FIELD OF THE INVENTION

The invention relates to a display apparatus such as an apparatus comprising an OLED display device, to a method of driving a display device and to a display driver device for such a display apparatus.

BACKGROUND

Driving of a display screen is described in an article titled "Video processing for active-matrix polymer OLED TV" by F.P.M. Budzelaar et al, published in the Journal of the SID, Volume 14, Issue 5, May 2006, pp. 461-466. As described in this article, OLED displays offer various possibilities for improving image quality. OLED displays have the capability to produce locally high peak luminance levels, which can be used by enhancing the brightness of highlights, i.e. by detecting small areas wherein the brightness is above a threshold and enhancing the brightness in these areas. Furthermore, peak brightness in images can be increased by applying a dynamically variable gain factor to the image content, based on the maximum brightness in successive images. Increased brightness improves perceived image quality.

Budzelaar et al note that limits are set on the allowable brightness adjustments by global and local panel temperature. OLEDs suffer damage when their temperature becomes too high. The gain should not be increased to a level where the resulting OLED driving results in excessive heating of the display. Similarly, enhancement of the brightness of highlights should be limited to avoid high local temperatures.

SUMMARY

Among others, it is an object to provide for improving displayed brightness information with reduced risk of overheating.

According to one aspect a display driver device according to claim 1 is provided. The device comprises an initial gain selection module configured to select an indication of a gain as a function of position in the images, dependent on a position dependent estimate of a temperature of the display screen. In an embodiment, the estimate of the temperature may be an estimated that has been computed on the basis of the content of the displayed images. In another embodiment, the estimate may be based on measurements from temperature sensors coupled to the display screen. As used herein "estimate" is used to refer to measurement results as well as model based results and combinations thereof. The gain is selected to reduce the gain in an area with higher estimated temperature relative to gain in areas with lower estimated temperature.

A low pass filter converts this indication of the gain into a modified indication of the gain, which is supplied to a gain control input of a gain control circuit that applies the gain to a current image. The low pass filter provides for application of a controllable spatial filter response function to the indication of the gain. The spatial filter response function is controlled dependent on a content of a current image, to limit blurring of the indication of the gain at positions of edges in the current image. A bilateral filter response may be used for example.

It has been found that the image content of the images affects the display screen temperature without necessarily resulting in a one to one relation between image values in an image and temperature. For example, when the successive images show a moving high brightness object, display screen temperature may rise more at image positions that show the object in many images than at image positions near the edge of the object that show the object only in a few images, for example only after the object has moved to start covering those positions. If the temperature estimate were used directly to control the gain, this would result into brightness artifacts within the object. By modifying the gain using image edge limited low pass filtering such artifacts are reduced.

In an embodiment the display driver circuit has a memory for storing data representing an estimate of current temperature as a function of position on a display screen and an update circuit configured to update the stored data successively dependent on successive ones of images, the initial gain selection module being configured to derive the position dependent estimate of the temperature dependent on the stored data. Thus temperatures estimates may be provided without requiring temperature sensors and/or for future time points. In a further embodiment the update circuit is configured to update the stored data dependent on the successive ones of images according to a result obtainable by applying the low pass filtered gain to the images. Thus a more accurate estimate is obtained.

In an embodiment the update circuit configured to update the stored data by adding a first term in proportion to modeled heat dissipated by the display screen as a function of the images, subtracting a second term in proportion to modeled heat loss to an ambient of the display screen and a third term in proportion to modeled thermal diffusion the display screen. Thus a realistic estimate may be obtained.

A display apparatus, comprising the display driver circuit and a display screen coupled to the video output of the display driver circuit may be provided. In an embodiment the display screen may be an OLED display screen. Furthermore a method of displaying a series of images on a display screen is provided and a computer program product to control display of a series of images.

BRIEF DESCRIPTION OF THE DRAWING

These and other objects and advantageous aspects will become apparent from a description of exemplary embodiments, using the following figures.

