How do I get the DC coefficient from a jpg using the jpg library? - c++

Question

I am new to this stuff, but I need to get the dc-coefficient from a jpeg using the jpeg library?
I was told as a hint that the corresponding function is in jdhuff.c, but I can't find it. I tried to find a decent article about the jpg library where I can get this, but no success so far.
So I hope you guys can help me a bit and point me to either some documentation or have a hint.
So, here is what I know:
A jpg picture consists of 8x8 Blocks. That are 64 Pixels. 63 of it are named AC and 1 is named DC. Thats the coefficient. The position is at array[0][0].
But how do I exactly read that with the jpg library? I am using C++.
edit:
This is what I have so far:
read_jpeg::read_jpeg( const std::string& filename )
{
FILE* fp = NULL; // File-Pointer
jpeg_decompress_struct cinfo; // jpeg decompression parameters
JSAMPARRAY buffer; // Output row-buffer
int row_stride = 0; // physical row width
my_error_mgr jerr; // Custom Error Manager
// Set Error Manager
cinfo.err = jpeg_std_error(&jerr.pub);
jerr.pub.error_exit = my_error_exit;
// Handle longjump
if (setjmp(jerr.setjmp_buffer)) {
// JPEG has signaled an error. Clean up and throw an exception.
jpeg_destroy_decompress(&cinfo);
fclose(fp);
throw std::runtime_error("Error: jpeg has reported an error.");
}
// Open the file
if ( (fp = fopen(filename.c_str(), "rb")) == NULL )
{
std::stringstream ss;
ss << "Error: Cannot read '" << filename.c_str() << "' from the specified location!";
throw std::runtime_error(ss.str());
}
// Initialize jpeg decompression
jpeg_create_decompress(&cinfo);
// Show jpeg where to read the data
jpeg_stdio_src(&cinfo, fp);
// Read the header
jpeg_read_header(&cinfo, TRUE);
// Decompress the file
jpeg_start_decompress(&cinfo);
// JSAMPLEs per row in output buffer
row_stride = cinfo.output_width * cinfo.output_components;
// Make a one-row-high sample array
buffer = (*cinfo.mem->alloc_sarray)((j_common_ptr) &cinfo, JPOOL_IMAGE, row_stride, 1);
// Read image using jpgs counter
while (cinfo.output_scanline < cinfo.output_height)
{
// Read the image
jpeg_read_scanlines(&cinfo, buffer, 1);
}
// Finish the decompress
jpeg_finish_decompress(&cinfo);
// Release memory
jpeg_destroy_decompress(&cinfo);
// Close the file
fclose(fp);
}

Answer 1

This is not possible using the standard API. With libjpeg API the closest you can get is raw pixel data of Y/Cb/Cr channels.
To get coefficients' data you'd need to hack the decode_mcu function (or its callers) to save the data decoded there.