Adaptive pixel-level color enhancement for a digital camera

Abstract

An adaptive color enhancer applies different scale factors to different pixels in a digital image. More color enhancement occurs for bright pixels and for dim pixels than for average-intensity pixels. Also, more color enhancement is applied to the more colorful pixels while less color enhancement is applied to dull, less-colorful pixels. Rather than enhance all pixels to the same extent, the bright, colorful pixels are enhanced further than the average. Likewise, dim areas are color enhanced more than average. A calculation unit receives a YUV pixel. The Y value is compared to range limits and a piece-wise-linear (PWL) function generates an intermediate scale factor. The absolute values of the U and V color values are combined to create a colorfulness factor. The colorfulness factor is also used with a PWL function and the intermediate scale factor to generate a final scale factor for that pixel. The final scale factor is then multiplied by the U and V values of the pixel to generate a color-corrected pixel.

1. A color enhancer comprising:
 a pixel input receiving a YUV pixel that includes a Y component indicating brightness but not color of the pixel, and U and V components indicating a color but not brightness of the pixel; 
 a calculator, receiving pre-set scale factors, the pre-set scale factors being constant for a range of pixels in an image, for generating an enhancement factor from the Y component and from the U and V components and the pre-set scale factors; 
 wherein the enhancement factor varies with the Y, U, and V components of the YUV pixel, the enhancement factor being a different value for different pixels in the image; 
 a first multiplier, receiving the U component from the pixel input, for multiplying the enhancement factor from the calculator by the U component to generate an adjusted U component; 
 a second multiplier, receiving the V component from the pixel input, for multiplying the enhancement factor from the calculator by the V component to generate an adjusted V component; and 
 an output, coupled to the pixel input, the first multiplier, and the second multiplier, for outputting the Y component from the pixel input with the adjusted U and V components from the first and second multiplier as a color-enhanced pixel; 
 wherein the calculator further comprises:
 a combiner, receiving the U and V components from the pixel input, for combining the U and V components to generate a colorfulness factor, the colorfulness factor representing a degree of color for the pixel; 
 a function generator, receiving the Y component and the colorfulness factor and the pre-set scale factors, for generating the enhancement factor wherein the enhancement factor is larger for more colorful pixels having large colorfulness factors than for less colorful pixels having small colorfulness factors, 
 wherein more color enhancement occurs for more colorful pixels; 
 wherein the function generator comprises:
 a first stage, receiving the Y component from the pixel input, for generating an intermediate factor that is a function of the Y component; 
 a second stage, receiving the intermediate factor from the first stage and receiving the colorfulness factor from the combiner, for generating the enhancement factor that is a function of the colorfulness factor; 
 wherein the first stage outputs a maximum pre-set factor as the intermediate factor when the Y component is above an upper Y limit or below a lower Y limit, 
 wherein the first stage outputs a combination of the maximum pre-set factor and a minimum pre-set factor when the Y component is between the upper Y limit and the lower Y limit, 
 whereby pixels are color enhanced with the enhancement factor that is generated for each pixel that varies with the Y, U, and V components of the YUV pixel, and whereby both brightness and color of the YUV pixel can determine the enhancement factor for the YUV pixel, and whereby color enhancement is reduced when the Y component is between the upper Y limit and the lower Y limit. 
 
 
 
     
     
       2. The color enhancer of  claim 1  wherein the function generator also generates the enhancement factor wherein the enhancement factor is larger for brighter pixels having a large Y component than for average pixels having a moderate Y component, 
       wherein bright pixels are color enhanced more than other pixels. 
     
     
       3. The color enhancer of  claim 1  wherein the second stage outputs the maximum pre-set factor as the enhancement factor when the colorfulness factor is above an upper color limit;
 wherein the second stage outputs the intermediate factor from the first stage when the colorfulness factor is below a lower color limit; 
 wherein the second stage outputs a combination of the intermediate factor from the first stage and the maximum pre-set factor when the colorfulness factor is between the upper color limit and a lower color limit, 
 whereby color enhancement is maximum for colorful pixels but reduced for dull pixels. 
 
