Enhancing contrast

1. A method of preparing a video signal for a display, comprising:

processing the video signal by:

detecting ambient light levels; and

performing local contrast enhancement processing on the video signal based on the detected ambient light levels to produce a processed video signal, wherein the local contrast enhancement processing comprises a spatial filtering operation to determine luminance on a pixel-by-pixel basis; and

presenting the processed video signal to the display.

2. The method of claim 1 wherein luminance is associated with object shapes and textures.

3. The method of claim 1 , wherein the spatial filtering operation uses a spatial filter that comprises a two dimensional spatial filter.

4. The method of claim 1 , wherein the filtering operation uses a spatial filter that varies based on detected ambient light levels.

5. The method of claim 1 , wherein the local contrast enhancement processing further comprises determining an enhancement for each pixel depending on pixel image brightness and detected ambient light levels.

6. The method of claim 5 , wherein the local contrast enhancement processing is characterized by a static gain.

7. The method of claim 6 , wherein the static gain increases when the pixel image brightness of a pixel is less than one of the detected ambient light levels and decreases when the pixel image brightness is greater than one of the detected ambient light levels.

8. The method of claim 5 , wherein the local contrast enhancement processing is characterized by a dynamic gain.

9. The method of claim 5 , further comprising adjusting the local contrast enhancement processing based on a comparison of pixel image brightness to one of the detected ambient light levels.

10. The method of claim 9 , wherein the adjusting of the enhancement processing is controlled by an enhancement control variable.

11. The method of claim 10 , wherein the enhancement control variable has a functional representation with values increasing when the pixel image brightness is less than one of the detected ambient light levels and decreasing when the pixel image brightness is greater than one of the detected ambient light levels.

12. The method of claim 5 , wherein the local contrast enhancement processing comprises a low-pass processing component and a high-pass processing component.

13. The method of claim 1 , further comprising performing a luminance adjustment process dependent on a comparison of a pixel image brightness with one of the detected ambient light levels, the luminance adjustment process being performed after the local contrast enhancement processing.

14. The method of claim 13 , wherein the luminance adjustment processing is controlled in accordance with an input-output luminance mapping table dependent on one of the detected ambient light levels.

15. The method of claim 14 , wherein the input-output luminance mapping table comprises a set of mapping curves generated by a functional representation of remapping input luminance onto an adjusted output luminance.

16. The method of claim 15 , wherein the functional representation is a gamma function.

17. The method of claim 1 , further comprising performing a luminance adjustment process dependent on a comparison of a pixel image brightness with one of the detected ambient light levels, the luminance adjustment process being performed before the local contrast enhancement processing.

18. The method of claim 17 , wherein the luminance adjustment processing is controlled in accordance with an input-output luminance mapping table dependent on one of the detected ambient light levels.

19. The method of claim 1 , further comprising performing two luminance adjustment processes dependent on a comparison of a pixel image brightness with one of the detected ambient light levels, the luminance adjustment processes being performed before and after the local contrast enhancement processing.

20. The method of claim 19 , wherein the two luminance adjustment processes are substantially complementary.

21. The method of claim 19 , wherein the luminance adjustment processing is controlled in accordance with an input-output luminance mapping table dependent on the one of the detected ambient light levels.

22. A video processing system having a display and video signal comprising:

a device for detecting ambient light levels on the display and for providing a detected ambient light level signal;

a processor responsive to the detected ambient light level signal for:

processing the video signal, and

performing pixel-by-pixel contrast enhancement processing on the video signal based on the detected ambient light levels to provide a processed video signal, the contrast enhancement processing comprising performing a spatial filtering operation on the video signal to determine luminance on a pixel-by-pixel basis; and

a display device having an active screen area for presenting the processed video signal on the display and surrounding areas.

23. The video processing system of claim 22 , wherein the device for detecting the ambient light levels comprises a camera for imaging at least one of the active screen area and the surrounding areas of the display device.

24. The video processing system of claim 22 , wherein the device for detecting the ambient light levels comprises one or more light sensors located near the active screen area of the display device.

25. The video processing system of claim 22 , wherein the spatial filtering is performed using a spatial filter comprising a 2-D high-pass filter.

26. The video processing system of claim 22 , wherein the spatial filtering is performed using a spatial filter comprising an all-pass filter and a 2-D low-pass filter.

27. The video processing system of claim 26 , wherein the spatial filtering is performed using a spacial filter comprising a 2-D low-pass filter responsive to the detected ambient light level signal.

28. The video processing system of claim 22 , wherein the processor comprises a comparator for:

comparing pixel image brightness to one of the detected ambient light levels: and

providing a pixel based contrast enhancement processing signal based on the comparison.