FIELD OF THE INVENTION

The invention relates to the field of image retargeting.

BACKGROUND OF THE INVENTION

The recent developments in the field of display technologies have seen great diversity in display sizes and same content is required to be displayed in different dimensions and aspect ratio for different devices. Typically, videos recorded for the old 4:3 ratio of CRT television are now displayed on 16:9 wide screen TV.

There is thus a need of algorithm that could adapt images to displays different than originally intended for.

Basic image resizing techniques are linear scaling or cropping. However, these techniques lead to image quality degradation due to loss of details, anisotropic squish or stretch, suppression of region outside the cropping window, etc.

Hence effective adaptation of images considering the image content is needed. Such an intelligent adaptation is known in the art as "Image retargeting" or "Video retargeting" if video is considered.

For modifying "intelligently" an image, numerous methods use a saliency map which defines an information value for each pixel.

For instance, document EP 1 968 008 discloses a method for content-aware image retargeting which is known as "Seam Carving". A saliency map, also called an energy image, from a source image is generated according to an energy function, often a luminance gradient function. From the energy image, one or more seams are determined according to a minimizing function such that each seam has a minimal energy. Each seam is applied to the source image by suppressing or duplicating the seam to obtain a target image that preserves content but with a different aspect ratio.

This technique was extended to video retargeting by defining a 2D seam surface in a 3D video space-time cube. The intersection of the surface with each frame defines a seam in the sense of the document. The manifold seam surface allows the seam to change adaptively over time, maintaining temporal coherence. Whichever the retargeting method used, the computation of a saliency map is a computer intensive operation. The better quality of the rescaling obtained by these content aware retargeting methods creates a need for better processing power usage. SUMMARY OF THE INVENTION

It would advantageous to achieve a method and apparatus which reduce the cost of computation whilst maintaining the high quality achieved by the content aware rescaling methods.

To better address one or more of these concerns, in a first aspect of the invention, a method for retargeting an image in a system comprising a transmitter connected to at least one receiver through a communication network, comprises:

computing by the transmitter an image saliency map;

transmitting the image and the saliency map from the transmitter to at least one receiver through the communication network;

- retargeting by at least one receiver the transmitted image based on the transmitted saliency map.

By computing the saliency map at the transmitter level, this computer intensive operation may be mutualised between many receivers. Furthermore, a unique saliency map is usable whatever the final aspect ratio is. Therefore, the aspect ratio of each receiver does not need to be the same.

The method has also the advantage to transfer the computation on the transmitter which is generally a high-end professional equipment. At the opposite, the receiver is generally a general purpose public equipment such as a mobile phone or a TV set for which the manufacturing cost must be kept as low as possible.

The computation of the saliency map may be also done well in advance and the saliency map is stored into the transmitter until a transmission is requested to smooth over time the computation needs.

In a particular embodiment, the saliency map comprises two ID saliency curves for horizontal and vertical scaling respectively.

This transformation of the saliency map reduces significantly the quantity of data to transmit through the communication network.

In a second aspect of the invention, a system for retargeting an image comprises:

a transmitter comprising: - a saliency map calculator for computing a saliency map of the image; and

- a network interface for transmitting the image and the saliency map onto a communication network;

a receiver comprising:

- a receptor connected to the communication network for receiving the transmitted image and the transmitted saliency map;

- an image modifier for retargeting the transmitted image based on the transmitted saliency map.

In a third aspect of the invention, a transmitter in a system for retargeting an image comprises:

a saliency map calculator for computing a saliency map of the image; and a network interface for transmitting the image and the saliency map onto a communication network;

In a fourth aspect of the invention, a receiver in a system for retargeting an image comprises:

a receptor connected to the communication network for receiving the transmitted image and the transmitted saliency map;

an image modifier for retargeting the transmitted image based on the transmitted saliency map.

Depending on the type of image, a particular embodiment may be preferred as easier to adapt or as giving a better result. Aspects of these particular embodiments may be combined or modified as appropriate or desired, however.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment described hereafter where:

Fig. 1 is a schematic view of a system according to an embodiment of the invention;

Fig. 2 is a flowchart of a retargeting method according to a first embodiment of the invention;

Fig. 3 is a flowchart of a retargeting method according to a second

embodiment of the invention;

Fig. 4 shows three different local magnification curves; Fig. 5 shows an image divided into eight vertical sections and four horizontal sections;

Fig. 6 illustrates the usage of a nonlinear position transformation curve;

Fig. 7 shows a scaled image obtained by the nonlinear position transformation curve of Figure 6;

Figs. 8-11 show different magnification curves obtained by using the method in accordance with an embodiment of the invention;

Fig. 12 is a flow chart illustrating a first variant of the second embodiment for correcting aspect ratio in accordance with the invention;

Fig. 13 is a flow chart illustrating a second variant of the second embodiment for correcting aspect ratio in accordance with the present invention; and

Fig. 14 is another flow chart illustrating the variant of Fig. 13.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In reference to Figure 1, a communication network 1, such as Internet, connects a transmitter 3 and at least one receiver 5.

