The picture shows the difference pixel-by-pixel between an image and its JPEG compressed version: white=no-difference; black=big-difference The original image is Image:Lichtenstein img processing test.png, that I have renamed castle.png on my hard-disk to make it simpler. Then I have converted it to JPEG using the "convert" utility by ImageMagik, with the following command:

convert -quality 50 castle.png castle.jpg

Then I have used the following Matlab code:

%read original image
original=imread('castle.png');
%read compressed image
compressed=imread('castle.jpg');

original=im2double(original);
compressed=im2double(compressed);

error=abs(original-compressed);

%amplify the error for each color layer
error(:,:,1)=imadjust(error(:,:,1));
error(:,:,2)=imadjust(error(:,:,2));
error(:,:,3)=imadjust(error(:,:,3));

%invert the picture
error=1-error;

%write it to a file
imwrite(error,'castle_error.png');