Re: [FFmpeg-devel] [PATCH 6/6] lavc/h264_ps: always include the stop bit in [s|p]ps->data

[Ridley Combs <rcombs@rcombs.me>]

> On May 9, 2022, at 23:26, Andreas Rheinhardt <andreas.rheinhardt@outlook.com> wrote:
> 
> rcombs:
>> From: rcombs <rcombs@rcombs.me>
>> 
>> The VideoToolbox hwaccel needs the entire NAL (including the stop bit),
>> but ff_h2645_packet_split may remove it. Detect this case by looking for
>> bit counts divisible by 8 and insert a stop-bit-only 0x80 byte.
>> 
>> Signed-off-by: rcombs <rcombs@rcombs.me>
>> ---
>> libavcodec/h264_ps.c | 8 ++++++++
>> 1 file changed, 8 insertions(+)
>> 
>> diff --git a/libavcodec/h264_ps.c b/libavcodec/h264_ps.c
>> index 051f06692c..e16da68dec 100644
>> --- a/libavcodec/h264_ps.c
>> +++ b/libavcodec/h264_ps.c
>> @@ -351,6 +351,10 @@ int ff_h264_decode_seq_parameter_set(GetBitContext *gb, AVCodecContext *avctx,
>> }
>> memcpy(sps->data, gb->buffer, sps->data_size);
>> 
>> + // Re-add the removed stop bit (may be used by hwaccels).
>> + if (!(gb->size_in_bits & 7) && sps->data_size < sizeof(sps->data))
>> + sps->data[sps->data_size++] = 0x80;
>> +
>> profile_idc = get_bits(gb, 8);
>> constraint_set_flags |= get_bits1(gb) << 0; // constraint_set0_flag
>> constraint_set_flags |= get_bits1(gb) << 1; // constraint_set1_flag
>> @@ -775,6 +779,10 @@ int ff_h264_decode_picture_parameter_set(GetBitContext *gb, AVCodecContext *avct
>> }
>> memcpy(pps->data, gb->buffer, pps->data_size);
>> 
>> + // Re-add the removed stop bit (may be used by hwaccels).
>> + if (!(bit_length & 7) && pps->data_size < sizeof(pps->data))
>> + pps->data[pps->data_size++] = 0x80;
>> +
>> pps->sps_id = get_ue_golomb_31(gb);
>> if ((unsigned)pps->sps_id >= MAX_SPS_COUNT ||
>> !ps->sps_list[pps->sps_id]) {
> 
> Modifying these functions affects more than just VideoToolbox. Are you
> sure that the other users of this file are ok with this change?
> 
> - Andreas

It passes FATE, and only adds a byte containing the stop bit (which would be present prior to this commit in 7 of 8 possible cases), so I believe it should be fine (and conceptually including the stop bit makes more sense IMO; being inconsistent about it like this is wacky).

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