the sake of simplicity.

VII. Innovations for Base Color Index Map Mode.

[0101] This section presents various innovations for base color index map ("BCIM") mode. Some of the innovations relate to reuse of a coefficient coding syntax structure for either transform coefficients or index map data in BCIM mode, while others relate to mapping or prediction of index values in BCIM mode. Still others relate to handling of exception values in BCIM mode. These innovations can facilitate BCIM mode that is more effective in terms of rate-distortion performance and/or computational efficiency of encoding and decoding. In particular, using BCIM mode can improve rate-distortion performance when encoding certain "artificially" created video content such as screen-capture content. Screen-capture content typically includes repeated structures (e.g., graphics, text characters), which provide opportunities for intra BC prediction to improve performance. Screen capture content is usually encoded in a format (e.g., YUV 4:4:4 or RGB 4:4:4) with high chroma sampling resolution, although it may also be encoded in a format with lower chroma sampling resolution (e.g., YUV 4:2:0).

A. Base Color Index Map Mode - Introduction.

[0102] In BCIM mode, a video encoder or image encoder encodes sample values using index values that represent base colors. Each of the index values is associated with a different value ("base color") among the sample values. During encoding, the sample values are replaced with corresponding index values. The encoder encodes and signals a table of index values and corresponding base colors ("base color table") as well as the arrangement of index values that represent the sample values ("index map"). A video decoder or image decoder receives and decodes the table of index values and corresponding base colors. Using that base color table, the decoder replaces index values of the index map with base colors for the original sample values.

[0103] Figure 7 shows a block (710) of sample values s in a two-dimensional arrangement with dimensions for 0 < i < 7 and 0 <j < 7. In Figure 7, the sample values s represent intensity or brightness values for screen capture content. The sample values s include sections of uniform values and strong patterns. The block (710) includes sample values 26, 85, 41, 127, 168 and 200.

[0104] The encoder creates a base color table (720) that assigns index values to corresponding base colors. In the example of Figure 7, the index value 0 is assigned to the sample value 200, the index value 1 is assigned to the sample value 168, and so on. The encoder can assign index values to base colors according to their likelihood of occurrence in the picture, such that more common sample values have lower index values and less common sample values have higher index values, which tends to result in more efficient coding if lower index values are represented with fewer bits. Alternatively, the encoder can assign index values to base colors according to order of appearance as a block is scanned, relying on later processes such as prediction to exploit redundancy among the index values of the index map. The base color table (720) can be implemented as a look-up table or other data structure.

[0105] Figure 7 shows a block (730) in which sample values s are replaced with corresponding index values n. The process of replacing sample values with index values is lossless. Alternatively, in a lossy compression variation, a sample value can be replaced with the index value representing the base color closest to the sample value, if an exact match is not available. This can reduce the size of the base color table (720) but also introduce perceptible distortion. Another approach to handling sample values not represented with index values (so-called exception values) is described below.

[0106] The encoder encodes and outputs the base color table (720) as well as an index map with elements representing the block (730) of index values n. For example, the encoder uses a coefficient coding syntax structure to represent elements of the block (730), as described below. As part of the encoding, the index values n for the block (730) can be processed with further mapping operations and/or prediction.

[0107] During decoding, the decoder receives and decodes the base color table (720) and data for the index map. For example, the decoder decodes elements of the block (730) from a coefficient coding syntax structure, as described below. As part of the decoding, the index values n for the block (730) can be processed with further mapping operations and/or prediction. The decoder then reverses the mapping process using the base color table (720), as illustrated in Figure 7.

[0108] In the example of Figure 7, the encoder creates a base color table (720) for the sample values of an 8x8 block. Alternatively, the block has another size {e.g., 4x4, 16x16, 32x32 or 64x64). More generally, the block is an m x n block, where m and n can have an equal value or have different values. Or, the encoder can create a base color mapping for the sample values of a slice, a tile, an entire picture, a group of pictures, or a video sequence. B. Reusing a Coefficient Coding Syntax Structure.

[0109] In some example implementations, a video encoder or image encoder uses a syntax structure normally used to signal transform coefficient information to instead signal BCIM mode data. In particular, the encoder reuses a coefficient coding syntax structure to signal index map information in BCIM mode.

