Color transformation for processing digital images

1. A method of transforming the color appearance of a plurality of digital images including the steps of:

a) receiving a plurality of digital images from a capture medium wherein each digital image includes a plurality of pixel values relating to at least three basic colors;

b) calculating a color correction transform by using:

i) a non-linear adjustment that is independent of the digital images and which corrects an under-exposure condition as a function of the capture medium; and

ii) a linear functional relationship dependent upon the pixels of the basic colors, the linear functional relationship defining an exposure-level-dependent estimate of gray corresponding to the photographic response of the capture medium; and

c) using the color correction transform to modify the pixels of the plurality of digital images wherein the pixels within a digital image are transformed with varying degrees of color modification as a function of exposure.

2. The method of claim 1 wherein the linear functional relationship is calculated using a least squares best fit line based on pixel values of the digital images.

3. The method of claim 1 wherein the color correction transform incorporates a non-linear component color transform that raises the contrast of pixels that relates to an under-exposure condition.

4. The method of claim 1 further including the step of calculating color balance values uniquely for each digital image, and using the color balance values to modify the color appearance of the corresponding digital image.

5. The method of claim 1 wherein calculating the color correction transform further includes:

calculating a first component color transform that incorporates the exposure-level-dependent relationship;

modifying the pixels of the digital images with the first component color transform;

calculating a second component color transform; and

further modifying the pixels of the digital images with the second component color transform.

6. The method of claim 1 wherein the digital images are derived from an original photographic film strip.

7. The method of claim 6 further including the steps of:

determining a minimum pixel value for each of the plurality of pixel colors; and

using the minimum pixel values to calculate the color correction transform.

8. The method of claim 7 wherein the pixels included in the plurality of digital images are used to determine the minimum pixel values.

9. The method of claim 7 wherein the pixels derived from inter-frame gap regions of the original photographic film strip are used to determine the minimum pixel values.

10. The method of claim 1 wherein step b) further includes:

analyzing the digital images to detect the likelihood of each digital image having been derived from a scene photographed with an artificial illumination light source; and

rejecting the pixels of digital images determined to have a high likelihood from contributing to the calculation of the color correction transform.

11. The method of claim 10 further including the steps of:

calculating a frame average pixel value for each color and for each digital images;

determining a minimum pixel value for each of the plurality of pixel colors; and

using the difference between the minimum pixel values and the frame average pixel values corresponding to an individual digital image to calculate the likelihood of each individual digital image having been derived from a scene photographed with an artificial illumination source.

12. The method of claim 1 wherein step b) further includes:

receiving nonpixel data for the plurality of digital images;

using the nonpixel data to indicate the likelihood of the digital image having been derived from a scene photographed with an artificial illumination source; and

using the pixels of digital images indicated to have a high likelihood to adjust the calculation of the color correction transform.

13. The method of claim 11 further including the steps of:

using the frame average pixel values and the minimum pixel values to calculate an order average pixel value for each of the plurality of colors;

calculating an initial gray point from a weighted average of the minimum pixel values and the order average pixel values; and

using the initial gray point to calculate the color correction transform.

14. The method of claim 1 further including the steps of calculating a measure of color saturation for pixels of the digital images and using the measure of color saturation to vary the degree to which the pixel contributes to the calculation of the color correction transform.

15. The method of claim 13 further including the steps of using the initial gray point to calculate a measure of color saturation for pixels of the digital images and using the measure of color saturation to vary the degree to which the pixel contributes to the calculation of the color correction transform.

16. The method of claim 1 wherein step b) further includes calculating a measure of spatial activity for pixels of the digital images and using the measure of spatial activity to vary the degree to which the pixel contributes to the calculation of the color correction transform.

17. The method of claim 13 further including the steps of calculating a measure of spatial activity for pixels of the digital images and using the measure of spatial activity to vary the degree to which the pixel contributes to the calculation of the color correction transform.

18. The method of claim 1 wherein step b) further includes calculating a measure of relative pixel brightness for pixels of the digital images and using the measure of relative pixel brightness to vary the degree to which the pixel contributes to the calculation of the color correction transform.