     
     
       4. The color enhancer of  claim 3  wherein the combination of the intermediate factor from the first stage and the maximum pre-set factor is a linear combination that depends on the colorfulness factor of the YUV pixel. 
     
     
       5. The color enhancer of  claim 1  wherein the combiner comprises:
 a first absolute generator, receiving the U component, for generating an absolute value of the U component; 
 a second absolute generator, receiving the V component, for generating an absolute value of the V component; 
 an adder, receiving the absolute values of the U and V components, for outputting the colorfulness factor, 
 whereby the colorfulness factor combines absolute values of U and V components. 
 
     
     
       6. A method for enhancing pixels in a digital image comprising:
 for each pixel in a digital image:
 receiving a current pixel value that includes a color value and a brightness value; 
 comparing the brightness value to a first brightness limit, and outputting a maximum factor as an intermediate factor when the brightness value exceeds the first brightness limit; 
 comparing the brightness value to a second brightness limit, and outputting the maximum factor as the intermediate factor when the brightness value is less than the second brightness limit; 
 using a first function to generate the intermediate factor when the brightness value is between the first and second brightness limits; 
 comparing the color value to a first color limit, and outputting the maximum factor as an enhancement factor when the color value exceeds the first color limit; 
 comparing the color value to a second color limit, and outputting the intermediate factor as the enhancement factor when the color value is less than the second color limit; 
 using a second function to generate the enhancement factor when the color value is between the first and second color limits; 
 adjusting the color value with the enhancement factor to generate an adjusted color value; and 
 replacing the color value with the adjusted color value for the current pixel value, 
 
 whereby color is adjusted in response to the brightness value and color value of each pixel in the digital image. 
 
     
     
       7. The method of  claim 6  further comprising:
 generating a colorfulness factor from the color value for each pixel by:
 generating a first absolute value of a first color component for the pixel; 
 generating a second absolute value of a second color component for the pixel; 
 adding the first and second absolute values to generate the colorfulness factor; 
 using the colorfulness factor as the color value when comparing to the first and second color limits, 
 whereby the colorfulness factor is used for comparison. 
 
 
     
     
       8. The method of  claim 7  further comprising:
 using a linear approximation of a minimum factor and the maximum factor as the first function to generate the intermediate factor wherein the intermediate factor is the minimum factor at a point between the first and second brightness limits but the intermediate factor is the maximum factor at the first and second brightness limits; 
 using a linear approximation of the intermediate factor and the maximum factor as the second function to generate the enhancement factor, 
 whereby linear functions are employed. 
 
     
     
       9. The method of  claim 8  further comprising:
 before a new digital image is processed, storing pre-set values to programmable registers, the pre-set values including the maximum factor, the minimum factor, the first and second brightness limits, and the first and second color limits, 
 whereby pre-set values are constant for pixels the digital image but different 
 enhancement factors are generated for each pixel within the digital image. 
 
     
     
       10. A color enhancer unit for color enhancing pixels in a digital image, the color enhancer comprising:
 a calculation unit configured to receive a brightness component and two or more color components corresponding to a pixel of the digital image, wherein the calculation unit comprises:   a combination unit configured to combine the color components to generate a color factor:   a first stage calculator configured to generate an intermediate factor, wherein the intermediate factor:
 is a maximum pre-set factor when the brightness component is above an upper brightness limit; 
 is the maximum pre-set factor when the brightness component is below a lower brightness limit; and 
 is a combination of the maximum pre-set factor and a minimum pre-set factor when the brightness component is between the upper brightness limit and the lower brightness limit; and 
   a second stage calculator configured to receive the intermediate factor and the color factor, and to generate an enhancement factor, wherein the enhancement factor:
 is the maximum pre-set factor when the color factor is above an upper color factor; 
 is the intermediate factor when the color factor is below a lower color factor; and 
 is a combination of the maximum pre-set factor and the intermediate factor when the color factor is between the upper color factor and lower color factor; 
   wherein the color enhancer unit is configured to color enhance certain pixels in the digital image more than other pixels.   
     