[0110] Figure 8 shows the process flow (800) of this approach during encoding. The encoder receives a block of spatial-domain values (810). The spatial-domain values (810) can be sample values. Or, the spatial-domain values (810) can be prediction residuals after inter-picture prediction or intra-picture prediction. The encoder can apply encoding operations in BCIM mode or apply operations in non-BCIM mode, using the same coefficient coding syntax structure in both modes.

[0111] For BCIM mode, the encoder maps (812) the spatial-domain values (810) (e.g., sample values) to index values (830), replacing spatial-domain values (810) with

corresponding index values (830). The encoder further maps (832) the index values to packed index values (840) for an index map, for example, as described in the next section. The mapping operations (812, 832) in the BCIM mode do not include quantization or transform operations (except for encoding of exception values, in some implementations). The encoder encodes (842) a base color table and elements of the index map, using a coefficient coding syntax structure (890) to signal the packed index values (840) of the index map.

[0112] For non-BCIM mode, the encoder applies (814) a frequency transform to the spatial-domain values (810) (e.g., residual values), producing transform coefficients (860), which the encoder quantizes (862). Alternatively, the transform and/or quantization are skipped. The quantized transform coefficients (870) are then encoded using the coefficient coding syntax structure (890).

[0113] In some example implementations that follow HEVC syntax, the coefficient coding syntax structure is a residual_coding syntax structure. See, e.g., sections 7.3.8.11 and 7.4.9.1 1 of JCTVC-N1005. In the BCIM mode, the encoder processes the elements of the index map (e.g., index values, packed index values, index residual values, or packed index residual values, as described below) as if they were quantized transform coefficients.

Specifically, the encoder signals the position of a last non-zero coefficient in the block, which can be a 4x4 block, 8x8 block, 16x16 block or 32x32 block. For each 4x4 group of values in the block, the encoder signals a flag indicating whether the 4x4 group has at least one nonzero coefficient. For a 4x4 group that has at least one non-zero coefficient, the encoder signals flags indicating which coefficients have non-zero values, then signals level

information and (in most cases) sign information for the coefficients with non-zero values. The encoder uses CABAC to encode various elements of the residual coding syntax structure. Alternatively, another coefficient coding syntax structure is used. For example, a coefficient coding syntax structure that follows syntax of the H.264 standard, VC-1 standard or another standard or proprietary format can be used.

[01 14] Figure 9 shows the process flow (900) of this approach during decoding. The decoder receives a coefficient coding syntax structure (990). The decoder can apply operations in BCIM mode or apply operations in non-BCIM mode, using the same coefficient coding syntax structure in both modes.

[01 15] For BCIM mode, the decoder decodes (942) a base color table and, using the coefficient coding syntax structure (990), also decodes (942) elements of an index map. This produces packed index values (940) for the index map. The decoder maps (932) packed index values (940) to index values (930) for the index map, for example, as described in the next section. The encoder then maps (912) the index values (930) to spatial-domain values (910) (e.g., sample values) for a block. The mapping operations (912, 932) in the BCIM mode do not include inverse quantization or inverse transform operations (except for decoding of exception values, in some implementations).

[01 16] For non-BCIM mode, the decoder decodes (972) quantized transform coefficients (970) that have been signaled using the coefficient coding syntax structure (990). The decoder can inverse quantize (962) the quantized transform coefficients, then apply (914) an inverse frequency transform to reconstruct spatial-domain values (910) (e.g., prediction residuals). Alternatively, the inverse quantization and/or inverse transform are skipped. The decoder can then perform inter-picture prediction or intra-picture prediction (not shown) and combine reconstructed residual values with the predicted values (not shown).

[0117] Figure 10 illustrates a generalized technique (1000) for encoding that uses a coefficient coding syntax structure to selectively represent elements of an index map or transform coefficients. The technique (1000) can be performed by an encoder as described with reference to Figure 3 or Figures 5a and 5b, or by another encoder.

[0118] The encoder encodes (1010) data, using a coefficient coding syntax structure for a block to selectively represent elements of an index map for the block or transform

coefficients for the block. For example, if the coefficient coding structure is used for transform coefficients in a non-BCIM mode, the encoder (a) computes residual values for the block, (b) performs a frequency transform on the residual values to produce the transform coefficients, (c) quantizes the transform coefficients, and (d) encodes the resulting values in the coefficient coding syntax structure. Or, if the coefficient coding structure is used for an index map in a BCIM mode, the encoder (a) maps sample values for the block to index values, each of the index values representing a base color, (b) maps the index values to packed index values or packed index residual values, and (c) encodes the resulting values in the coefficient coding syntax structure.

