[FFmpeg-cvslog] avcodec/dovi_rpu{enc,dec}: fix ms_weight handling

[Niklas Haas]

ffmpeg | branch: master | Niklas Haas <git at haasn.dev> | Sat May 25 18:05:04 2024 +0200| [6225ad5c19751ed55555ac9ffb738c2e4acc1f92] | committer: Niklas Haas

avcodec/dovi_rpu{enc,dec}: fix ms_weight handling

The code as written was wrong. In the spec, these fields are treated
merely as plain integers in the range 0 to 4095. The only difference
between L2 and L8 is that L2.ms_weight also accepts an additional value
of -1, hence the extra sign bit. While it's likely that these are still
shifted integers in disguise, since all real-world samples seem to use
a value of 2048 here, the offset used in the code was wrong.

In addition, because the l8.ms_weight struct member is unsigned, these
wrong shifting semantics ended up overflowing the field, leading to
undefined behavior when transcoding. Fortunately, the damage was
relatively contained in practice, because it just corrupts the coding of
this field, which is ignored in practice in all implementations I have
seen.

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=6225ad5c19751ed55555ac9ffb738c2e4acc1f92
---

 libavcodec/dovi_rpudec.c | 4 ++--
 libavcodec/dovi_rpuenc.c | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/libavcodec/dovi_rpudec.c b/libavcodec/dovi_rpudec.c
index 7c7eda9d09..a477dbd4e3 100644
--- a/libavcodec/dovi_rpudec.c
+++ b/libavcodec/dovi_rpudec.c
@@ -142,7 +142,7 @@ static void parse_ext_v1(DOVIContext *s, GetBitContext *gb, AVDOVIDmData *dm)
         dm->l2.trim_power = get_bits(gb, 12);
         dm->l2.trim_chroma_weight = get_bits(gb, 12);
         dm->l2.trim_saturation_gain = get_bits(gb, 12);
-        dm->l2.ms_weight = get_bits(gb, 13) - 8192;
+        dm->l2.ms_weight = get_sbits(gb, 13);
         break;
     case 4:
         dm->l4.anchor_pq = get_bits(gb, 12);
@@ -197,7 +197,7 @@ static void parse_ext_v2(DOVIContext *s, GetBitContext *gb, AVDOVIDmData *dm,
         dm->l8.trim_power = get_bits(gb, 12);
         dm->l8.trim_chroma_weight = get_bits(gb, 12);
         dm->l8.trim_saturation_gain = get_bits(gb, 12);
-        dm->l8.ms_weight = get_bits(gb, 12) - 8192;
+        dm->l8.ms_weight = get_bits(gb, 12);
         if (ext_block_length < 12)
             break;
         dm->l8.target_mid_contrast = get_bits(gb, 12);
diff --git a/libavcodec/dovi_rpuenc.c b/libavcodec/dovi_rpuenc.c
index 3c3e0f84c0..dacb8b54e7 100644
--- a/libavcodec/dovi_rpuenc.c
+++ b/libavcodec/dovi_rpuenc.c
@@ -276,7 +276,7 @@ static void generate_ext_v1(PutBitContext *pb, const AVDOVIDmData *dm)
         put_bits(pb, 12, dm->l2.trim_power);
         put_bits(pb, 12, dm->l2.trim_chroma_weight);
         put_bits(pb, 12, dm->l2.trim_saturation_gain);
-        put_bits(pb, 13, dm->l2.ms_weight + 8192);
+        put_sbits(pb, 13, dm->l2.ms_weight);
         break;
     case 4:
         put_bits(pb, 12, dm->l4.anchor_pq);
@@ -374,7 +374,7 @@ static void generate_ext_v2(PutBitContext *pb, const AVDOVIDmData *dm)
         put_bits(pb, 12, dm->l8.trim_power);
         put_bits(pb, 12, dm->l8.trim_chroma_weight);
         put_bits(pb, 12, dm->l8.trim_saturation_gain);
-        put_bits(pb, 12, dm->l8.ms_weight + 8192);
+        put_bits(pb, 12, dm->l8.ms_weight);
         if (ext_block_length < 12)
             break;
         put_bits(pb, 12, dm->l8.target_mid_contrast);