     
       11. The color enhancer unit of claim 10, wherein the pixel of the digital image is a YUV pixel including a Y component indicating a brightness of the pixel, and U and V components indicating a color of the pixel. 
     
     
       12. The color enhancer unit of claim 10, wherein the calculation unit is configured to receive at least two scale factors, wherein the calculation unit is configured to generate the enhancement factor based on the brightness component, the two or more color components, and the scale factors. 
     
     
       13. The color enhancer unit of claim 10, wherein the combination unit is configured to generate the color component by summing at least an absolute value of a first of the two or more color components with an absolute value of a second of the two or more color components. 
     
     
       14. The color enhancer unit of claim 10, wherein the first stage calculator is configured to output a linear combination of the maximum pre-set factor and the minimum pre-set factor as the intermediate factor when the brightness component is between the upper brightness limit and the lower brightness limit. 
     
     
       15. The color enhancer unit of claim 10, wherein the first stage calculator is configured to output a non-linear combination of the maximum pre-set factor and the minimum pre-set factor as the intermediate factor when the brightness component is between the upper brightness limit and the lower brightness limit. 
     
     
       16. The color enhancer unit of claim 10, wherein the second stage calculator is configured to output a linear combination of the intermediate factor and the maximum pre-set factor as the enhancement factor when the color factor is between the upper color factor and the lower color factor. 
     
     
       17. The color enhancer unit of claim 10, wherein the maximum and minimum pre-set factors, the upper and lower brightness limits, and the upper and lower color factors are programmable. 
     
     
       18. The color enhancer unit of claim 10, wherein the combination unit is configured to generate an absolute value of each of the color components, wherein the combination unit is further configured to combine the absolute values of the color components to generate the color factor. 
     
     
       19. The color enhancer unit of claim 10, further comprising a multiplier unit configured to receive the color components corresponding to the pixel, wherein the multiplier unit is configured to multiply each of the color components by the enhancement factor to generate adjusted color components. 
     
     
       20. The color enhancer of claim 19, further comprising an output coupled to the multiplier unit, wherein the output is configured to output the brightness component and the adjusted color components as a color-enhanced pixel. 
     
     
       21. An apparatus for color enhancing pixels in a digital image, the apparatus comprising:
 a calculation means for receiving a brightness component and two or more color components corresponding to a pixel of the digital image, wherein the calculation unit comprises:   a combination means for combining the color components to generate a color factor;   a first stage calculation means for generating an intermediate factor, wherein the intermediate factor:
 is a maximum pre-set factor when the brightness component is above an upper brightness limit; 
 is the maximum pre-set factor when the brightness component is below a lower brightness limit; and 
 is a combination of the maximum pre-set factor and a minimum pre-set factor when the brightness component is between the upper brightness limit and the lower brightness limit; and 
   a second stage calculation means for receiving the intermediate factor and the color factor, and for generating an enhancement factor, wherein the enhancement factor:
 is the maximum pre-set factor when the color factor is above an upper color factor; 
 is the intermediate factor when the color factor is below a lower color factor; and 
 is a combination of the maximum pre-set factor and the intermediate factor when the color factor is between the upper color factor and lower color factor; 
   wherein the apparatus is configured to color enhance certain pixels in the digital image more than other pixels.   
     
     
       22. A digital camera comprising:
 a processing unit configured to generate a plurality of pixels corresponding to a digital image;   a color enhancer unit configured to color enhance pixels in the digital image, the color enhance comprising:   a calculation unit configured to receive a brightness component and two or more color components corresponding to a pixel of the digital image, wherein the calculation unit comprises:   a combination unit configured to combine the color components to generate a color factor:   a first stage calculator configured to generate an intermediate factor, wherein the intermediate factor:
 is a maximum pre-set factor when the brightness component is above an upper brightness limit; 
 is the maximum pre-set factor when the brightness component is below a lower brightness limit; and 
 is a combination of the maximum pre-set factor and a minimum pre-set factor when the brightness component is between the upper brightness limit and the lower brightness limit; and 
   a second stage calculator configured to receive the intermediate factor and the color factor, and to generate an enhancement factor, wherein the enhancement:
 is the maximum pre-set factor when the color factor is above an upper color factor; 
 is the intermediate factor when the color factor is below a lower color factor; and 
 is a combination of the maximum pre-set factor and the intermediate factor when the color factor is between the upper color factor and lower color factor; 
   wherein the color enhancer unit is configured to color enhance certain pixels in the digital image more than other pixels.   
     
