When I try combining channels for RGB compressed image, the Image coloring if "off" and somewhat "yellow-ish"

Question

Kayla Kloster 2017-4-28

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https://ww2.mathworks.cn/matlabcentral/answers/337774-when-i-try-combining-channels-for-rgb-compressed-image-the-image-coloring-if-off-and-somewhat-ye

评论： Kayla Kloster 2017-4-28

I am trying to use DCT image compression on an RGB image by applying DCT to each separate channel and then recombining the channels in my final image. However, when I run my code, the output image is not fully colored, and I don't understand why. This is what the image looks like when using Matlab's DCT functions

This is what the image looks like after implementing my DCT image compression:

The following is my code:

clear all; close all;
X_int = imread('wombat.jpg');
redChannel = X_int(:, :, 1);
greenChannel = X_int(:, :, 2);
blueChannel = X_int(:, :, 3);
N=8;
Z=256;
 %DCT FOR RED CHANNEL
 B=zeros(256,256);
  for r=1:N:256
    for c=1:N:256
        B(r:r+N-1,c:c+N-1)=redChannel(r:r+N-1,c:c+N-1);
          [m n] = size(B);
       %CREATE DCT
        DCT=zeros(n);
          for i=1:m
            for j=1:n
              if i==1
                DCT(i,j) = ((1/n)^(1/2));
              else
                DCT(i,j) = ((2/n)^(1/2))*cos((pi*(2*j-1)*(i-1))/(2*n));
              end
          end
      end
      %APPLY DCT
        Y=DCT*B*DCT';
       %Zero out sub-antidiagonal entries
        p=.1;
        for i=1:n
            for j=1:n
             if i+j>p*2*n
                   Y(i,j) = 0;
             end
            end
        end
    ImageR = DCT'*Y*DCT;
    end
  end
%DCT FOR GREEN CHANNEL
BG=zeros(256,256);
for r=1:N:256
  for c=1:N:256
      BG(r:r+N-1,c:c+N-1)=greenChannel(r:r+N-1,c:c+N-1);
      [s l] = size(BG);
        %CREATE DCT
        DCT=zeros(l);
           for i=1:s
             for j=1:l
                if i==1
                   DCT(i,j) = ((1/l)^(1/2));
                else
                   DCT(i,j) = ((2/l)^(1/2))*cos((pi*(2*j-1)*(i-1))/(2*l));
                end
            end
         end
      %Apply DCT 
      YG=DCT*B*DCT';
      %Zero out sub-antidiagonal entries
      p=.1;
      for i=1:n
          for j=1:n
           if i+j>p*2*n
                 YG(i,j) = 0;
           end
          end
      end
  ImageG = DCT'*YG*DCT;
  end
end
%DCT FOR BLUE CHANNEL
BB = zeros(256,256);
 for r=1:N:256
   for c=1:N:256
       BB(r:r+N-1,c:c+N-1)=blueChannel(r:r+N-1,c:c+N-1);
       [q w] = size(BB);
    %Function to return DCT matrix of any size
     DCT=zeros(w);
        for i=1:q
          for j=1:w
            if i==1
              DCT(i,j) = ((1/w)^(1/2));
            else
              DCT(i,j) = ((2/w)^(1/2))*cos((pi*(2*j-1)*(i-1))/(2*w));
            end
         end 
       end
      %Apply DCT 
      YB=DCT*BB*DCT';
      %Zero out sub-antidiagonal entries
      p=.1;
      for i=1:n
          for j=1:n
           if i+j>p*2*n
                 YB(i,j) = 0;
           end
          end
      end
     ImageB = DCT'*YB*DCT;
    end
end
   %CONCATENATE THE 3 RGB CHANNELS
    red = uint8(ImageR);
    green = uint8(ImageG);
    blue = uint8(ImageB);
    Isvd = cat(3,red,green,blue);
      figure
      imshow(Isvd);
      imwrite(Isvd,'my_output_image.jpg');

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Stephen23 2017-4-28

@Kayla Kloster: please do not keep adding the same question repeatedly. You can simply edit the question instead.

Stephen23 2017-4-28

编辑：Stephen23 2017-4-28

@Kayla Kloster: Rather than using nested loops, you should consider writing vectorized code, which would be much simpler and easier to understand:

https://www.mathworks.com/help/matlab/matlab_prog/vectorization.html

https://www.mathworks.com/matlabcentral/answers/228557-experts-of-matlab-how-did-you-learn-any-advice-for-beginner-intermediate-users

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Answer 1

Jan 2017-4-28

2
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/337774-when-i-try-combining-channels-for-rgb-compressed-image-the-image-coloring-if-off-and-somewhat-ye#answer_264878

编辑：Jan 2017-4-28

在 MATLAB Online 中打开

Your code contain 3 almost equal sections. Such redundancies are prone to bugs, because it is confusing to edit them. Prefer to use a loop over the 3 channels and call the same code section.

This confusing repeating of code sections decrease the readability and typos like this are almost invisible:

%DCT FOR GREEN CHANNEL
BG =zeros(256,256);
for r=1:N:256
  for c=1:N:256
    BG(r:r+N-1,c:c+N-1)=greenChannel(r:r+N-1,c:c+N-1);
    ...
      %Apply DCT 
      YG=DCT*B*DCT';
  %          ^

You do not want "B" here, but BG.

The names for the different channels are not smart: B for red, BG for green, BB for blue.

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Kayla Kloster 2017-4-28

Thanks so so much!! That was the issue. Yes I have noticed it have been extremely difficult to debug, so I will work on using a for loop instead of repeated sections.

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