how can i apply exact dct2 on 8x8 blocks and display them in subplots? - matlab

Question

I=imread('ft.jpg');
[a b]=size(I);
figure;imshow(I);
j=rgb2ycbcr(I);
[m n]=size(j);
figure;
imshow(j);
ca=mat2cell(j,8*ones(1,size(j,1)/8),8*ones(1,size(j,2)/8),3);
p = 1;
figure;
figure;
X=ones(m,n,8,8);
for c=1:size(ca,1)
for r=1:size(ca,2)
temp=zeros(8,8,'uint8');
temp(:,:)=X(c,r,:,:);
temp=temp-128;
temp=dct2(temp);
subplot(size(ca,1),size(ca,2),temp(:,:)); %// Change
imshow(ca{c,r});
p=p+1;
end
end
the error is:
Error using ==> subplot at 309
Illegal plot number.
Error in ==> project at 22
subplot(size(ca,1),size(ca,2),temp(:,:)); %// Change

Answer 1

That's because you're not calling subplot properly. It needs p as the parameter for the third argument, not temp. p determines which slot you want to place your figure in. You putting in a doublevector as the third parameter makes no sense. Also, ca contains 8 x 8 pixel blocks, and you'd like to show the DCT of each block. Your current code does not do this. Actually, X is what you're trying to find the DCT of, and it's all ones.... doesn't make much sense.
You probably want to show the DCT of each block inside the figure as well, not ca.... and so you need to do this:
for c=1:size(ca,1)
for r=1:size(ca,2)
temp = double(ca{c,r}); %// Change - cast to double for precision
temp=temp-128;
temp=dct2(temp);
subplot(size(ca,1),size(ca,2),p); %// Change
imshow(temp,[]); %// Change here too
p=p+1;
end
end
Note that I did imshow(temp,[]); so that we can contrast stretch the block so that the minimum value gets mapped to black while the maximum value gets mapped to white. Everything else is a shade of gray in between. Be advised that this doesn't change the image itself. It only changes it for display.
Suggestion
You should read up on my post on how subplot works here: How does subplot work and what is the difference between subplot(121) and subplot(1,2,1) in MATLAB?
Once you read through this post, you'll have a better understanding of how each parameter works for subplot, and how you can display graphs / images in a figure.
Minor Note
Looks like you took that code from this post but you didn't copy and paste properly :) That %// Change comment, as well as the variable name ca looked oddly familiar.