Blockproc error when including BorderSize argument

I'm working on the exercise by Matlab on dct https://www.mathworks.com/help/images/discrete-cosine-transform.html

and i ran the following code:

I = imread('cameraman.tif'); 
I = im2double(I);
imshow(I)

T = dctmtx(8);
dct = @(block_struct) T * block_struct.data * T';
B = blockproc(I,[8 8],dct,'BorderSize',[4 4],'Trim',false) %modification made here

The code was from the link above, but i included the 'BorderSize' criteria to allow for overlapping of the blocks. Upon running the code, i get the error saying:

Error using blockprocFunDispatcher
BLOCKPROC encountered an error while evaluating the user-supplied function handle,FUN.

Error in blockprocInMemory
[u1_output fun_nargout] = blockprocFunDispatcher(fun,block_struct,...

Error in blockproc
    result_image = blockprocInMemory(source,fun,options);

What could be the issue causing the error message above? If i remove 'BorderSize',[4 4], 'Trim',false it works as normal like in the link but am i using it wrongly?

[Adam]

T is hard-coded to be of size 8 by 8.

You have now added a [4 4] border to your 8 by 8 blocks so they cannot multiply by T any more because they are not the same size.

[Stewart Tan]

So is it not possible to apply dct with included borders? Do you have any suggestion on this?

Answer 1

[Walter Roberson]

I = imread('cameraman.tif'); 
I = im2double(I);
T = dctmtx(8);
dct = @(block_struct) T * block_struct.data * T';
B = blockproc(I,[4 4],dct,'BorderSize',[2 2],'Trim',false, 'padpartial', true); %modification made here
size(I)
size(B)

Note that B comes out larger than I because you have trim set to false.

Notice the pad partial being needed: without it then when you reach the edge of the image, you run off the edge. blockproc() keeps moving the window until the last of the [4 4] windows is at the right (or bottom) edge, leaving the specified border padding "hanging over" the end of the image.