Methods and systems for converting images from low dynamic range to high dynamic range
Aspects of the invention provide systems and methods for converting a digital image represented in a lower bit depth representation to a higher bit depth representation. A saturation region is identified, where a color model value of the pixels in the saturation region is above an upper saturation threshold or below a lower saturation threshold. The color model value for each pixel in the saturation region is then adjusted by a corresponding adjustment. The magnitude of the adjustment for each pixel is based on characteristics of the image data.
1. A signal processing unit configured to convert image data from a lower bit depth representation to a higher bit depth representation, the signal processing unit comprising:
a processor configured to:
identify pixels in a saturation region wherein a color model value of each individual pixel in the saturation region is one of: above an upper saturation threshold and below a lower saturation threshold; and
adjust the color model value of one or more of the individual pixels in the saturation region by a corresponding adjustment, a magnitude of each adjustment dependent, at least in part, on a number of pixels between the corresponding individual pixel and an edge of the saturation region.
2. A signal processing unit according to claim 1 wherein the processor is configured to identify pixels in the saturation region by scanning the color model values of pixels in the image data along a first scan axis to determine a start pixel at one edge of a saturation region and an end pixel at another edge of the saturation region.
3. A signal processing unit according to claim 1 wherein the processor is configured to determine the magnitude of each adjustment based, at least in part, on a function of the number of pixels between the corresponding individual pixel and the edge of the saturation region, wherein the function has an extremum in the saturation region.
4. A signal processing unit according to claim 3 wherein the processor is configured to identify pixels in the saturation region by scanning the color model values of pixels in the image data along a first scan axis to determine a start pixel at one edge of a saturation region and an end pixel at another edge of the saturation region.
5. A signal processing unit according to claim 4 wherein the function comprises a parabola and the processor is configured to set a vertex of the parabola to be substantially midway between the start pixel and the end pixel.
6. A signal processing unit according to claim 5 wherein the magnitude of the function at the extremum increases with a number of pixels between the start pixel and the end pixel.
7. A signal processing unit according to claim 2 wherein the processor is configured to determine the magnitude of each adjustment on the basis of at least one of: a gradient between the color model values of the start pixel and at least one pixel preceding the start pixel and a gradient between the color model values of the end pixel and at least one pixel following the end pixel.
8. A signal processing unit according to claim 1 wherein the processor is configured to determine a number of pixels in the saturation region and to determine the magnitude of an adjustment based, at least in part, on the number of pixels in the saturation region.
9. A signal processing unit according to claim 1 wherein the processor is configured to determine the magnitude of an adjustment based, at least in part, on detecting a lens flare pattern surrounding the saturation region; and predict color model values of pixels in the saturation region based at least in part upon a model of the lens flare pattern.
10. A signal processing unit according to claim 1 wherein the processor is configured to adjust the color model values of the pixels in the image data to form an intermediate higher-bit-depth representation of the image data and wherein adjusting the color model value of individual pixels in the saturation region is performed on pixels of the intermediate higher bit depth representation.
11. A signal processing unit according to claim 1 wherein the processor is configured to scale the color model values of individual pixels in the saturation region by corresponding scaling factors.
12. A signal processing unit according to claim 1 wherein the image data comprises data derived from an NTSC color television signal.
13. A television comprising a signal processing unit according to claim 1 .
14. A signal processing unit according to claim 1 wherein the processor is configured to determine the magnitude of each adjustment based, at least in part, on a gradient between the color model values of at least one pixel on the edge of the saturation region and at least one pixel outside of the saturation region.