The transmitter 3 comprises a network interface 7 to connect the transmitter 3 to the communication network 1 and to transmit data to the receivers 5.

The transmitter 3 comprises also a calculator 9 and a storage area 11 and a variety of input sources 13 transporting video in a compressed format, such as MPEG-2, MPEG-4 or other formats. A decoder 15 generates from the received video raw image data IMD-I, a stream of pictures, one picture per frame to be used by the calculator 9.

The receivers 5 comprise a receptor 17 which may be a network interface similar to the network interface 7 of the transmitter 3. The receptor 17 connects the receiver 5 to the communication network 1 to receive the data transmitted by the transmitter 3.

The receivers 5 comprise also an image modifier 19 to resize a received image to a new aspect ratio adapted to the display 21.

Figure 2 illustrates the relationship between the transmitter 3 and at least one receiver 5 in the form of a flowchart where each rectangle in a column refers to a step executed in the corresponding apparatus.

At step 31, the transmitter 3 receives a video stream through one of its input sources 13. The video stream is decoded, step 33, to a stream of raw images sent to the calculator 9. The calculator 9 computes, step 35, the saliency map of the images by using some energy function. For instance, the calculator may use a saliency map computed according to EP 1 968 008.

Each image with its saliency map is transmitted, step 37, to the receiver 5. To optimize the throughput rate of the transmission, the image and its saliency map may be compressed by using well-known algorithms. For instance, and to avoid a compression step, the transmitted image may be the compressed image received by the input sources 13.

Therefore, the decoded image is used only to compute the saliency map.

The receiver 5 receives, step 39, the image and its saliency map and, if necessary, uncompresses them. The image modifier 19 retargets, step 41, the image to the desired aspect ratio by using the transmitted saliency map. The retargeting method used by the image modifier is chosen to be compatible with the saliency map. For instance, if the saliency map was computed according to EP 1 968 008, the retargeting method would be a seam carving method.

The retargeted image is then displayed, step 43, onto the display 21.

In a variant, figure 3, the saliency map computed at step 35 is transformed, step 51, into two ID saliency curves for horizontal and vertical scaling respectively before being transmitted. This transformation reduces substantially the quantity of information to transmit at step 53.

The image and the two ID saliency curves are received, step 55, and the ID saliency curves are then used by the receiver to calculate, step 57, scaling curves to apply to the received image for retargeting it, step 59.

Figure 4 shows three exemplary scaling curves and more specifically magnification curves describing local magnification. These curves are: one linear scaling curve with constant magnification multiplier, one linear scaling curve with negative multiplier and the so called "bathtub" curve. The shape of the bathtub curve is such that the unity scaling is used at the centre of the image, whereas the magnification increases toward the edges of the image. Unity scaling at the centre of the image means that the objects at the centre of the image remain undistorted. Usually magnification is between 0.5 and 2.0

Figures 5, 6 and 7 illustrate nonlinear image scaling by use of a nonlinear scaling curve and more specifically position transformation (or mapping) curve. The position transformation curve results as the integral of the magnification curve. Figure 5 shows an image 61 divided into eight vertical sections with equal width and four horizontal sections with equal width. There is also shown a line 63 from one corner of the image to the opposite corner.

To arrive at a nonlinearly scaled image 67 as shown in Figure 7, a nonlinear position transformation curve 65 is used as shown in Figure 6. Figure 6 clearly shows an image scaling curve, i.e. position transformation curve, which results as the integral of the magnification curve. A horizontal/vertical magnification curve has to be integrated over the horizontal/vertical positions to result in a horizontal/vertical position transformation curve. In the figure it can further be seen that near the edges of the image, the vertical sections become narrower, whereas close to the centre, the horizontal sections remain unchanged. By using this kind of curve, it is assumed that the most important information of the image is located near the centre of the image. In Figure 7, the straight line 63 of a slant angle is displayed as a curve 69 due to the nonlinear scaling in the horizontal direction.

However, it is to be noted that the most relevant information is not always located near the centre of the image. For this purpose different scaling curves can be advantageously used.

To determine the scaling curves to use, information about the local saliency, i.e. the saliency of each pixel, is accumulated in one direction (horizontal or vertical) as will be explained later in more detail. The accumulated local saliency is used to calculate costs for different scaling curves. In this example, a set of initial horizontal and/or vertical scaling curves is defined that include the standard curve, i.e. the "bathtub" curve, but also some curves that might be suitable in cases where the standard curve fails. This happens mainly when most important object(s) are near the side panels of the screen. The number of stored initial scaling curves is at least 2, but smaller than the number of pixels in the image. In most applications the usage of 3-10 initial scaling curves suffices.

Given the salient features or local saliency of the current image, a "cost" for each of these initial curves can be calculated. The cost of a scaling curve depends on the position of essential objects such as faces, moving objects, etc., in the image, such that the cost increases the more the local scaling factor differs from unity scaling (scaling factor 1) particularly at the position of these essential objects. In other words, a high number of salient features in locations where the scaling factor differs from 1 leads to a high cost value. For the calculation of the cost values, the salient features in locations where the scaling factor is 1 can be neglected.

