if I dct a gray image and then Idct, isnt it should be the original gray image?

A=imread('yachae.jpg');
B=rgb2gray(A);
DCTB=dct2(B);
IDCTB=idct2(DCTB);
imshow(IDCTB);

isnt it should be the original image if I dct a gray image and idct back?

but the result is a white image...where should i fix?

Answer 1

[Walter Roberson]

No, it should not be the same. You are not doing an integer transform, so you are taking in a integer image and outputing a floating point result that is in approximately the same value range but is double precision instead of integer data type. imshow() applied to a double precision matrix assumes that the data is in the range 0 to 1 and that anything outside that range should saturate to the boundary.

Try

imshow(IDCTB, [])