[FFmpeg-devel] [PATCH] avcodec/h264_metadata_bsf: Allow zeroing constraint_set4_flag and constraint_set5_flag

[Derek Buitenhuis]

These bits are reserved in earlie versions of the H.264 spec, and
some poor hardware decoders require they are zero. Thus, it is useful
to be able to zero these on streams that may have them set. The result
is still a valid H.264 bitstream.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis at gmail.com>
---
e.g. x264 wrote these bits for several months before reverting that
functionality, since it broke several hardware decoders.
---
 doc/bitstream_filters.texi     | 5 +++++
 libavcodec/h264_metadata_bsf.c | 8 ++++++++
 2 files changed, 13 insertions(+)

diff --git a/doc/bitstream_filters.texi b/doc/bitstream_filters.texi
index 60e729484d..81220638d4 100644
--- a/doc/bitstream_filters.texi
+++ b/doc/bitstream_filters.texi
@@ -253,6 +253,11 @@ Set whether the stream has fixed framerate - typically this indicates
 that the framerate is exactly half the tick rate, but the exact
 meaning is dependent on interlacing and the picture structure (see
 H.264 section E.2.1 and table E-6).
+ at item zero_new_constraint_set_flags
+Zero constraint_set4_flag and constraint_set5_flag in the SPS. These
+bits were reserved in a previous version of the H.264 spec, and thus
+some hardware encoders require these to be zero. The result of zeroing
+this is still a valid bitstream.
 
 @item crop_left
 @item crop_right
diff --git a/libavcodec/h264_metadata_bsf.c b/libavcodec/h264_metadata_bsf.c
index ef74cba560..452a8ec5dc 100644
--- a/libavcodec/h264_metadata_bsf.c
+++ b/libavcodec/h264_metadata_bsf.c
@@ -61,6 +61,7 @@ typedef struct H264MetadataContext {
 
     AVRational tick_rate;
     int fixed_frame_rate_flag;
+    int zero_new_constraint_set_flags;
 
     int crop_left;
     int crop_right;
@@ -228,6 +229,10 @@ static int h264_metadata_update_sps(AVBSFContext *bsf,
         need_vui = 1;
     }
     SET_VUI_FIELD(fixed_frame_rate_flag);
+    if (ctx->zero_new_constraint_set_flags) {
+        sps->constraint_set4_flag = 0;
+        sps->constraint_set5_flag = 0;
+    }
 
     if (sps->separate_colour_plane_flag || sps->chroma_format_idc == 0) {
         crop_unit_x = 1;
@@ -618,6 +623,9 @@ static const AVOption h264_metadata_options[] = {
     { "fixed_frame_rate_flag", "Set VUI fixed frame rate flag",
         OFFSET(fixed_frame_rate_flag), AV_OPT_TYPE_INT,
         { .i64 = -1 }, -1, 1, FLAGS },
+    { "zero_new_constraint_set_flags", "Set constraint_set4_flag / constraint_set5_flag to zero",
+        OFFSET(zero_new_constraint_set_flags), AV_OPT_TYPE_BOOL,
+        { .i64 = 0 }, 0, 1, FLAGS },
 
     { "crop_left", "Set left border crop offset",
         OFFSET(crop_left), AV_OPT_TYPE_INT,
-- 
2.32.0