Fixing illumination issues in CIE-L*a*b*

The color distortion observed in the example of the previous section was caused by the high correlation between the R, G, and B channels in the case of the RGB image. This is another reason, why in many cases, a safer choice of color channel for color image processing tasks is CIE-L*a*b*. Let's try to alter the function written in the previous section, so that it converts the RGB input image to the CIE-L*a*b* color space, apply the chosen enhancement method only in the Lightness channel (so that the colors remain unaffected) and then transform the resulting image back to RGB:

function output = ColorContrastEnhanceLab(input, method) 

% Function for color contrast enhancement of input image in L*a*b* 
% Inputs:
% input  - Input image
% method – Enhancement method selection (0: histeq
% 1: adapthisteq
% 2: imadjust)
% Output:   
% output - Output image (enhanced)

cform = makecform('srgb2lab'), % Make the transform structure
img_lab = applycform(input,cform); % Apply transform to L*a*b*

switch method

% Apply chosen method in the Lightness channel (img_lab(:,:,1))

case 0
  img_lab (:,:,1) = histeq(img_lab (:,:,1));
case 1
  img_lab (:,:,1) = adapthisteq(img_lab (:,:,1));
case 2
  img_lab (:,:,1) = imadjust(img_lab (:,:,1));
end

cform = makecform('lab2srgb'), % Make the inverse transform structure
output = applycform(img_lab, cform); % Apply transform to RGB

Now we can test our new function to see if our results have improved:

>> img = imread('Steps.bmp'),
>> img1 = ColorContrastEnhanceLab(img, 0);
>> img2 = ColorContrastEnhanceLab(img, 1);
>> img3 = ColorContrastEnhanceLab(img, 2); 
>> subplot(1,4,1),imshow(img),title('Original image')
>> subplot(1,4,2),imshow(img1),title('CIE – L*a*b* Color histeq')
>> subplot(1,4,3),imshow(img2),title('CIE – L*a*b* Color adapthisteq')
>> subplot(1,4,4),imshow(img3),title('CIE – L*a*b* Color imadjust')
Fixing illumination issues in CIE-L*a*b*

Success! No color distortion this time. The adapthisteq result also seems to have added more detail to the original photograph this time, enhancing the shadowed areas in a way that is quite pleasing to the eye.

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