MATLAB: How to make an image take different RGB values

arraysdigital image processingimage processingMATLAB

I have an image with NxMx3 uint8 colors but I want the colors of the image to be replaced with another array of 256×3(Each column corresponds to R,G,B values) colors. How can I map the values from the array to the image?

I was advised that I need to split the 3 channels and then map them independently

Best Answer

LUT = randi([0, 255], 256, 3, 'uint8');       % The look up table
Img = randi([0, 255], 640, 480, 3, 'uint8');  % The image
NewImg = cat(3, LUT(Img(:, :, 1) + 1, 1), ...
                LUT(Img(:, :, 2) + 1, 2), ...  % [EDITED, 3 ==> 2]
                LUT(Img(:, :, 3) + 1, 3));

This replaces the original colors by the ones from the look up table by a simple indexing. See this example:

LUT = [2, 4, 8]
Img = [0, 1; 2, 1];
LUT(Img + 1)

Related Solutions

MATLAB: How to Store this attached rgbimage with a colormap. I tried the following code but could not write the image with colormap.

Your are writing the image in JPEG format and imwrite converts indexed images to RGB before writing data to JPEG files, because the JPEG format does not support indexed images.

Use the code below

[img, ~] = imread('yourImage.jpg');
[ind1, map1] = rgb2ind(img, 32);
imwrite(ind1, map1, 'yourImageNew.png'); % Don't change the extension to .jpg or .jpeg
[img2, map2] = imread('yourImageNew.png');

MATLAB: How to use RGB2IND with the colormap approximation method to convert images with a pre-specified colormap in Image Processing Toolbox 3.2 (R13)

To convert an RGB image into an indexed image using a pre-specified colormap, use the DITHER function rather than the RGB2IND function, specifiying the following values for the Qm and Qe parameters to DITHER: Qm = 8 and Qe = 7.

The following example illustrates this procedure.

% Create a colormap.
gmap = [0:255;0:255;0:255]'/255;
% Create an RGB image.
rgb1 = zeros(256,256,3);                
rgb1(:,:,1) = repmat([0:255]/255,256,1);
rgb1(:,:,2) = repmat([0:255]/255,256,1);
rgb1(:,:,3) = repmat([0:255]/255,256,1);
% Use DITHER to convert the image,
% specifying the colormap and values
% for the Qe and Qm parameters.
rgb2 = dither(rgb1,gmap,8,7);          
% To test this solution, convert the indexed image back into 
% an RGB image and find the difference between the
% original image and the newly created image.
rgb3 = ind2rgb(rgb2, gmap);             
diff = sum(sum(sum(rgb1-rgb3)))

The reason for using DITHER instead of RGB2IND is that when running RGB2IND with a specified colormap and the 'nodither' option, the function uses a method that quantizes the search space. Specifically, RGB2IND uses the following code:

dither(rgb,map,Qm,Qe)

With Qm = 5 and Qe = 4. Since Qe < Qm, no dithering is performed, which is the same when you specify the ‘nodither’ option with RGB2IND. Since Qm < 8, several of the 2^8 colors in the colormap get grouped together to perform the search. This is done to get reasonable performance since a brute-force nearest neighbor search is quite expensive to do for every pixel in an image.

Hence, to get the exact same colors back, using DITHER with Qm = 8 and Qe = 7. However, this is computationally more expensive than the previous method as the syntax forces DITHER to search through the entire colormap.

Best Answer

Related Solutions

MATLAB: How to Store this attached rgbimage with a colormap. I tried the following code but could not write the image with colormap.

MATLAB: How to use RGB2IND with the colormap approximation method to convert images with a pre-specified colormap in Image Processing Toolbox 3.2 (R13)

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