Adjusting the color, brightness, and tone scale of rendered digital images

1. A method of processing a digital image to produce an enhanced digital image, comprising the steps of:

a) receiving a source digital image;

b) receiving a rendering tone scale function wherein the rendering tone scale function is a nonlinear function that has an absolute magnitude slope in the middle of the function domain that is greater than or equal to the absolute magnitude slope at either extreme of the function domain;

c) using the rendering tone scale function to calculate a de-rendering tone scale function;

d) receiving or calculating an adjustment tone scale function;

e) combining the de-rendering, rendering, and adjustment tone scale functions to form an enhancement tone scale function; and

f) using the enhancement tone scale function and the source digital image to produce an enhanced digital image.

2. The method of claim 1 wherein step c) includes calculating the de-rendering tone scale function as the mathematical inverse of the rendering tone scale function.

3. The method of claim 1 wherein step e) includes combining the tone scale functions has the form E( )=R 2 (A(R 1 ( ))) wherein E( ) denotes the enhancement tone scale function, R 2 ( ) denotes the rendering tone scale function, R 1 ( ) denotes the de-rendering tone scale function, and A( ) denotes the adjustment tone scale function.

4. The method of claim 1 wherein step f) the enhancement tone scale function has the form q=E(p) wherein E( ) denotes the enhancement tone scale function, p denotes a pixel value of the source digital image, and q denotes a pixel value of the enhanced digital image.

5. The method of claim 1 further including the steps of:

i) receiving a plurality of rendering tone scale functions;

ii) receiving an image source identification tag indicating the origin of the source digital image; and

iii) selecting the rendering tone scale function to be used for processing the source digital image from the plurality of rendering tone scale functions on the basis of the image source identification tag.

6. The method of claim 1 further including the step of using the pixel values of the source digital image to calculate the adjustment tone scale function.

7. The method of claim 1 wherein step d) further includes the steps of:

i) receiving an adjustment parameter from a user interface; and

ii) using the adjustment parameter to calculate the adjustment tone scale function.

8. The method of claim 1 wherein the adjustment tone scale function is calculated using a linear equation.

9. The method of claim 1 wherein the adjustment tone scale function is calculated using a nonlinear equation.

10. The method of claim 1 wherein the source digital image has pixels of more than one color and a separate adjustment tone scale function and enhancement tone scale function is calculated for each of the colors of the source digital image.

11. The method of claim 1 wherein step e) further includes the steps of:

i) generating a first exponential tone scale function;

ii) generating a second exponential tone scale function as the mathematical inverse of the first exponential tone scale function; and

iii) combining the de-rendering, rendering, adjustment, first exponential, and second exponential tone scale functions to form an enhancement tone scale function.

12. The method of claim 11 wherein step e) includes combining the tone scale functions using the formula E( )=F 2 (R 2 (A(R 1 (F 1 ( ))))) where E( ) denotes the enhancement tone scale function, R 2 ( ) denotes the rendering tone scale function, R 1 ( ) denotes the de-rendering tone scale function, A( ) denotes the adjustment tone scale function, F 1 ( ) denotes the first exponential tone scale function, and F 2 ( ) denotes the second tone scale function.

13. The method of claim 11 wherein step f) the enhancement tone scale function has the form q=E(p) wherein E( ) denotes the enhancement tone scale function, p denotes a pixel value of the source digital image, and q denotes a pixel value of the enhanced digital image.

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

i) receiving a plurality of rendering tone scale functions;

ii) receiving an image source identification tag indicating the origin of the source digital image; and

iii) selecting the rendering tone scale function to be used for processing the source digital image from the plurality of rendering tone scale functions on the basis of the image source identification tag.

15. The method of claim 11 further including the step of using the pixel values of the source digital image to calculate the adjustment tone scale function.

16. The method of claim 11 wherein step d) further includes the steps of:

i) receiving an adjustment parameter from a user interface; and

ii) using the adjustment parameter to calculate the adjustment tone scale function.

17. The method of claim 11 wherein the adjustment tone scale function is calculated using a linear equation.

18. The method of claim 11 wherein the adjustment tone scale function is calculated using a nonlinear equation.

19. The method of claim 11 wherein the source digital image has pixels of more than one color and a separate adjustment tone scale function and enhancement tone scale function is calculated for each of the colors of the source digital image.

20. The method of claim 1 wherein step f) further includes the steps of:

i) using the de-rendering tone scale function and the source digital image to calculate a de-rendered digital image;

ii) using the adjustment tone scale function and the de-rendered digital image to calculate an adjusted de-rendered digital image; and

iii) using the rendering tone scale function and the adjusted de-rendered digital image to calculate the enhanced digital image.

21. The method of claim 11 wherein step f) further includes the steps of:

i) using the first exponential tone scale function and the de-rendering tone scale function to calculate a combined de-rendering tone scale function;

ii) using the second exponential tone scale function and the rendering tone scale function to calculate a combined rendering tone scale function;

iii) using the combined de-rendering tone scale function and the source digital image to calculate a de-rendered digital image;

iv) using the adjustment tone scale function and the de-rendered digital image to calculate an adjusted de-rendered digital image; and

iii) using the combined rendering tone scale function and the adjusted de-rendered digital image to calculate the enhanced digital image.

22. The method of claim 20 further including the step of using a spatial filter to apply the adjustment tone scale function to the de-rendered digital image to calculate an adjusted de-rendered digital image.

23. The method of claim 21 further including the step of using a spatial filter to apply the adjustment tone scale function to the de-rendered digital image to calculate an adjusted de-rendered digital image.

24. Apparatus for processing a source digital image using a rendering tone scale function, wherein the rendering tone scale function has an absolute magnitude slope in the middle of the function domain that is greater than or equal to the absolute magnitude slope at either extreme of the function domain, to produce an enhanced digital image comprising:

a) means for calculating a de-rendering tone scale function as a function of the rendering tone scale function;

b) means for calculating an adjustment tone scale function;

c) means for combining the de-rendering, rendering, and adjustment tone scale functions to form an enhancement tone scale function; and

d) means for using the enhancement tone scale function and the source digital image to produce an enhanced digital image.