[0119] For the BCIM mode, the index map can include packed index values for the block, where each of the packed index values is an index value that represents a base color and is packed into the coefficient coding syntax structure. Or, the index map can include packed index residual values for the block, wherein each of the packed index residual values represents a base color and is packed into the coefficient coding syntax structure. A packed index residual value can be an index residual value based on the difference between an index value and a predicted index value, or the packed index residual value can be based on the difference between a packed index value and a predicted packed index value. The index map can also include one or more exception values not representing any base color, which can be processed as described below.

[0120] The encoder outputs (1020) the encoded data. For example, for implementations that follow HEVC syntax, the encoded data can include a residual coding syntax structure for elements of the index map or transform coefficients. Alternatively, the encoded data is formatted in some other way. The encoder can repeat the technique (1000) for another block.

[0121] Figure 11 illustrates a generalized technique (1100) for decoding that uses a coefficient coding syntax structure that selectively represents elements of an index map or transform coefficients. The technique (1100) can be performed by a decoder as described with reference to Figure 4 or Figure 6, or by another decoder.

[0122] The decoder receives (1110) encoded data. For example, for implementations that follow HEVC syntax, the encoded data can include a residual coding syntax structure for elements of the index map or transform coefficients. Alternatively, the encoded data is formatted in some other way.

[0123] The decoder decodes (1120) the encoded data, using a coefficient coding syntax structure for a block that selectively represents elements of an index map for the block or transform coefficients for the block. For example, if the coefficient coding structure is used for transform coefficients in a non-BCIM mode, the decoder (a) decodes values for transform coefficients from the coefficient coding syntax structure, (b) inverse quantizes the transform coefficients, (c) performs an inverse frequency transform on the transform coefficients to produce residual values for the block, and (d) combines the residual values with prediction values to reconstruct sample values for the block. Or, if the coefficient coding structure is used for an index map in a BCIM mode, the decoder (a) decodes values for an index map from the coefficient coding syntax structure, (b) maps packed index values or packed index residual values to index values, each of the index values representing a base color, and (c) maps the index values to sample values for the block.

[0124] For the BCIM mode, the index map can include packed index values for the block, where each of the packed index values is an index value that represents a base color and is packed into the coefficient coding syntax structure. Or, the index map can include packed index residual values for the block, wherein each of the packed index residual values represents a base color and is packed into the coefficient coding syntax structure. A packed index residual value can be an index residual value based on the difference between an index value and a predicted index value, or the packed index residual value can be based on the difference between a packed index value and a predicted packed index value. The index map can also include one or more exception values not representing any base color, which can be processed as described below.

[0125] The decoder can repeat the technique (1100) for another block.

[0126] Figure 12 illustrates a more specific example technique (1200) for decoding that uses a coefficient coding syntax structure that selectively represents elements of an index map or transform coefficients. The technique (1200) can be performed by a decoder as described with reference to Figure 4 or Figure 6, or by another decoder.

[0127] The decoder receives (1210) a coefficient coding syntax structure for a block. For example, for implementations that follow HEVC syntax, the coefficient coding syntax structure is a residual_coding syntax structure. Alternatively, the coefficient coding syntax structure is organized in some other way.

[0128] The decoder checks (1220) whether the mode of the block is BCIM. For example, the decoder checks the value of one or more syntax elements for the block. For

implementations that follow HEVC syntax, the syntax element(s) that indicate whether the block is a BCIM-mode block can be signaled for a PU. Alternatively, the syntax element(s) are signaled for a CU, CB, PB, TU or TB.

[0129] If the block is a BCIM block, the decoder decodes (1240) elements of an index map from the coefficient coding structure for the block. Otherwise, the decoder decodes (1230) transform coefficients from the coefficient coding syntax structure for the block. For example, the decoder performs operations as described with reference to Figure 9 or 11.

[0130] The decoder checks (1250) whether to continue with the next block. If so, the decoder receives (1210) the coefficient coding syntax structure for the next block.

