jpeg_read_coefficients() shall return the contents of the JPEG image file as DCT coefficients. The entire image shall be read as a set of virtual coefficient-block arrays, one array per component. The return value is a pointer to an array of virtual-array descriptors.
Each block in the block arrays shall contain quantized coefficient values in normal array order and not JPEG zigzag order. The block arrays shall contain only DCT blocks containing real data; any entirely-dummy blocks added to fill out interleaved MCUs at the right or bottom edges of the image shall be discarded during reading and shall not be stored in the block arrays. The size of each block array can be determined from the width_in_blocks and height_in_blocks fields of the component's comp_info entry.
jpeg_read_coefficients() shall return a pointer to an array of virtual-array descriptors containing DCT coefficients.