how to reconstruct image in jpeg compression?

i am doing image compression using JPEG but i cannot reconstruct the image after inverse DCT. I used the following code...

if true
  % code
end
% Restore back image
%----------------------------------------------------------
recon_image=double(recon_image);
recon_image=recon_image + 128 * (ones(size(recon_image)));
recon_image = mat2gray(recon_image);
I1=I1 + 128 * (ones(size(I1)));
I1 = mat2gray(I1); 
row = row - pad_rows;
coln = coln - pad_colns;
recon_image = recon_image(1:row, 1:coln);
recon_image = mat2gray(recon_image);