I = imread('peppers.png');

bs = 8

fun = @dct2;
J = blkproc(I,[bs bs],fun);
figure(1);
subplot(1,2,1); imshow(I, [])
subplot(1,2,2); imshow(J, [])

q = 20;
Jc= round(J/q)*q;

save 'peppers_orig.mat' I
save 'peppers_J.mat' J
save 'peppers_Jc.mat' Jc

fun = @idct2;
R  = blkproc(J,[bs bs],fun);
Rc = blkproc(Jc,[bs bs],fun);

figure(2)
subplot(1,2,1);imshow(R, [])
subplot(1,2,2);imshow(Rc, [])

x = linspace( min(J(:)), max(J(:)), 1000);
h  = hist(J(:), x);
hc = hist(Jc(:), x);
figure(3)
plot(x,log(h+1),'r', x,log(hc+1),'b')

figure(4); imshow(Jc-J, [])
max(abs(Jc(:)-J(:)))