jpeg_write_coefficients() shall write the contents of the JPEG file as DCT coefficients. The DCT coefficients shall be obtained from the array of virtual block array descriptors pointed to be "coeff_arrays". The number of arrays to be written is obtained from cinfo.num_components. Note: The virtual arrays may not be realized before jpeg_write_coefficients() is called. The virtual arrays requested from the compression object's memory manager may be realized by calling jpeg_write_coefficients() as a side-effect.