Special start-of-pass hook. This is called by jpeg_write_scanlines if call_pass_startup is true. In single-pass processing, we need this hook because we don't want to write frame/scan headers during jpeg_start_compress; we want to let the application write COM markers etc. between jpeg_start_compress and the jpeg_write_scanlines loop. In multi-pass processing, this routine is not used.
public void pass_startup()
{
m_cinfo.m_master.m_call_pass_startup = false; /* reset flag so call only once */
m_cinfo.m_marker.write_frame_header();
m_cinfo.m_marker.write_scan_header();
}
