Combined color cast removal and contrast enhancement for digital color images

• US 6,181,321 B1
• Filed: 04/23/1997
• Issued: 01/30/2001
• Est. Priority Date: 04/23/1997
• Status: Expired due to Term

• Current Assignee: Canon Kabushiki Kaisha (Canon Inc.)
• Original Assignee: Canon Kabushiki Kaisha (Canon Inc.)
• Inventors: Kohler, Timothy L., Zhao, Jun, Hui, Jonathan
• Primary Examiner(s): Shankar, Vijay
• Assistant Examiner(s): Frenel, Vanel
• Application Number:
  US08/844,933
• Time in Patent Office: 1,378 Days
• Field of Search: 345/20, 345/22, 345/152, 345/150, 348/557, 348/562, 348/577, 348/673, 348/675, 348/679
• US Class Current: 345/617
• CPC Class Codes: H04N 1/6027 Correction or control of colour gradation or colour contrast H04N1/6058 takes precedence
• CPC Class Codes: H04N 1/6077 Colour balance, e.g. colour cast correction

Abstract

An original image is processed while permitting a user to control an amount of color cast removal and an amount of color contrast enhancement performed on the original image. A user interface, which includes a control element having a single degree of freedom, is provided. User adjustment of the control element is responded to by simultaneously varying the degree of color cast removal and the amount of color contrast enhancement performed on the original image. In a further embodiment, an original image is processed so as to automatically remove color cast from the original image. According to this embodiment contrast is reduced for certain very dark pixels and very bright pixels, and enhanced for the remainder. In a still further embodiment, an original image is processed so as to automatically enhance color contrast in the original image. According to this embodiment, a first mapping of intensity levels is performed if the image is not both highkey and lowkey, and a second mapping is performed if the image is both highkey and lowkey.

Claims

1. A method for digitally processing a digital image under control of a user so as to simultaneously control an amount of color cast removal and an amount of color contrast enhancement performed on the digital image, said method comprising:

providing a user interface which includes a control element having a single degree of freedom, the single degree of freedom for permitting coupled control over color cast removal and contrast enhancement;

digitally processing the digital image; and

responding to user adjustment of the control element by simultaneous and coupled variation of a degree of color cast removal and an amount of color contrast enhancement performed on the digital image in said digital processing step.

2. A method according to claim 1, wherein the user interface comprises a graphical user interface.

3. A method according to claim 2, wherein the control element comprises a sliding bar.

4. A method according to claim 1, wherein at one extreme setting of the control element a maximum amount of contrast enhancement is performed, and at the other extreme setting a maximum degree of color cast removal is performed.

5. A method according to claim 1, wherein color cast removal processing is performed when the control element is set below a predetermined setting level and contrast enhancement is performed when the control element is set above the predetermined setting level.

6. A method according to claim 5, wherein the color cast removal processing includes contrast enhancement.

7. A method according to claim 6, wherein the amount of contrast enhancement and the degree of color cast removal are simultaneously varied by varying a cutoff point at which contrast enhancement is performed.

8. A method for digitally processing an original image so as to automatically remove color cast from the original image, said method comprising:

obtaining pixel intensity levels for each color component i in an original image;

separately mapping the pixel intensity levels for each color component i to new pixel intensity levels for that color component by (a) determining a value x₀ based on the darkest d_i percent of pixels, (b) mapping pixels darker than x₀ to a small first range of dark intensity levels, (c) determining a value x₁ based on the brightest b_i percent of pixels, (d) mapping pixels brighter than x₁ to a small second range of bright intensity levels, and (e) mapping the remainder of the pixels to fall between the first and second ranges, where d_i and b_i are numbers pre-selected for each color component i; and

creating a new image using the new pixel intensity levels for each of the color components.

9. A method according to claim 8, wherein each d_i is approximately equal to 5%.

10. A method according to claim 8, wherein each b_i is approximately equal to 3%.

11. A method according to claim 8, wherein all the d_is are approximately equal to 5% and all the b_is are approximately equal to 3%.

12. A method according to claim 8, wherein all pixels brighter than x₁ are mapped to the brightest intensity level available and all pixels darker than x₀ are mapped to zero intensity level.

13. A method according to claim 8, wherein the mapping of the remainder of the pixels is linear.

14. A method according to claim 8, wherein each d_i and each b_i is set in accordance with the setting of a user-adjustable control having a single degree of freedom.

15. A method for digitally processing each of multiple different color components of a digital image so as to automatically enhance color contrast in the digital image, said method comprising:

designating the digital image as highkey only if, for each color component in the digital image, there exists a sufficient percentage of pixels having a pixel intensity level for that color component which is greater than or equal to a first threshold intensity value;

designating the digital image as lowkey only if, for each color component in the digital image, there exists a sufficient percentage of pixels having a pixel intensity level for that color component which is less than or equal to a second threshold intensity value;

performing a first mapping for each color component of the digital image if the digital image is not both highkey and lowkey; and

performing a second mapping for each color component of the digital image if the digital image is both highkey and lowkey,

wherein the second mapping is different than the first mapping, and wherein the second mapping enhances contrast for a range of the brightest pixels and a range of the darkest pixels and reduces contrast for pixels having an intensity between the ranges of darkest and brightest pixels for which contrast is enhanced.

16. A method according to claim 15, wherein the original image is designated as highkey only if, for each color component in the original image, there are at least approximately 10% of the pixels having a pixel intensity level for that color component which is greater than or equal to approximately 90% of the maximum intensity level available.

17. A method according to claim 15, wherein the original image is designated as lowkey only if, for each color component in the original image, there are at least approximately 10% of the pixels having a pixel intensity level for that color component which is less than or equal to approximately 14% of the maximum intensity level available.

18. A method according to claim 15, wherein the first mapping enhances contrast for all pixels in the original image other than small ranges of the darkest and brightest pixels in the original image.

19. A method according to claim 18, wherein the first mapping includes:

(a) mapping the darkest d_i percent of pixels to a small first range of dark intensity levels, (b) mapping the brightest b_i percent of pixels to a small second range of bright intensity levels, and (c) linearly mapping the remainder of the pixels to fall between the first and second ranges, where d_i and b_i are numbers pre-selected for each color component i.

20. A method according to claim 15, wherein the second mapping includes:

(a) mapping the darkest d_i percent of pixels to a small first range of dark intensity levels, (b) mapping the brightest b_i percent of pixels to a small second range of bright intensity levels.

21. A method according to claim 20, wherein each d_i and each b_i is approximately equal to 0.5%.

22. A method according to claim 20, wherein the second mapping step further includes a step of locating an interval t on a pixel intensity distribution histogram for the current color component which is likely to be low and flat, reducing contrast for the interval t, and enhancing contrast for the portions between d_i and the beginning of the interval t and between b_i and the end of the interval t.

23. A method according to claim 15, wherein each of the first and second mappings includes at least one linear mapping.

24. A method according to claim 20, wherein each d_i and each b_i is set in accordance with the setting of a user-adjustable control having a single degree of freedom.