The scaling curve, i.e. the position transformation curve, to be used in the actual image rescaling is calculated as a weighted average of the individual curves where the weights are inversely related to the aforementioned cost. This means that the weights are decreasing with increasing cost of a predefined scaling curve. All candidate curves (both horizontal and vertical scaling curves) individually cause the desired aspect ratio change. In this case when the sum of the weights equals 1 the resulting curves will also lead to the desired aspect ratio change. In case the input video sequence has a good temporal stability (no scene change), the weights will only change gradually causing also the output retargeted video to be temporally stable. In the event of low temporal stability of the input video (scene change), the output can react immediately to the updated cost without remaining effects from the previous scene. Consequently, the so much appreciated temporal stability of the proposed rescaling method does not prohibit rapid adaptation to the new shot. Moreover, by selecting the initial curves more or less ambitiously (i.e. the curves differ from the standard curve) it can be guaranteed that the artifacts of the aspect ratio correction are modest.

Tables 1-4 illustrate concrete examples for calculating the correct magnification curve to be used in the image scaling. In the tables each column represents a specific horizontal location in the image. For simplicity the predefined scaling curves in these examples use only two different magnification values, namely values 1 and 2. These magnification values can also be referred to as local magnification values or local scaling curves in more general terms. Thus, the predefined scaling curves can be considered as consisting of several local scaling curves that can be considered as glued together. The predefined set of scaling curves contains three scaling curves in each example. The quality figure shown in the tables is inversely related to the cost values, which are calculated for each curve in the predefined set by taking into account the local saliency in the image as was explained above. For the final scaling curve, for each location 7 the resulting magnification in one direction, i.e. horizontal or vertical, can be calculated by using the following formula:

Number of _ curves

^QUALITY _FIGURE _{Curve X} ^■ MAGNIFICATION _{Locatlon Y} .

X=I

The resulting final magnification curves for Tables 1, 2, 3 and 4 are shown in figures 8, 9, 10 and 11, respectively. The curves shown in these figures exhibit abrupt changes in magnification and for this reason in order to avoid unacceptable distortions, these curves may have to be smoothed e.g. by filtering or starting from smooth curves.

To transform the saliency map into two ID saliency curves, Figure 12, the local saliency, i.e. the saliency of each pixel, is accumulated, step 71, in a first direction, such as vertical direction, for obtaining a one-dimensional projection in a second direction (horizontal in this example) of the two-dimensional saliency map. The projection into one direction also covers the situation where the projection takes place over columns (or rows) that are wider than a single pixel line. In other words, the projection projects the local saliency of individual pixels in the received image, or a combination of local saliencies over a group of pixels orthogonal to the accumulation direction. The combination of local saliencies can for instance allow using a median or weighted average.

A set of initial scaling curves is obtained, step 73. Costs are calculated, step 75, for the different initial curves as explained above by taking into account the local saliency in the image. And a new scaling curve is calculated, step 77, based on the calculated costs. Finally, the image is rescaled, step 79, in a second direction (horizontal direction in this example) by applying the new scaling curve. The image is now ready to be displayed to the user. The second direction is substantially orthogonal to the first direction. It is to be noted that in the example above scaling was applied in just one direction, but is equally possibly to apply scaling in both horizontal and vertical directions.

In another embodiment of this variant, Figure 13, the one-dimensional projection in the second direction is obtained, step 71. In this projection, peaks indicate the location of the salient features. Next the created projection is inverted, step 81, to obtain a local magnification factor profile. The inversion is done since for salient features a magnification factor close to one (i.e. no extra magnification) is desirable. The local magnification profile is also advantageously smoothed; step 83. And the local magnification profile is then used as scaling curve, step 85-89, for retargeting the image. An example of the method is shown at Figure 14.

The local magnification profile may be computed at the transmitter level as it is independent of the final aspect ratio. In that case, it has the same role as the ID saliency curve. Consequently, the term "transmitted saliency map" needs to be understood, in this document, as comprising all data conveying saliency information to the receiver,

independently of the final aspect ratio to be applied to the image.

The method may be implemented by a computer program product that is able to implement any of the method steps as described above when loaded and run on computer means of an image resizing apparatus. The computer program may be stored/distributed on a suitable medium supplied together with or as a part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless

telecommunication systems.

An integrated circuit may be arranged to perform any of the method steps in accordance with the disclosed embodiments.

While the invention has been illustrated and described in details in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiment.

For instance, the receiver 5 may be a part of a TV set or be integrated into a set top box connected to a TV set through a HDMI interface, for instance. But the receiver may also be a part of a mobile terminal able to receive and display video streams.

Other variations to the disclosed embodiments can be understood and effected by those skilled on the art in practicing the claimed invention, from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements and the indefinite article "a" or "an" does not exclude a plurality.