Hello,
I am a tiny bit stuck with decompressing a 3 channel color jpg into spatial domain. To directly access the jpg's DCT coefficients and quanitsation tables I've opened the jpg file via Phil Sallee's jpg toolbox's jpeg_read() function as follows and tried decompressing it as follows:
addpath('jpeg_toolbox-2/');
path = '/data/test.jpg';
img = jpeg_read(path);
Lum = img.coef_arrays{img.comp_info(1).component_id};
ChromCr = img.coef_arrays{img.comp_info(2).component_id};
ChromCb = img.coef_arrays{img.comp_info(3).component_id};
Lum_quant_table = img.quant_tables{img.comp_info(1).quant_tbl_no};
Chrom_quant_table = img.quant_tables{img.comp_info(2).quant_tbl_no};
fun = @(x)x.data .*Lum_quant_table;
lum_spatial = blockproc(Lum,[8 8],fun);
fun2 = @(x)x.data .*Chrom_quant_table;
cr_spatial = blockproc(ChromCr,[8 8],fun2);
cb_spatial = blockproc(ChromCb,[8 8],fun2);
fun3=@(x)idct2(x.data);
lum_spatialf = blockproc(lum_spatial,[8 8],fun3);
cr_spatialf = blockproc(cr_spatial,[8 8],fun3);
cb_spatialf = blockproc(cb_spatial,[8 8],fun3);
f = cat(3, lum_spatialf, cr_spatialf, cb_spatialf);
f = ycbcr2rgb(f);
imshow(f);
save('data/test4.mat','f', '-v6');
(The first part - extracting the coefficents and tables - is taken from the book Steganography in Digital Media: Principles, Algorithms, and Applications.)
It seems like I am missing something since the decompression does not quite look right...
For grayscale images I manage to do the decompression:
img = jpeg_read(path);
fun = @(x)x.data .*img.quant_tables{1};
spatial = blockproc(img.coef_arrays{1},[8 8],fun);
fun=@(x)idct2(x.data);
f = blockproc(spatial,[8 8],fun);
imshow(f);
save('data/test8.mat','f', '-v6');
but for color images I don't get it to work.
Thank you very much in advance!
