[FFmpeg-cvslog] avcodec/mpegvideo_dec: Simplify check for unquantizing inter blocks

[Andreas Rheinhardt]

ffmpeg | branch: master | Andreas Rheinhardt <andreas.rheinhardt at outlook.com> | Tue May 20 23:58:34 2025 +0200| [778c0a48fa4656fe603231e775322bc4e3f5b962] | committer: Andreas Rheinhardt

avcodec/mpegvideo_dec: Simplify check for unquantizing inter blocks

Just ensure that dct_unquantize_inter is set iff it is used
and check for the function pointer instead.

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

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=778c0a48fa4656fe603231e775322bc4e3f5b962
---

 libavcodec/h263dec.c       | 3 ++-
 libavcodec/mpeg4videodec.c | 6 ++++--
 libavcodec/mpeg4videodec.h | 2 ++
 libavcodec/mpegvideo_dec.c | 8 +++-----
 libavcodec/msmpeg4dec.c    | 3 +++
 5 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/libavcodec/h263dec.c b/libavcodec/h263dec.c
index f59012fda4..37abf3382e 100644
--- a/libavcodec/h263dec.c
+++ b/libavcodec/h263dec.c
@@ -110,7 +110,8 @@ av_cold int ff_h263_decode_init(AVCodecContext *avctx)
     ff_mpv_unquantize_init(&unquant_dsp_ctx,
                            avctx->flags & AV_CODEC_FLAG_BITEXACT, 0);
     // dct_unquantize defaults for H.263;
-    // they might change on a per-frame basis for MPEG-4.
+    // they might change on a per-frame basis for MPEG-4;
+    // dct_unquantize_inter will be unset for MSMPEG4 codecs later.
     s->dct_unquantize_intra = unquant_dsp_ctx.dct_unquantize_h263_intra;
     s->dct_unquantize_inter = unquant_dsp_ctx.dct_unquantize_h263_inter;
 
diff --git a/libavcodec/mpeg4videodec.c b/libavcodec/mpeg4videodec.c
index c4eed9136a..936711d72e 100644
--- a/libavcodec/mpeg4videodec.c
+++ b/libavcodec/mpeg4videodec.c
@@ -3417,6 +3417,8 @@ static int decode_vop_header(Mpeg4DecContext *ctx, GetBitContext *gb,
 
     s->dct_unquantize_intra = s->mpeg_quant ? ctx->dct_unquantize_mpeg2_intra
                                             : ctx->dct_unquantize_h263_intra;
+    // The following tells ff_mpv_reconstruct_mb() to unquantize iff mpeg_quant
+    s->dct_unquantize_inter = s->mpeg_quant ? ctx->dct_unquantize_mpeg2_inter : NULL;
 
 end:
     /* detect buggy encoders which don't set the low_delay flag
@@ -3961,8 +3963,8 @@ static av_cold int decode_init(AVCodecContext *avctx)
     ctx->dct_unquantize_h263_intra  = unquant_dsp_ctx.dct_unquantize_h263_intra;
     ctx->dct_unquantize_mpeg2_intra = unquant_dsp_ctx.dct_unquantize_mpeg2_intra;
     // dct_unquantize_inter is only used with MPEG-2 quantizers,
-    // so we can already set dct_unquantize_inter here once and for all.
-    s->dct_unquantize_inter = unquant_dsp_ctx.dct_unquantize_mpeg2_inter;
+    // so that is all we keep.
+    ctx->dct_unquantize_mpeg2_inter = unquant_dsp_ctx.dct_unquantize_mpeg2_inter;
 
     s->y_dc_scale_table = ff_mpeg4_y_dc_scale_table;
     s->c_dc_scale_table = ff_mpeg4_c_dc_scale_table;
diff --git a/libavcodec/mpeg4videodec.h b/libavcodec/mpeg4videodec.h
index 57a2f81816..111d60ba10 100644
--- a/libavcodec/mpeg4videodec.h
+++ b/libavcodec/mpeg4videodec.h
@@ -91,6 +91,8 @@ typedef struct Mpeg4DecContext {
 
     Mpeg4VideoDSPContext mdsp;
 
+    void (*dct_unquantize_mpeg2_inter)(MpegEncContext *s,
+                                       int16_t *block, int n, int qscale);
     void (*dct_unquantize_mpeg2_intra)(MpegEncContext *s,
                                        int16_t *block, int n, int qscale);
     void (*dct_unquantize_h263_intra)(MpegEncContext *s,
diff --git a/libavcodec/mpegvideo_dec.c b/libavcodec/mpegvideo_dec.c
index c85c7db722..f8551b93c8 100644
--- a/libavcodec/mpegvideo_dec.c
+++ b/libavcodec/mpegvideo_dec.c
@@ -960,11 +960,8 @@ void mpv_reconstruct_mb_internal(MpegEncContext *s, int16_t block[12][64],
         }
 
         /* add dct residue */
-        if (!(IS_MPEG12_H261(s) || s->msmpeg4_version != MSMP4_UNUSED ||
-              (s->codec_id == AV_CODEC_ID_MPEG4 && !s->mpeg_quant))) {
+        if (is_mpeg12 != DEFINITELY_MPEG12_H261 && s->dct_unquantize_inter) {
             // H.263, H.263+, H.263I, FLV, RV10, RV20 and MPEG-4 with MPEG-2 quantization
-            // Also RV30, RV40 when performing error resilience, but
-            // all blocks are skipped in this case.
             add_dequant_dct(s, block[0], 0, dest_y                          , dct_linesize, s->qscale);
             add_dequant_dct(s, block[1], 1, dest_y              + block_size, dct_linesize, s->qscale);
             add_dequant_dct(s, block[2], 2, dest_y + dct_offset             , dct_linesize, s->qscale);
@@ -978,7 +975,8 @@ void mpv_reconstruct_mb_internal(MpegEncContext *s, int16_t block[12][64],
         } else if (is_mpeg12 == DEFINITELY_MPEG12_H261 || lowres_flag || (s->codec_id != AV_CODEC_ID_WMV2)) {
             // H.261, MPEG-1, MPEG-2, MPEG-4 with H.263 quantization,
             // MSMP4V1-3 and WMV1.
-            // Also the VC-1 family when performing error resilience
+            // Also RV30, RV40 and the VC-1 family when performing error resilience,
+            // but all blocks are skipped in this case.
             add_dct(s, block[0], 0, dest_y                          , dct_linesize);
             add_dct(s, block[1], 1, dest_y              + block_size, dct_linesize);
             add_dct(s, block[2], 2, dest_y + dct_offset             , dct_linesize);
diff --git a/libavcodec/msmpeg4dec.c b/libavcodec/msmpeg4dec.c
index ddb990b1a0..df5ab5186e 100644
--- a/libavcodec/msmpeg4dec.c
+++ b/libavcodec/msmpeg4dec.c
@@ -366,6 +366,9 @@ av_cold int ff_msmpeg4_decode_init(AVCodecContext *avctx)
     if (ff_h263_decode_init(avctx) < 0)
         return -1;
 
+    // We unquantize inter blocks as we parse them.
+    s->dct_unquantize_inter = NULL;
+
     ff_msmpeg4_common_init(s);
 
     switch (s->msmpeg4_version) {