for (unsigned c = 0; c < num_components; ++c) {
unsigned cn = component_num[c];
unsigned stride = image->num_blocks_horizontal * image->hsample[cn] * DCTSIZE;
for (unsigned c = 0; c < num_components; ++c) {
unsigned cn = component_num[c];
unsigned stride = image->num_blocks_horizontal * image->hsample[cn] * DCTSIZE;