C. Mapping Index Values to Packed Index Values.

[0131] To signal index values as if they were transform coefficients, an encoder maps the index values n to packed index values n _pac ked- During decoding, a decoder maps packed index values n _pac ked back to index values n. The encoder and decoder can follow various strategies when mapping between index values n and packed index values n _pac ked-

[0132] For example, the encoder can simply use an index value as the packed index value. That is, n _pac ked = n during encoding, and n = n _pac ked during decoding. Aside from its simplicity, one advantage of this approach is that sign values for the packed index values n _P acked can be omitted from signaled encoded data, since the packed index values are always non-negative.

[0133] As a second example approach, the encoder can map an index value n to a packed index value n _pac ked as follows.

if n%2 is 0 then n _pac ked = -{nil); and

otherwise, n _pac ked = (n+\)/2.

where % represents a MOD operator such that x%y is the remainder when x is divided by y, and / represents division with truncation of the result towards zero. The decoder can map the packed index value n _pac ked back to the index value n as follows.

if necked is < 0, n = and

otherwise, n = 2*n _pac ked- .

[0134] Figure 13 shows mapping of index values to packed index values, and mapping of packed index values back to index values, according to this approach. Index values n in a block (1310) are mapped to packed index values n _pac ked in a block (1330) during encoding, and mapped backed to index values n in the block (1310) during decoding. The index values ranging from 0 ... 5 are mapped to packed index values ranging from -2 ... 3, and vice versa.

[0135] In a variation of the second example approach, the encoder can map even index values to non-negative numbers, and map odd numbers to negative values, with the decoder reversing that mapping. For the mapping during encoding:

if n%2 is 0 then n _pac ked = (n/2); and

otherwise, n _pa cked = -(n+\)/2.

In decoding:

if npacked is > 0, n = 2*n _pac ked, and

otherwise, n = -2 _aC(b¾r l . [0136] As a third example approach, index values for different color components (e.g. , luma and chroma components for video in YUV 4:4:4 format) can be mapped into a single packed index value for a pixel. For example, a luma sample is mapped to a luma index value n _y , and two corresponding chroma samples are mapped to two chroma index values n _u and n _v . The index values n _y , n _u and n _v are then mapped to a single packed index value. If the number of bits used to represent each of the index values n _y , n _u and n _v is nuts, the single packed index value ripacked, combined (having 3* ribits bits) can be determined as follows.

Wpacked, combined Wy (2*?¾¾) ^"†" flu Kbits ttv

where « represents a left bit shift operation. For decoding, the index values n _y , n _u and n _v can be reconstructed using bit mask operations and bit shifting operations to identify the values of the appropriate bits of the packed index value n _pa cked, combined, as follows.

fty Wpacked, combined » (2*n _bi ts)

u = (npacked, combined » bits) & ((1« Kbits) " 1)

Wpacked, combined & ((l«n _b its) - 1)

where » represents a right bit shift operation, and & represents a bitwise AND operation. The ordering of index values n _y , n _u and n _v can be varied in the mapping operation during encoding, with corresponding changes to the mapping operations during decoding. Also, the index values n _y , n _u and n _v can have different numbers of bits, with corresponding changes to the mapping operations during encoding and decoding.

[0137] Alternatively, if there are county base colors for the Y component, count _u base colors for the U component, and count _v base colors for the V component, the single packed index value can be determined as follows.

npacked combined = «v * (county * coun + n _u * county + n _y .

For decoding, the index values n _y , n _u and n _v can be reconstructed as follows.

W _y ^packed, combined /o COUnt _y

n _u = (npacked, combined I COUnty) % COUnt _u

n _v — npacked, combined I (COUnty * COlint _u )

where / represent division with truncation of the result towards zero. Again, the ordering of index values n _y , n _u and n _v can be varied, with corresponding changes to which count variables are used. [0138] Alternatively, the encoder and decoder use a different approach when mapping between index values n and packed index values n _pac ked-

[0139] Figure 14 illustrates a technique (1400) for mapping of index values for base colors during encoding. The technique (1400) can be performed by an encoder as described with reference to Figure 3 or Figures 5a and 5b, or by another encoder.

[0140] The encoder maps (1410) a sample value to an index value for a base color. For example, the encoder uses a base color table as described above to map a luma or chroma sample value to an index value.