19. The method of claim 14 wherein step b) further includes calculating a measure of relative pixel brightness for pixels of the digital images and using the measure of relative pixel brightness to vary the degree to which the pixel contributes to the calculation of the color correction transform.

20. The method of claim 1 wherein step b) further includes the steps of calculating a measure of pixel exposure for pixels of the digital images and using the measure of pixel exposure to vary the degree to which the pixel contributes to the calculation of the color correction transform.

21. The method of claim 14 wherein step b) further includes the steps of calculating a measure of pixel exposure for pixels of the digital images and using the measure of pixel exposure to vary the degree to which the pixel contributes to the calculation of the color correction transform.

22. The method of claim 3 wherein the slope coefficients of the least squares best fit line are modified by a constant proportional factor.

23. The method of claim 3 wherein the slope coefficients of the least squares best fit line do not exceed a predetermined value.

24. The method of claim 3 further including the steps of calculating a measure of confidence corresponding to the best fit line and using the measure of confidence to modify the slope coefficients of the least squares best fit line.

25. The method of claim 1 further including scanning to produce the density values of an original photographic film strip that has data recorded uniquely identifying the type of photographic film: reading the data recorded indicating of the type of photographic film material; and selectively excluding the step of applying the color correction transform to the digital images.

26. The method of claim 8 wherein the digital images used to produce the color correction transform are derived from multiple photographic film strips that relate to different consumers.

27. The method of claim 1 wherein the digital images are derived from a digital camera.

28. The method of claim 1 wherein a spatial filter is used to apply the color correction transform.

29. A method of transforming the color appearance of a plurality of digital images, comprising the steps of:

a) receiving a plurality of source digital images from a capture medium wherein each source digital image includes a plurality of pixel values relating to at least three basic colors;

b) calculating a first color transform T 1 [ ] that incorporates a functional relationship dependent upon the pixels of the basic colors, the functional relationship defining an exposure-level-dependent estimate of gray corresponding to the photographic response of the capture medium;

c) calculating a second color transform T 2 [ ] that incorporates a non-linear adjustment that is independent of the pixels of the source digital images and relates to an under-exposure condition and adjusts the color of the under-exposed pixels;

d) calculating a third color transform T 3 [ ] that incorporates a non-linear adjustment that is independent of the pixels of the source digital images and raises the contrast of pixels that relate to an under-exposure condition;

e) combining the color transforms T 1 [ ], T 2 [ ], and T 3 [ ] to calculate a color transform T 4 [ ]; and

f) using the color transform T 4 [ ] and the source digital images to produce a set of color corrected digital images.

30. The method of claim 29 wherein step f) further includes the steps of:

i) using the color transform T 4 [ ] and the digital images to calculate intermediate digital images;

ii) calculating a fifth color transform T 5 [ ] that incorporates a functional relationship dependent upon the pixels of the basic colors, the functional relationship defining an exposure-level-dependent estimate of gray; and

iii) using the fifth color transform T 5 [ ] and the intermediate digital images to produce the set of color corrected digital images.

31. Apparatus for transforming the color appearance of a plurality of digital images, comprising:

a) means for receiving a plurality of source digital images from a capture medium wherein each source digital image includes a plurality of pixel values relating to at least three basic colors;

b) means for calculating a first color transform T 1 [ ] that incorporates a functional relationship dependent upon the pixels of the basic colors, the functional relationship defining an exposure-level-dependent estimate of gray corresponding to the photographic response of the capture medium;

c) means for calculating a second color transform T 2 [ ] that incorporates a non-linear adjustment that is independent of the pixels of the source digital images and relates to an under-exposure condition and adjusts the color of the under-exposed pixels;

d) means for calculating a third color transform T 3 [ ] that incorporates a non-linear adjustment that is independent of the pixels of the source digital images and raises the contrast of pixels that relate to an under-exposure condition;

e) means for combining the color transforms T 1 [ ], T 2 [ ], and T 3 [ ] to calculate a color transform T 4 [ ]; and

f) means for using the color transform T 4 [ ] and the source digital images to produce a set of color corrected digital images.

32. A computer storage product having at least one computer storage medium having instructions stored therein causing one or more computers to perform the method of claim 1 .