my DCT2 algorithm results in blank image, please help

have a grey image lenna512.bmp as im, were asked to do the two-dimensional DCT of all the 8 X 8 non-overlapping blocks of the image im, and merge the left-top pixel of all blocks after the DCT transformation to get a smaller image ims.

this is what I did:

A = double(imread('lenna512.bmp'));
B = zeros(64,64);
for i = 1:64
    for j = 1:64
        %Split the original image into non-overlapping 8*8blocks
        %making a total of 64*64=4096
        C = A((8*i-7):8*i, (8*j-7):8*j);
        %Call the build-in function to perform a 
        %2-dimentional Fourier transform on each block
        D = dct2(C);
        %Fill the top left element of the 
        %completed block to a new matrix
        B(i,j) = D(1,1);
    end
end
%imwrite(uint8(B),'ims.bmp');
imshow(uint8(B));

it shows a blank image without any details, please tell me how to fix it

[liam xu]

sorry it's not the fourier transform it's a cosine transform

Answer 1

[Chunru]

A = rgb2gray(imread('Lenna.png'));
figure; imshow(A);

whos

  Name          Size                Bytes  Class    Attributes

  A           512x512              262144  uint8              
  I           512x512x3            786432  uint8              
  cmdout        1x33                   66  char               

B = zeros(64,64);
for i = 1:64
    for j = 1:64
        %Split the original image into non-overlapping 8*8blocks
        %making a total of 64*64=4096
        C = A((8*i-7):8*i, (8*j-7):8*j);
        %Call the build-in function to perform a 
        %2-dimentional Fourier transform on each block
        D = dct2(double(C));
        %Fill the top left element of the 
        %completed block to a new matrix
        B(i,j) = D(1,1);
    end
end
%imwrite(uint8(B),'ims.bmp');
%imshow(uint8(B));
figure; imagesc(B)

[liam xu]

thank you

i got it, just use imshow(,[])