Method for processing highlights and saturated regions in a digital image
A method for performing highlight restoration on a digital image includes comparing the pixels in the image with a saturation level value to identify saturated pixels. A saturation map of saturated pixels is generated. Each selected saturated pixel is identified as a restorable pixel only if at least one color channel of the pixel is unsaturated. For each restorable pixel, a group of the closest unsaturated pixels above, below, to the left, and to the right of the select saturated pixel is identified. A replacement pixel value is generated for each saturated color channel of the restorable pixel, using a combination of the pixel values of the unsaturated color channels of the restorable pixel and the pixel values of the corresponding color channels of the nearby unsaturated pixels.
1. A method for restoring pixels in a digital image having saturated values in a color channel, comprising:
creating a saturation map of the image by comparing the pixel values with a predetermined threshold;
analyzing the image to select pixels having unsaturated values in the color channel;
creating only a single table of correction factors for the entire image by selecting pixels from the entire image having an unsaturated value in a selected color channel and for each selected pixel, calculating at least two values as a function of at least one color channel, a first value being an index into the single table of correction factors, and a second value being a correction factor stored in the single table of correction factors at a location pointed to by the index, the number of entries in the single table of correction factors being set as smaller than or equal to the number of bits used to represent each color channel;
for each pixel in the image having unsaturated values in the color channel, calculating an index pointer to the single table of correction factors, and applying a correction factor stored in the single table of correction factors at a location pointed to by the index pointer to the saturated value of the color channel to obtain a corrected value; and
replacing the saturated value in the color channel with the corrected value.
2. The method of claim 1 wherein creating the single table of correction factors comprises creating more than one table of correction factors.
3. The method of claim 1 further comprising selecting only pixels having a preselected characteristic for inclusion in the single table of correction factors.
4. The method of claim 3 wherein the preselected characteristic is pixels that have a luminance value larger than a predetermined threshold and do not have any of their color channels saturated.
5. The method of claim 1 wherein the number of entries in the single table of correction factors is smaller than or equal to the number of bits used to represent each color channel.
6. The method of claim 1 wherein the index into the single table of correction factors is a function of at least one of the color channels not being restored.
7. The method of claim 1 wherein the index into the single table of correction factors is a function of the color channel being restored.
8. The method of claim 1 wherein the index into the single table of correction factors is a function of the color channel being restored and at least one other color channel.
9. The method of claim 1 wherein correction factors to be stored in the single table of correction factors are calculated as a scaled value of the color channel being corrected.
10. The method of claim 1 wherein correction factors to be stored in the single table of correction factors are calculated as a ratio of the color channel to be corrected to one of the other color channels in the pixel.
11. The method of claim 1 wherein correction factors to be stored in the single table of correction factors are calculated as a difference between the color channel to be corrected and one of the other color channels in the pixel.
12. The method of claim 1 wherein correction factors to be stored in the single table of correction factors are calculated as a function of the color channel to be corrected and all other color channels in the pixel.