[0141] The encoder then maps (1420) the index value to at least part of a packed index value for a coefficient coding syntax structure. For example, the encoder sets the packed index value based on the index value, e.g., as in the first mapping approach described above. Or, as another example, if the index value is even, the encoder sets the packed index value based on the index value divided by two, but otherwise the encoder sets the packed index value based on the index value plus one, then divided by two {e.g., as in the second mapping approach described above). Or, as another example, the encoder sets part of the packed index value using the index value, e.g. , as in the third mapping approach described above, where the packed index values is set based on a luma index value and multiple chroma index values.

[0142] The encoder can then determine a packed index residual value based on the packed index value and a predicted packed index value, as described below. Or, the index values that are mapped to packed index residual values can be index residual values.

[0143] The encoder checks (1430) whether to continue with the next sample value. If so, the encoder maps (1410) the next sample values to the next index value. The encoder can proceed in a raster scan pattern through the sample values of a block. Or, for the third approach, the encoder can map sample values of corresponding positions in a luma block and chroma blocks to index values, then map those index values to a packed index value.

[0144] Figure 15 illustrates a technique (1500) for mapping of index values for base colors during decoding. The technique (1500) can be performed by a decoder as described with reference to Figure 4 or Figure 6, or by another decoder. [0145] The decoder maps (1510) at least part of a packed index value for a coefficient coding syntax structure to an index value for a base color.

For example, the decoder sets the index value based on the packed index value, e.g. , as in the first mapping approach described above. Or, as another example, if the packed index value has a given relation to zero, the decoder sets the index value based on the packed index value times two, but otherwise the decoder sets the index value based on the packed index value times two minus one (e.g., as in the second mapping approach described above). Or, as another example, the decoder sets the index value using part of the packed index value, e.g., as in the third mapping approach described above, where a luma index value and multiple chroma index values are set based on the packed index value.

[0146] The decoder maps (1520) the index value to a sample value. For example, the decoder uses a base color table as described above to map a luma or chroma index value to a sample value.

[0147] Before the mapping operations, the decoder can predict a given packed index value then combined a packed index residual value with the predicted packed index value to reconstruct the packed index value. Or, the index values that are reconstructed can be index residual values.

[0148] The decoder checks (1530) whether to continue with the next index value. If so, the decoder maps (1510) at least part of a packed index value for the coefficient coding syntax structure to the next index value for a base color. The decoder can proceed in a raster scan pattern through the index values for a block. Or, for the third approach, the decoder can map a packed index value to multiple index values for corresponding positions in a luma block and chroma blocks, then map those index values to sample values.

D. Prediction of Elements of Index Map.

[0149] An encoder and decoder can also use prediction of elements of an index map. When prediction works well, many of the index residual values (or packed index residual values) have a value of zero. The prediction improves coding efficiency in typical entropy coding schemes because values closer to zero are encoded using fewer bits.

[0150] Figure 16 shows a block (1610) of index values n with dimensions i,j, for 0 < i < 7 and 0 <j < 7. Alternatively, the prediction is applied to a block of another size {e.g., 4x4, 16x16 or 32x32). During encoding, the encoder predicts a given index value n(i,j) from one or more neighboring index values, on an index value-by-index value basis. The direction of prediction can be horizontal, vertical, diagonal in right-downward direction, or some other direction. In Figure 16, for example, the direction of prediction is vertical, producing a block (1630) of index residual values n _reS id- The index value n(i,j) is predicted using the preceding index value in the same column n{i,j-Y). The index residual value is simply the difference between the two values. n _resi d(i,j) = n{i,j) - n(i,j-\). Similarly, for horizontal prediction, the index residual value is the difference between the index value and its left neighboring index value as the predicted index value: n _resi d(i,j) = n{i,j) - n(i-\,j). For diagonal prediction at a 45 degree downward angle, the index residual value can be calculated as n _reS id{i,j) = n(i,j) - (n(i,j-\)+ n(i-\,j))»\, or it can be calculated as n _res id{i ) = n( j) - n(i-\,j-Y). Prediction in other diagonal directions can similarly blend neighboring index values depending on the angle of prediction. For prediction at the edge of a block {e.g., i < 0 and/or j < 0), the neighboring index value can have a value of zero or be given a default value {e.g., number of index values divided by two). During decoding, the decoder determines the same predicted index value (depending on the direction of prediction) and combines the predicted index value with the index residual value. For vertical prediction, n{i,j) = n _res id{i ) + n(i,j-l). For horizontal prediction, n{i,j) = n _resi Ji,j) + n{i-\,j). For diagonal prediction at a 45 degree downward angle, n{i,j) = n _resid (i,j) + {n{i,j-Y)+ n{i-\,j))»\, or n{i,j) = n _resid {i,j) + n{i-\,j- 1). Prediction in other diagonal directions can similarly blend neighboring index values depending on the angle of prediction.

