Image Compression: How to use blockproc function with dct2 inside

For the uploaded image below with 536x704 pixel size, my code reads the image, converts to ycbcr and subsamples, then with dct tries to make 8x8 block with 4:2;0 format and choose the top 6th row of the luminance.

My code:

I = imread('Flooded_house.jpg','jpg');

%converting to ycbcr

YCbCr = rgb2ycbcr(I);

Y = YCbCr(:,:,1);

Cb = YCbCr(:,:,2);

Cr = YCbCr(:,:,3);

%sub-sampling

Sub_Cb = Cb(1:2:end, 1:2:end);

Sub_Cr = Cr(1:2:end, 1:2:end);

%Now computing the 8x8 block DCT transform coefficients in 4:2:0 format

DCT_Trans = @(block_struct) dct2(block_struct.data);

DCT_Y = blockproc(Y, [8 8], DCT_Trans);

DCT_Cb = blockproc(Sub_Cb, [8 8], DCT_Trans, 'PadPartialBlocks', true);

DCT_Cr = blockproc(Sub_Cr, [8 8], DCT_Trans, 'PadPartialBlocks', true);

%Part (a)

%putting the first block of the DCT image

First_Block = DCT_Trans(1:8,1:8);

imshow(First_Block)

%computing the first 2 blocks in the 6th row from the top of the luminance

%component

Sixth_Block = DCT_Trans(41:48,1:8);

imshow(Sixth_Block)

Sixth_Block_p2 = DCT_Trans(41:48,9:16);

imshow(Sixth_Block_p2)

Error Code:

Error using Encoder_a>@(block_struct)dct2(block_struct.data)

Too many input arguments.

Error in Encoder_a (line 31)

First_Block = DCT_Trans(1:8,1:8);

Answer 1

[Abolfazl Chaman Motlagh]

you created DCT_Trans function for blockprop . it's a function which takes structure as input. if you pass structure S to it's input, it takes dct of S.data. so the line

First_Block = DCT_Trans(1:8,1:8);

and all lines like this, return error. because (1:8,1:8) are two input. and they are not structure.

if you wanna call your transformed image you should call DCT_Y, DCT_Cb and DCT_Cr .

don't you mean :

First_Block = DCT_Y(1:8,1:8);

???