Error while compressing the image using dct2()

Hi!

I am trying to compress image using DCT :

close all;
x=imread('image.jpeg');
figure,imshow(x);
F=dct2(x);
figure,imshow(F*0.01);
ff=idct2(F);
figure,imshow(ff/255);
[r,c]=size(x);
DF=zeros(r,c);
DFF=DF;
IDF=DF;
IDFF=DF;
depth=4;
N=8;

for i=1:N:r
  for j=1:N:c
    f=original(i:i+N-1,j:j+N-1);
    df=dct2(f);
    DF(i:i+N-1,j:j+N-1)=df;
    dff=idct2(df);
    DFF(i:i+N-1,j:j+N-1)=dff;
    
    
    df(:,N:-1:depth+1)=0;
    df(N:-1:depth+1,:)=0;
    IDF(i:i+N-1,j:j+N-1)=df;
    dff=idct2(df);
    IDFF(i:i+N-1,j:j+N-1)=dff;
   end
end
figure,imshow(DF/255);
figure,imshow(DFF);
A=DFF/255;
figure,imshow(A);
imwrite(A,'img.jpeg');
B=IDFF/255;
imwrite(B,'img_comp.jpeg');
figure,imshow(B);

Got this code from Online sources.

I am getting the following errors:

Matrix dimensions must agree.

Error in dct (line 76)
  b = W .* fft(y);

Error in dct2 (line 50)
    b = dct(dct(arg1).').';

Error in image_compression2 (line 4)
F=dct2(x);

Please help me resolve this Issue. Thanking you in Anticipation.

[Sriram Tadavarty]

There are pretty unclear terms in your code. What is the image.jpeg and original? If the image is colored, convert to gray image using rgb2gray and then perform the task, you will not see the error that is posted here. You can look at the example provided in this page.

[Walter Roberson]

jpeg are nearly always rgb even when they look grayscale.

[anuja tidke]

Thanks ! This helped :)

[Sriram Tadavarty]

Glad that it helped. I will place this answer and it helps others too.

Answer 1

[Sriram Tadavarty]

Hi Anuja,

This error can be solved by perfoming the RGB to gray conversion using rgb2gray function. To know the usage of dct2 with a example, look this page.

Hope this helps.

Regards,

Sriram