[0151] In Figure 16, prediction is applied before index values are mapped to packed index values. Alternatively, prediction is applied after index values are mapped to packed index values.

[0152] Figure 17 shows a block (1710) of packed index values n _pac ked with dimensions i, j, for 0 < i < 7 and 0 <j < 7. Alternatively, the prediction is applied to a block of another size (e.g. , 4x4, 16x16 or 32x32). During encoding, the encoder predicts a given packed index value ripackeJi ) from one or more neighboring packed index values, on a packed index value-by-packed index value basis. The direction of prediction can be horizontal, vertical, diagonal in right-downward direction, or some other direction. In Figure 17, for example, the direction of prediction is vertical, producing a block (1730) of packed index residual values packed, resid- The packed index value n _pac ked(i,j) is predicted using the preceding packed index value in the same column The packed index residual value is simply the difference between the two values, ripacked resiJj ) = n _pa cked{i ) - n _pa cked(i -l). Similarly, for horizontal prediction, the packed index residual value is the difference between the packed index value and its left neighboring packed index value as the predicted index value: n _pa cked resid j) = ripackeaii,]) - , j)■ For diagonal prediction at a 45 degree downward angle, the packed index residual value can be calculated as n _pa cked resiJjJ) = n _pac ked{i ) - (n _pa cked(i - 1)+ n _P ackedii- ,j))» , or it can be calculated as n _pa cked residii ) = n _pac ked{i,j) - n _pa cked(i- i ,j-i). Prediction in other diagonal directions can similarly blend neighboring packed index values depending on the angle of prediction. For prediction at the edge of a block (e.g. , i < 0 and/or j < 0), the neighboring packed index value can have a value of zero or be given a default value. During decoding, the decoder determines the same predicted packed index value (depending on the direction of prediction) and combines the predicted packed index value with the packed index residual value. For vertical prediction, n _pa cked(i,j) ⁼ ripacked resid j) + ripackeai j- ^)- For horizontal prediction, n _pa cked(i,j) = n _pa ckedresid(i ) + n _pa cked(i- ^ ,j)- For diagonal prediction at a 45 degree downward angle, n _pac ked{i ) = n _pa cked,resid(i ) + {n _pa cked(i - 1)+ , or ripackeaii ) = n _pa cked,re _S id(i,j) + n _packe d(i-l ,j- l). Prediction in other diagonal directions can similarly blend neighboring packed index values depending on the angle of prediction.

[0153] In the preceding examples of prediction, subtraction operations are used during encoding, and addition operations are used during decoding. Alternatively, an encoder and decoder can use bitwise exclusive OR (XOR) operations in prediction.

[0154] When packed index values are predicted, the encoder determines a packed index residual value as a packed index value XOR its predicted packed index value. When index values are predicted, the encoder determines an index residual value as an index value XOR its predicted index value. The residual value is signaled to the decoder. The predicted value can depend on the direction of prediction (e.g. , horizontal, vertical, diagonal).

[0155] When packed index values are predicted, the decoder determines a packed index value as its packed index residual value XOR the predicted packed index value. When index values are predicted, the decoder determines an index value as its index residual value XOR its predicted index value. Again, the predicted value can depend on the direction of prediction (e.g., horizontal, vertical, diagonal).

[0156] Figure 18 illustrates prediction using XOR operations during encoding and decoding. The packed index value is 7, and the predicted packed index value (based on one or more neighboring packed index values) is 6. During encoding, the encoder determines the packed index residual is 1 : 00000001 = 000001 1 1 XOR 000001 10. During decoding, the decoder reconstructs the packed index value 7 from the packed index residual value and predicted packed index value: 000001 1 1 = 00000001 XOR 000001 10.

