You did not follow this code from that page link you gave us:
I = imread('cameraman.tif');
I = im2double(I);
T = dctmtx(8);
dct = @(block_struct) T * block_struct.data * T';
B = blockproc(I,[8 8],dct);
mask = [1 1 1 1 0 0 0 0
1 1 1 0 0 0 0 0
1 1 0 0 0 0 0 0
1 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0];
B2 = blockproc(B,[8 8],@(block_struct) mask .* block_struct.data);
invdct = @(block_struct) T' * block_struct.data * T;
I2 = blockproc(B2,[8 8],invdct);
imshow(I), figure, imshow(I2)
It runs fine, though if I had written it, I would have used more descriptive variable names and used subplot. Anyway, you must have altered it somehow but either forgot to tell us how, or don't want to tell us how. So we can only guess and I think Walter's guess that you read in a color image is a good one.
To get a gray scale image, you can do this:
or you can use one of the color channels, which can be extracted like this:
redChannel = rgbImage(:, :, 1);
greenChannel = rgbImage(:, :, 2);
blueChannel = rgbImage(:, :, 3);
Then use one of those variables instead of "I".