     
       23. The digital camera of claim 22, wherein the pixel of the digital image is a YUV pixel including a Y component indicating a brightness of the pixel, and U and V components indicating a color of the pixel. 
     
     
       24. The digital camera of claim 22, wherein the combination unit is configured to generate the color factor by summing absolute values of the two or more color components. 
     
     
       25. The digital camera of claim 22, wherein the first stage calculator is configured to output a linear combination of the maximum pre-set factor and the minimum pre-set factor as the intermediate factor when the brightness component is between the upper brightness limit and the lower brightness limit. 
     
     
       26. The digital camera of claim 22, wherein the first stage calculator is configured to output a non-linear combination of the maximum pre-set factor and the minimum pre-set factor as the intermediate factor when the brightness component is between the upper brightness limit and the lower brightness limit. 
     
     
       27. The digital camera of claim 28, wherein the second stage calculator is configured to output a linear combination of the intermediate factor and the maximum pre-set factor as the enhancement factor when the color factor is between the upper color factor and the lower color factor. 
     
     
       28. A method for color enhancing pixels in a digital image, the method comprising:
 combining two or more color components corresponding to a pixel of the digital image to generate a color factor;   generating an intermediate factor, wherein the intermediate factor:
 is a maximum pre-set factor when a brightness component is above an upper brightness limit; 
 is the maximum pre-set factor when the brightness component is below a lower brightness limit; and 
 is a combination of the maximum pre-set factor and a minimum pre-set factor when the brightness component is between the upper brightness limit and the lower brightness limit; and 
   generating an enhancement factor, wherein the enhancement factor:
 is the maximum pre-set factor when the color factor is above an upper color factor; 
 is the intermediate factor when the color factor is below a lower color factor; and 
 is a combination of the maximum pre-set factor and the intermediate factor when the color factor is between the upper color factor and lower color factor. 
   
     
     
       29. The method of claim 28, wherein the pixel of the digital image is a YUV pixel including a Y component indicating a brightness of the pixel, and U and V components indicating a color of the pixel. 
     
     
       30. The method of claim 28, further comprising using a digital signal processor to generate the intermediate factor according to a non-linear function. 
     
     
       31. The method of claim 28, further comprising outputting a linear combination of the maximum pre-set factor and the minimum pre-set factor as the intermediate factor when the brightness component is between the upper brightness limit and the lower brightness limit. 
     
     
       32. The method of claim 28, further comprising generating the enhancement factor according to a non-linear function. 
     
     
       33. The method of claim 28, further comprising generating the color factor by taking at least two absolute values of at least two of the two or more color components. 
     
     
       34. The method of claim 28, further comprising outputting a linear combination of the intermediate factor and the maximum pre-set factor as the enhancement factor when the color factor is between the upper color factor and the lower color factor. 
     
     
       35. The method of claim 28, wherein the maximum and minimum pre-set factors, the upper and lower brightness limits, and the upper and lower color factors are programmable. 
     
     
       36. The method of claim 28, further comprising generating an absolute value of each of the color components, and combining the absolute values of the color components to generate the color factor. 
     
     
       37. The method of claim 28, further comprising multiplying each of the color components by the enhancement factor to generate adjusted color components. 
     
     
       38. The method of claim 37, further comprising outputting the brightness component and the adjusted color components as a color-enhanced pixel. 
     
     
       39. The method of claim 28, wherein said color enhancing is performed on a pixel-by-pixel basis.