Function mpeg_decode_slice

libavcodec/mpeg12dec.c:1359–1637

* Decode a slice. * Mpeg12SliceContext.c.mb_y must be set to the MB row from the startcode. * @return DECODE_SLICE_ERROR if the slice is damaged, * DECODE_SLICE_OK if this slice is OK */

 *         DECODE_SLICE_OK if this slice is OK
 */
static int mpeg_decode_slice(Mpeg12SliceContext *const s, int mb_y,
                             const uint8_t **buf, int buf_size)
{
    AVCodecContext *avctx = s->c.avctx;
    const int lowres      = s->c.avctx->lowres;
    const int field_pic   = s->c.picture_structure != PICT_FRAME;
    int ret;

    s->c.resync_mb_x =
    s->c.resync_mb_y = -1;

    av_assert0(mb_y < s->c.mb_height);

    ret = init_get_bits8(&s->gb, *buf, buf_size);
    if (ret < 0)
        return ret;

    if (s->c.codec_id != AV_CODEC_ID_MPEG1VIDEO && s->c.mb_height > 2800/16)
        skip_bits(&s->gb, 3);

    s->c.interlaced_dct = 0;

    s->c.qscale = mpeg_get_qscale(&s->gb, s->c.q_scale_type);

    if (s->c.qscale == 0) {
        av_log(s->c.avctx, AV_LOG_ERROR, "qscale == 0\n");
        return AVERROR_INVALIDDATA;
    }

    /* extra slice info */
    if (skip_1stop_8data_bits(&s->gb) < 0)
        return AVERROR_INVALIDDATA;

    s->c.mb_x = 0;

    if (mb_y == 0 && s->c.codec_tag == AV_RL32("SLIF")) {
        skip_bits1(&s->gb);
    } else {
        while (get_bits_left(&s->gb) > 0) {
            int code = get_vlc2(&s->gb, ff_mbincr_vlc,
                                MBINCR_VLC_BITS, 2);
            if (code < 0) {
                av_log(s->c.avctx, AV_LOG_ERROR, "first mb_incr damaged\n");
                return AVERROR_INVALIDDATA;
            }
            if (code >= 33) {
                if (code == 33)
                    s->c.mb_x += 33;
                /* otherwise, stuffing, nothing to do */
            } else {
                s->c.mb_x += code;
                break;
            }
        }
    }

    if (s->c.mb_x >= (unsigned) s->c.mb_width) {
        av_log(s->c.avctx, AV_LOG_ERROR, "initial skip overflow\n");