avcodec/mpeg4video: Skip unneeded element when parsing picture header

Namely, skip some elements that are only useful for a decoder
when calling ff_mpeg4_decode_picture_header() from the MPEG-4 parser.

In particular, this ensures that the VLCs need no longer be
initialized by the parser.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/h263dec.c

@@ -507,9 +507,9 @@ retry:
             GetBitContext gb;
 
             if (init_get_bits8(&gb, s->avctx->extradata, s->avctx->extradata_size) >= 0 )
-                ff_mpeg4_decode_picture_header(avctx->priv_data, &gb, 1);
+                ff_mpeg4_decode_picture_header(avctx->priv_data, &gb, 1, 0);
         }
-        ret = ff_mpeg4_decode_picture_header(avctx->priv_data, &s->gb, 0);
+        ret = ff_mpeg4_decode_picture_header(avctx->priv_data, &s->gb, 0, 0);
     } else if (CONFIG_H263I_DECODER && s->codec_id == AV_CODEC_ID_H263I) {
         ret = ff_intel_h263_decode_picture_header(s);
     } else if (CONFIG_FLV_DECODER && s->h263_flv) {

libavcodec/mpeg4video.h

@@ -160,7 +160,8 @@ void ff_mpeg4_pred_ac(MpegEncContext *s, int16_t *block, int n,
 void ff_set_mpeg4_time(MpegEncContext *s);
 int ff_mpeg4_encode_picture_header(MpegEncContext *s, int picture_number);
 
-int ff_mpeg4_decode_picture_header(Mpeg4DecContext *ctx, GetBitContext *gb, int header);
+int ff_mpeg4_decode_picture_header(Mpeg4DecContext *ctx, GetBitContext *gb,
+                                   int header, int parse_only);
 void ff_mpeg4_encode_video_packet_header(MpegEncContext *s);
 void ff_mpeg4_clean_buffers(MpegEncContext *s);
 void ff_mpeg4_stuffing(PutBitContext *pbc);

libavcodec/mpeg4video_parser.c

@@ -100,13 +100,13 @@ static int mpeg4_decode_header(AVCodecParserContext *s1, AVCodecContext *avctx,
 
     if (avctx->extradata_size && pc->first_picture) {
         init_get_bits(gb, avctx->extradata, avctx->extradata_size * 8);
-        ret = ff_mpeg4_decode_picture_header(dec_ctx, gb, 1);
+        ret = ff_mpeg4_decode_picture_header(dec_ctx, gb, 1, 1);
         if (ret < 0)
             av_log(avctx, AV_LOG_WARNING, "Failed to parse extradata\n");
     }
 
     init_get_bits(gb, buf, 8 * buf_size);
-    ret = ff_mpeg4_decode_picture_header(dec_ctx, gb, 0);
+    ret = ff_mpeg4_decode_picture_header(dec_ctx, gb, 0, 1);
     if (s->width && (!avctx->width || !avctx->height ||
                      !avctx->coded_width || !avctx->coded_height)) {
         ret = ff_set_dimensions(avctx, s->width, s->height);

libavcodec/mpeg4videodec.c

@@ -2844,7 +2844,8 @@ int ff_mpeg4_workaround_bugs(AVCodecContext *avctx)
     return 0;
 }
 
-static int decode_vop_header(Mpeg4DecContext *ctx, GetBitContext *gb)
+static int decode_vop_header(Mpeg4DecContext *ctx, GetBitContext *gb,
+                             int parse_only)
 {
     MpegEncContext *s = &ctx->m;
     int time_incr, time_increment;
@@ -3018,6 +3019,12 @@ static int decode_vop_header(Mpeg4DecContext *ctx, GetBitContext *gb)
         ff_init_scantable(s->idsp.idct_permutation, &s->intra_v_scantable, ff_alternate_vertical_scan);
     }
 
+    /* Skip at this point when only parsing since the remaining
+     * data is not useful for a parser and requires the
+     * sprite_trajectory VLC to be initialized. */
+    if (parse_only)
+        goto end;
+
     if (s->pict_type == AV_PICTURE_TYPE_S) {
         if((ctx->vol_sprite_usage == STATIC_SPRITE ||
             ctx->vol_sprite_usage == GMC_SPRITE)) {
@@ -3095,6 +3102,8 @@ static int decode_vop_header(Mpeg4DecContext *ctx, GetBitContext *gb)
             skip_bits(gb, 2);  // ref_select_code
         }
     }
+
+end:
     /* detect buggy encoders which don't set the low_delay flag
      * (divx4/xvid/opendivx). Note we cannot detect divx5 without B-frames
      * easily (although it's buggy too) */
@@ -3214,11 +3223,14 @@ static int decode_studiovisualobject(Mpeg4DecContext *ctx, GetBitContext *gb)
  * Decode MPEG-4 headers.
  *
  * @param  header If set the absence of a VOP is not treated as error; otherwise, it is treated as such.
+ * @param  parse_only If set, things only relevant to a decoder may be skipped;
+ *                    furthermore, the VLC tables may be uninitialized.
  * @return <0 if an error occurred
  *         FRAME_SKIPPED if a not coded VOP is found
  *         0 else
  */
-int ff_mpeg4_decode_picture_header(Mpeg4DecContext *ctx, GetBitContext *gb, int header)
+int ff_mpeg4_decode_picture_header(Mpeg4DecContext *ctx, GetBitContext *gb,
+                                   int header, int parse_only)
 {
     MpegEncContext *s = &ctx->m;
     unsigned startcode, v;
@@ -3371,7 +3383,7 @@ end:
         }
         return decode_studio_vop_header(ctx, gb);
     } else
-        return decode_vop_header(ctx, gb);
+        return decode_vop_header(ctx, gb, parse_only);
 }
 
 av_cold void ff_mpeg4videodec_static_init(void) {