int ff_mjpeg_decode_dqt(MJpegDecodeContext *s)
{
    int len, index, i, j;

    len = get_bits(&s->gb, 16) - 2;

    while (len >= 65) {
        /* only 8 bit precision handled */
        if (get_bits(&s->gb, 4) != 0) {
            av_log(s->avctx, AV_LOG_ERROR, "dqt: 16bit precision\n");
            return -1;
        }
        index = get_bits(&s->gb, 4);
        if (index >= 4)
            return -1;
        av_log(s->avctx, AV_LOG_DEBUG, "index=%d\n", index);
        /* read quant table */
        for (i = 0; i < 64; i++) {
            j = s->scantable.permutated[i];
            s->quant_matrixes[index][j] = get_bits(&s->gb, 8);
        }

        // XXX FIXME finetune, and perhaps add dc too
        s->qscale[index] = FFMAX(s->quant_matrixes[index][s->scantable.permutated[1]],
                                 s->quant_matrixes[index][s->scantable.permutated[8]]) >> 1;
        av_log(s->avctx, AV_LOG_DEBUG, "qscale[%d]: %d\n",
               index, s->qscale[index]);
        len -= 65;
    }
    return 0;
}