[0157] Instead of being applied to all bits of a value, predicted value and residual value, the prediction operations (whether subtraction, XOR or otherwise) can be applied to a subset of the bits. For example, the XOR operation can be applied to the lowest p bits of an index value and its predicted index value. Suppose p _mas k is (l«p)-l . The encoder determines an index residual value n _resi d(i,j) = (n(i,j)»p«p) + (n _pre dicted(i,j) & Pmask) XOR (n(i,j) & p _mas k), where n _pre dicted(i,j) can be n(i,j-\), or n(i-\ ,j) or some other predicted value depending on direction of prediction. The residual value is signaled to the decoder. The decoder

determines the index value n(i,j) = (n _resi d(i,j)»p«p) + (n _res id(i ) & Pmask) XOR (n _pre dictea{i, j) & pmask)- The same operations can be applied to predicted packed index values. The operations on partial bits can be applied to least significant bits, most significant bits or middle bits of the values. The bits subjected to prediction operations can be continuous or not continuous.

[0158] In implementations that follow HEVC syntax, the encoder and decoder can reuse a coding tool that is normally used to predict residual values to instead predict elements of an index map. In particular, when no transform is applied to residual values (but quantization may or may not be applied), the encoder and decoder can use the tool to predict residual values from neighboring residual values. The coding tool and syntax for signaling its use can also be applied to elements of an index map.

[0159] Figure 19 shows a technique (1900) for prediction of elements of an index map during encoding. The technique (1900) can be performed by an encoder as described with reference to Figure 3 or Figures 5a and 5b, or by another encoder.

[0160] The encoder encodes (1910) data, in particular, encoding elements of an index map for a block using prediction. For example, when the index map includes index residual values, the encoder predicts an index value that represents a base color, then determines an index residual value based on the index value and the predicted index value. Or, when the index map includes packed index residual values, the encoder predicts a packed index value (where the packed index value is an index value that represents a base color and is packed into a coefficient coding syntax structure), then determines a packed index residual value based on the packed index value and the predicted packed index value. The encoding with prediction can include subtraction operations, XOR operations or other operations, on all bits of the values or on partial bits of the values.

[0161] The encoder outputs (1920) the encoded data. The encoder checks (1930) whether to continue with the next block. If so, the encoder continues by encoding (1910) elements of an index map for the next block.

[0162] Figure 20 shows a technique (2000) for prediction of elements of an index map during decoding. The technique (2000) can be performed by a decoder as described with reference to Figure 4 or Figure 6, or by another decoder.

[0163] The decoder receives (2010) encoded data and decodes (2020) the encoded data. In particular, the decoder decodes elements of an index map for a block using prediction. For example, when the index map includes index residual values, the decoder predicts an index value that represents a base color, then combines an index residual value and the predicted index value. Or, when the index map includes the packed index residual values, the decoder predicts a packed index value (where the packed index value is an index value that represents a base color and is packed into a coefficient coding syntax structure), then combines a packed index residual value and the predicted packed index value. The decoding with prediction can include addition operations, XOR operations or other operations, on all bits of the values or on partial bits of the values.

[0164] The decoder checks (2030) whether to continue with the next block. If so, the decoder continues by receiving (2010) encoded data for the next block.

E. Handling of Exception Values.

[0165] When BCIM mode is used, in a base color table, an encoder and decoder can assign index values to every different sample value that is used in a block. In some cases, this can result in a base color table that is inefficient to signal, especially when some index values are used very infrequently. When a particular base color rarely appears in a block, it might not be worthwhile to assign an index value to that base color.

[0166] In some example implementations, an encoder and decoder can use any of several approaches to handle base colors that occur rarely in a block. An "exception" value with one of these rare base colors is not encoded or decoded using an index value.

[0167] In one approach, when an encoder identifies an exception value, the encoder determines the closest base color that is represented with an index value in the base color table. The encoder uses the index value for the closest base color to represent the exception value. A decoder reconstructs the exception value as the sample value associated with the index value in the base color table. This approach is a type of lossy compression and, in some cases, can lead to significant distortion.

[0168] In another approach, the encoder signals exception values directly as sample values, which may be quantized. For example, the encoder determines the highest packed index value h that is possible for a base color table, which depends on the approach used for mapping. When n _pac ked = n, the value h is the highest value of n _pac ked- When odd values of n are mapped according to n _pac ked = (n+\)/2, the value h is again the highest value of n _pac ked-

[0169] The encoder represents an exception value as e = s/q+h+\, where q is a quantization parameter, and / represents division with truncation towards zero. If no quantization is applied, q=\, and the division operation can be skipped. At the decoder side, if the packed index value n _pac ked is less than or equal to h, the decoder decodes a sample value s from the packed index value n _pac ked using mapping operations and a base color table, as described above. If the packed index value is greater than h, however, the packed index value is actually an exception value e, and the sample value is reconstructed as s " = (e-h-\)*q.