Figure 1 shows a display apparatus

Figure 2 shows a signal processing circuit

Figure 3 shows a flow-chart of signal processing

Figure 4 shows a display apparatus

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Figure 1 shows a display apparatus comprising a video input 10, a gain control circuit 12, a signal processing circuit 14 and a display screen 16. Gain control circuit 12 is coupled between video input 10 and display screen 16. Display screen 16 may be an OLED display screen for example. Gain control circuit 12 may comprise a multiplier with a first input coupled to video input 10, a second input coupled to signal processing circuit 14 and an output coupled to an image input of display screen 16. A video decoder and/or video processing circuit (not shown) may be present between a device input (not shown) and video input 10, or gain control circuit 12 may be a video decoder and/or video processing circuit that supports a gain control function. For the sake of exposition such a video decoder and/or video processing circuit will be referred to as a gain control circuit 12 as well. The display apparatus may comprise an image driver device (for example an integrated circuit) comprising gain control circuit 12, a signal processing circuit 14, the image driver device having a video output coupled to the output of gain control circuit 12, the display screen 16 being coupled to the video output external to the image driver device. Signal processing circuit 14 has an input coupled to video input 10 and an output coupled to a control input of gain control circuit 12. Figure 2 shows an embodiment of signal processing circuit 14. In this embodiment, signal processing circuit 14 comprises a memory 140 for storing data representing position dependent temperature estimates, an update module 142, an initial gain selection module 144, and a bilateral filter 146. By way of example, a memory 140 with a write input and a read output is shown. Update module 142 has inputs coupled to video input 10, an output of bilateral filter 146 and the read output of memory 140. Update module 142 has an output coupled to the write input of memory 140. Initial gain selection module 144 has an input coupled to the read output of memory 140. Bilateral filter 146 has inputs coupled to an output of initial gain selection module 144 and video input 10.

Update module 142, initial gain selection module 144, and bilateral filter 146 may be implemented using a programmable computer circuit by and program modules with instructions to make the programmable computer circuit perform their functions.

Alternatively, one or more of the update module 142, initial gain selection module 144, and bilateral filter 146 may be implemented using separate computer circuits or circuitry designed to perform the functions of update module 142, initial gain selection module 144, and/or bilateral filter 146. These functions will be described in terms of operation of signal processing circuit 14, but it should be understood that the steps of this operation describe steps performed by update module 142, initial gain selection module 144, and bilateral filter 146 to implement the functions.

In operation data representing a stream of images is supplied to display screen

16, to control display of these images by display screen 16. Gain control circuit 12 applies a gain to these images dependent on position in the images. Signal processing circuit 14 controls the gain, providing a position dependent gain that may be set differently for different ones of the images. The position dependent gain is computed dependent on data in memory 140 that represents an estimate of temperature of display screen 16 as a function of position.

As is known per se, display screens such as OLED display screens may be vulnerable to damage if heated to excessive temperatures during prolonged periods of time. Short lasting temperature excesses need not be damaging. The exact temperatures and exposure durations depend on the type of display. Given a display type, they can easily be established.

Figure 3 shows a flow-chart of an embodiment that includes computation of the position dependent gain. In a first step 31, initial gain selection module 144 maps position dependent temperature values from memory 140 to an intermediate signal d(r), wherein r represents position on display screen 16 or, equivalently, position in the images displayed on display screen 16. The intermediate signal indicates an initial selection of a position dependent gain to be applied to the images. As will be explained, this initial selection may be modified by bilateral filter 146.

In an embodiment initial gain selection module 144 may perform a thresholding function, producing a first value of the intermediate signal for image positions where the temperature values are below a threshold and a second value of the intermediate signal are above the threshold. The threshold may be set to a value below temperature values that are damaging for display screen 16 when display screen 16 is exposed to these temperature values for a prolonged period of time. Preferably the function applied to the temperature by initial gain selection module 144 provides a smoothed transition between the first and second values as a function of temperature. A logistics function applied to the temperature value may be used for example, which produces 90% of the difference between a maximum and minimum function value below the lower bound of the temperature values that are damaging for display screen 16. A look up table may be used to implement the mapping for example, but alternatively a computation may be used to compute intermediate signal values from the estimated temperature. The values of the intermediate signal may be gain factors that are suitable for being directly multiplied by image values to apply the gain, but this is not necessary. It may suffice that there is a one to one relation between such suitable gain factors and intermediate signal values. In one example the intermediate signal may represent temperature estimates.

