static void usage()
{
    std::cerr << "Usage: dct_compress infile outfile width height"
              << " {rgb|cmyk|gray}" << std::endl;
    exit(2);
}
static void usage()
{
    std::cerr << "Usage: dct_compress infile outfile width height"
              << " {rgb|cmyk|gray}" << std::endl;
    exit(2);
}