[0170] Figures 21a and 21b illustrate processing (2100) of exception values and index values for BCIM mode during encoding and decoding, respectively. In Figure 21a, a block (2110) of sample values s includes some sample values that are represented with index values for base colors: 85, 41, 200, 168, 127, 26. These values are encoded in BCIM mode using the base color table shown in Figure 21a.

[0171] The block (2110) also includes two sample values that occur infrequently in the block, and hence are not represented with index values for base colors. These two exception values - 179 and 29 - are encoded using a quantization parameter q (with q = 4 in Figures 21a and 21b) and highest packed index value h (with h = 3 in Figures 21a and 21b). The value of h depends on the range of index values. The value of q can change from block to block, slice to slice, tile to tile, picture to picture, etc. As shown in the block (2130) of packed index values and exception values e, the sample value 179 is encoded as the exception value 49 = (181/4) + 3 + 1, and the sample value 29 is encoded as the exception value 11 = (29/4) + 3 + 1.

[0172] As shown in Figure 21b, during decoding, any packed index value greater than 3 is processed as an exception value. The exception value 49 is reconstructed as the sample value 180 = (49 - 3 - 1)*4, and the exception value 11 is reconstructed as the sample value 28 = (11 - 3 - 1)*4. Most values in the block (2150) are decoded in BCIM mode using mapping operations and the base color table shown in Figure 21b.

[0173] Figure 22 shows a generalized technique (2200) for encoding using index values and exception values in a BCIM mode. The technique (2200) can be performed by an encoder as described with reference to Figure 3 or Figures 5a and 5b, or by another encoder.

[0174] The encoder encodes (2210) data for a block using one or more index values each representing a base color and one or more exception values not representing any base color. For example, the encoder encodes some of the sample values of the block using packed index values or packed index residual values, as described above. The encoder also encodes some of the sample values as exception values. For example, the encoder replaces a sample value with an exception value using a quantization parameter and a highest packed index value.

[0175] The encoder outputs (2220) the encoded data. The encoder checks (2230) whether to continue with the next block. If so, the encoder continues by encoding (2210) data for the next block.

[0176] Figure 23 shows a technique (2300) for decoding using index values and exception values in a BCIM mode. The technique (2300) can be performed by a decoder as described with reference to Figure 4 or Figure 6, or by another decoder.

[0177] The decoder receives (2310) encoded data and decodes (2320) the encoded data. In particular, the decoder decodes one or more index values each representing a base color. The one or more index values can be packed index values or packed index residual values. The decoder also decodes one or more exception values not representing any base color. For example, for one of the exception values, the decoder reconstructs a sample value using the exception value, a quantization parameter and a highest packed index value.

[0178] The decoder checks (2330) whether to continue with the next block. If so, the decoder continues by receiving (2310) encoded data for the next block.

[0179] Figure 24 shows a more specific technique (2400) for using index values and exception values during decoding of a BCIM-mode block. The technique (2400) can be performed by a decoder as described with reference to Figure 4 or Figure 6, or by another decoder.

[0180] For a packed index value of the block, the decoder checks (2520) whether the value exceeds a threshold. For example, the threshold is the highest possible value for the packed index value. If the packed index value exceeds the threshold, the decoder processes (2540) the value as an exception value. Otherwise, the decoder processes (2530) the value using mapping operations and a base color table. The decoder then checks (2550) whether to continue with another value in the block. If so, the decoder checks (2520) how to decode the next value.

[0181] In the foregoing examples, the threshold is based on a highest packed index residual. Alternatively, when packed index residual values are used, the threshold is based on a highest possible packed index residual value, which depends on the greatest possible difference between a packed index value and its predicted packed index value.

[0182] In view of the many possible embodiments to which the principles of the disclosed invention may be applied, it should be recognized that the illustrated embodiments are only preferred examples of the invention and should not be taken as limiting the scope of the invention. Rather, the scope of the invention is defined by the following claims. We therefore claim as our invention all that comes within the scope and spirit of these claims.