Although an embodiment is described wherein a current temperature estimate is used to control the gain, it should be appreciated that temperatures for other time points may be used as well. For example, in displays like OLEDs, display temperatures that would be damaging if they persisted are not damaging if they occur only temporarily for not more than a predetermined duration. Hence, reduction of the gain may be omitted for short lasting high temperatures. In another example, the gain may be reduced in advance, before the temperature becomes excessive in order to prevent a later temperature excess.

Accordingly, in an embodiment, the intermediate signal may be derived from a sum of temperatures for an image position (e.g. time averages and/or weighted sums), over a time interval (which may be an effective time interval of a weighted sum with a length determined by the weights). This may be implemented by configuring update module 142 to compute sums for different image positions and store them in memory 140 for use by initial gain selection module 144, or by keeping estimates for a plurality of temperatures in memory 140, initial gain selection module 144 taking the average or sum. In either case, initial gain selection module 144 may select the indication of the gain based on the sums of the temperatures. The effective time interval over which the sums (average) are taken and the threshold summed temperature above which the gain is reduced may be selected dependent on the properties of the display device. To control the gain for the current image, a time interval may be used that includes the time points of display of images that will be displayed subsequently, or a time interval may be used that includes no future times. This may be used to select whether gain reduction is used in advance of heating or only after some amount of heating that is not yet damaging.

In a second step 32, bilateral filter 146 spatially filters the intermediate signal under control of a video signal that represents the current image that is to be displayed. Thus a modified version of the intermediate signal is obtained. Bilateral filters are known per se. The output signal value B(r) of a bilateral filter for a position r may be computed according to B(r) = Sum d(r-dr) * H(dr) * G(I(r)-I(r-dr))/Norm wherein

Norm= Sum H(dr) * G(I(r)-I(r-dr))

Herein dr is a spatial position offset and H(dr) values for different dr are filter coefficients of a spatial low pass filter, for example a Gaussian as a function of dr. However, any type of low pass filter may be used. I(r) and I(r-dr) are image signal values for positions r and r-dr, for example luminance values or sums of RGB values. G is a weight factor that depends on image signal value difference, for example a Gaussian function of the signal value differences, or another function that decreases towards zero with increasing signal value differences. The sums are taken over offset dr in a predetermined support area of the filter H.

The bilateral filter is used to align edges in the intermediate signal with edges in the image signal. The bilateral filter has the effect of spatial blurring of the intermediate signal values in image areas where the image signal values are less different from each other that an amplitude bandwidth defined by the amplitude response function G, without extending the blurring effect over edges with a transition in the image signal values that exceeds the amplitude bandwidth. In a third step 33, the modified version of the intermediate signal from bilateral filter 146 is used to control the position dependent gain applied by gain control circuit 12. If the modified version of the intermediate signal represents gain factors directly, image values may be multiplied by values of the modified version of the intermediate signal, or a table of products may be used to look up the products based on image values and gain factors. But if the modified version of the intermediate signal indicates the gain factors indirectly, gain control circuit 12 may use the modified version to look up the products, or convert the modified version to gain factors for application to a multiplier.

By applying the gain, gain control circuit 12 modifies the relative size of the image values from video input 10 according to the gain factor. According to the gain, the image values may be reduced at image positions where the output of bilateral filter 146 corresponds to a value or values of the intermediate signal for relatively higher temperatures, relative to the image value at image positions where the output of bilateral filter 146 corresponds to the a value or values of the intermediate signal for relatively lower temperatures. In an embodiment, for all image positions all color components of the image signal from video input 10 are adjusted according to the gain factor for these positions.

Because bilateral filter 146 is used to control the gain factors, this has the effect of relatively reducing the gain in image areas with boundaries that are aligned with edges in the current image when the image areas contain image positions with excessive temperature. As may be noted, this has the effect that the gain may also be reduced at image positions without excessive temperature, such as image position that have more recently become brighter due to movement in the images, without yet causing the temperature to be raised to damaging levels at these positions.

