Rereading the spec, I think it only makes sense to update the
low_delay_hrd_flag parameter when the bitstream filter option for
fixed_frame_rate_flag is 1 to avoid clobbering the existing value in
other cases. I tried to work through all the scenarios below:
<fixed_frame_rate_flag>/<low_delay_hrd_flag> (BSF options) -> Resulting flags:
0/0 (tick_rate=10) -> 0/0
0/1 (tick_rate=10) -> 0/1
1/0 (tick_rate=10) -> 1/0
1/1 (tick_rate=10) -> 1/1
0/0 (fixed_frame_rate_flag=0) -> 0/1
0/1 (fixed_frame_rate_flag=0) -> 0/1
1/0 (fixed_frame_rate_flag=0) -> 0/0
1/1 (fixed_frame_rate_flag=0) -> 0/1
0/0 (fixed_frame_rate_flag=1) -> 1/0
0/1 (fixed_frame_rate_flag=1) -> 1/0
1/0 (fixed_frame_rate_flag=1) -> 1/0
1/1 (fixed_frame_rate_flag=1) -> 1/0
Signed-off-by: Keshav Varma <kesha...@gmail.com>
---
libavcodec/h264_metadata_bsf.c | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/libavcodec/h264_metadata_bsf.c b/libavcodec/h264_metadata_bsf.c
index 9df99cbae3..3e558a5166 100644
--- a/libavcodec/h264_metadata_bsf.c
+++ b/libavcodec/h264_metadata_bsf.c
@@ -228,7 +228,13 @@ static int h264_metadata_update_sps(AVBSFContext *bsf,
sps->vui.timing_info_present_flag = 1;
need_vui = 1;
}
+
+ // Set fixed frame rate flag and update low_delay_hrd_flag to match
SET_VUI_FIELD(fixed_frame_rate_flag);
+ if (ctx->fixed_frame_rate_flag) {
+ sps->vui.low_delay_hrd_flag = 0;
+ }
+
if (ctx->zero_new_constraint_set_flags) {
sps->constraint_set4_flag = 0;
sps->constraint_set5_flag = 0;
--
2.31.1
On Fri, Feb 4, 2022 at 3:00 PM Keshav Varma <kesha...@gmail.com> wrote:
>
> After changes like ef13faf, the h264_metadata bitstream filter stopped
> working when using the fixed_frame_rate_flag option on an input stream
> that doesn't contain VUI because the default inferred value of
> low_hrd_delay_flag seems to be 1. ffmpeg used to raise a warning, but
> proceeded anyway but now aborts after the other fixes since the output
> is rightfully invalid.
>
> I believe this change makes the bitstream filter conform to page 403
> of the ITU spec: "When fixed_frame_rate_flag is equal to 1,
> low_delay_hrd_flag shall be equal to 0. When low_delay_hrd_flag is not
> present, its value shall be inferred to be equal to 1 −
> fixed_frame_rate_flag."
>
> Signed-off-by: Keshav Varma <kesha...@gmail.com>
> ---
> libavcodec/h264_metadata_bsf.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/libavcodec/h264_metadata_bsf.c b/libavcodec/h264_metadata_bsf.c
> index 9df99cbae3..59a7eba546 100644
> --- a/libavcodec/h264_metadata_bsf.c
> +++ b/libavcodec/h264_metadata_bsf.c
> @@ -228,7 +228,13 @@ static int h264_metadata_update_sps(AVBSFContext *bsf,
> sps->vui.timing_info_present_flag = 1;
> need_vui = 1;
> }
> - SET_VUI_FIELD(fixed_frame_rate_flag);
> +
> + // Set fixed frame rate flag and update low_delay_hrd_flag to match
> + if (ctx->fixed_frame_rate_flag >= 0) {
> + sps->vui.fixed_frame_rate_flag = ctx->fixed_frame_rate_flag;
> + sps->vui.low_delay_hrd_flag = 1 - sps->vui.fixed_frame_rate_flag;
> + }
> +
> if (ctx->zero_new_constraint_set_flags) {
> sps->constraint_set4_flag = 0;
> sps->constraint_set5_flag = 0;
> --
> 2.31.1
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