matlab error using CAT, Dimensions of matrices being concatenated are not consistent

Question

I am trying to compute the PURE BLUE COLOR from an image and compare it to the original blue channel. Then I have to explain the advantages of using pure colors instead of RGB channels.

This Is my code:

>> RGB = double( imread('players.jpeg'))/255;
>> imagesc(RGB);

>> red = RGB( :,:,1);
>> green = RGB( :,:,2);
>> blue = RGB( :,:,3);
>> pure_BLUE = blue ./ (red+green+blue);
>> imagesc(pure_BLUE);

however when I use this:

>> imagesc(cat(3,pure_BLUE,zeros(240,320),zeros(240,320)));

I get the following error:

Error using cat Dimensions of matrices being concatenated are not consistent.

Not quite sure what goes wrong here and How I can fix this!!!!

Answer 1

It looks like the two arrays you're trying to concatenate are the wrong size compared to the image. The image appears to be 176x241 so you should try:

imagesc(cat(3,pure_BLUE,zeros(176,241),zeros(176,241)));

or a more general way is to use the size of the array itself:

imagesc(cat(3,pure_BLUE,zeros([size(pure_BLUE) 2])));