Subsequent steps are involved with updating of the estimated temperature in memory 140. In the illustrated embodiment a heating model is used. The temperature of display screen 16 can be modeled as the effect of net heat flow in terms of a sum of position dependent heat input dependent on the position dependent brightness of the displayed image, heat loss to the ambient and lateral thermal diffusion along the display screen.

The model may represent a grid of positions r on the display screen, which may be pixel positions for example, with vectors dx and dy between neigboring pixels along different directions. The position dependent heat input may be taken as a fraction of the power input, i.e. the fraction that is not converted into emitted light, according to the efficiency of the display. Different efficiencies may hold for different primary colors, e.g. for red, green and blue. The heat flow from a screen position r due to thermal diffusion may be modeled in terms of the temperature T(r) at that position and temperatures a the neighboring positions:

Diffusion = constant * ( 4*T(r) - T(r+dx) - T(r-dx) - T(r+dy) - T(r-dy))

The heat loss to the ambient may be modeled as Ambient loss = F(T(r) - Ta) Herein Ta is ambient temperature and the function F may be a linear function for example. The heat loss is position dependent because the temperature T(r) is position dependent. Current temperatures for a screen position may be computed for example by updating the current estimated temperature at the position represented by the data in memory 140 with an update dependent on the net heat flow for the position. The various parameters of the model may be determined from measurements on a sample display screen, for example by fitting the parameters to temperature measurements on that sample display screen obtained with different image sequences. Alternatively, the parameters may be derived from design data. It should be noted that it suffices that the model provides a reasonable approximation of temperature. Absolute accuracy is not required.

In an embodiment, the model may be used to predict future temperatures based on images that have yet to be displayed. This makes it possible to adapt the gain in advance. In a further embodiment the initial selection of the gain may be made iteratively, each time using a new iteratively selected gain to compute future resulting temperature. In this embodiment the intermediate signal may be changed to reduce the indicated gain locally at image positions where the temperature, or time summed value, is predicted to exceed a threshold value and the iterations may be repeated until the temperature, or time summed value nowhere exceeds the threshold value.

In a fourth step 34, update module 142 receives the output of bilateral filter 146 and the image values from video input 10 to compute the heat input. Update module 142 may compute the image values after application of the position dependent gain factor for example and the associated fraction of the drive power that will be produced as heat.

Optionally, may use an output signal of gain control circuit 12 to compute the heat supply, instead of the output of bilateral filter 146 and the image values from video input 10. In a fifth step 35, update module 142 reads the data representing the previous temperature from memory 140. In a sixth step 36, update module 142 computes heat flow due to diffusion and heat loss to the ambient from the data representing the previous temperature. In an embodiment, the display apparatus may contain an ambient temperature sensor (not shown) coupled to update module 142 to supply a measurement of ambient temperature for use in this computation. Alternatively, a worst case default value for ambient temperature may be used for this computation.

In a seventh step 37 update module 142 computes position dependent temperature updates from the computed heat flow, e.g. from the heat supply, heat diffusion and heat loss, applies the updates to the data representing the temperature and writes the updated temperatures to memory 140. After seventh step 37, the process repeats from first step 31.

It should be noted that the position dependent temperature depends on the content of a series of previous images. Therefore, unless all successively displayed images are the same, there is no one to one relation between temperature and image content of a currently displayed image. Moreover, even if the images are the same, there is no one to one relation between pixel values and pixel temperature, because of diffusion. Thus, pixels with the same image content may have a higher or lower temperature dependent on whether they are located remotely from or close to image edges both within the image and at the outer boundary of the image. In order to distinguish image positions with potentially damaging temperatures from those without such temperatures, a model including diffusion is used.

In the illustrated computation, temperature change as a function of time are computed for each screen position from the net heat flow and updates are performed for successive time points of display of successive images. But a one to one relation between the time points of modeled temperatures and the time points of images is not needed. That is, the update steps need not be part of the same loop as the first three steps that involve control of the displayed image. For example, the temperatures may be computed for more widely spaced time points, using sums of heat supply values for a plurality of images, or a subsampled set of images. This reduces required computational resources. As used herein, the use of successive images for the computation of the temperature estimates refers to use of directly successive images as well as to use of sub-sampled images that succeed each other with intervening images in between. Alternatively, the temperatures could be computed for time points at a shorter mutual temporal distance than the images to obtain a more accurate estimate of temperature. Similarly, the temperature estimates may be recorded only for more widely spaced image points than pixels in the images.

Although a specific example of signal processing circuit 14 has been shown, it should be appreciated that variations are possible because it suffices to have an approximate estimate of the temperature. For example, instead using the image after applying the gain factor as input for computing the update, the image before applying the gain factor may be used as an approximation. As another example, instead of accounting for diffusion by temperature differences a convolution may be used. As another example, the intermediate signal may represent temperatures, a non linear gain selection being applied to the filtered temperatures between bilateral filter 146 and gain control circuit 12.

Figure 4 shows an embodiment wherein a plurality of temperature sensors 40 (only part of the sensors shown) is provided, in thermal contact with respective different positions on display screen 16. Temperature sensors 40 are coupled to update module 142. In this embodiment, temperature data obtained from sensors 40 may be used as input for the computation of the position dependent gain without using the computation based on heat flow. In this case bilateral filter 146 may be used to align temperature sensor based input with image edges in the computation of the gain factors. As used herein, both temperature data derived from measured temperatures and model based temperatures data will be referred to as temperature estimates. Also in this case, a sum of temperatures for different time points may be computed and used as input for the computation of the position dependent gain instead of the instantaneous temperature.

In another embodiment, a combination of data from sensors and image signal data may be used to compute temperature estimates. Thus, for example, an interpolation of position dependent temperature between the positions of sensors 40 may be performed, using iterative adjustments of the position dependence of ambient loss used in the model directed at reducing the differences between model based and measured temperatures for the sensor positions.

Although an embodiment has been shown wherein the function of aligning gain adjustment with edges of image areas is performed by means of a specific filter response for bilateral filter 146, it should be appreciated that this function can be performed by other edge preserving filters. A bilateral filter is only one example of edge preserving filters that are known in the art. For example, bilateral filter 146 may be configured to simulate a gain factor diffusion process with position dependent diffusion coefficients that are reduced with increasing image intensity gradient in the current image from video input 10. That is the filtered output may be computed iteratively from d(r;n+l) = d(r;n) - D(r)*(4*d(r;n)- d(r+dx;n)-d(r-dx;n)-d(r+dy;n)-d(r-dy;n)), wherein d(r;n) is the gain value for position r at the nth iteration and D(r) is a position dependent diffusion coefficient for the gain factor, which is set dependent of the image content, D at a position r being increasingly reduced towards zero with increasing gradient in the image values in an image area containing r and/or set to zero upon detection of an edge in the image at the position r. In another example, a segmentation algorithm may be applied to the current image to segment the image into image areas and an average or peak intermediate signal value of each image area may be assigned to the image positions in the image area. Segmentation may be performed for example by thresholding, histogram comparison, connected component labeling etc.

Embodiments have been shown with a position dependent decrease of image intensity at locations determined from areas with higher position dependent temperature estimates after alignment of the edges of the areas with image edges, without other adjustments of the image intensity. However, it should be appreciated that additionally other adjustments may be made. For example, gain control may be combined with known highlight enhancement. A highlight area detector may be included, coupled to gain control circuit to enhance the gain in detected highlight areas of the image. An adder located between bilateral filter 146 and gain control circuit 12 may be used to increase the gain in the highlight areas for example. Thus, local intensity reduction and enhancement may be combined. Due to heat diffusion the heat from small high intensity highlights may not lead to excessive

temperatures. Update module 142 may be provided with information about the highlight areas and their heat supply and to use this information to compute the resulting temperature for use in gain reduction. Highlight detection may involve detection of image areas with less than a predetermined size and intensity above a first threshold, surrounded by an image area with intensity below a second threshold.

In an embodiment the described steps may be performed by a programmable computer under control of a program of instructions. The program may be provided in a computer readable medium, such as a magnetic or optical disc or a semi-conductor memory.

Other variations to the disclosed embodiments can be understood and effected by those skilled in 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 or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.