[FFmpeg-devel] [PATCH 4/4] avcodec/cbs_h265: add missing support for reserved_payload_extension_data SEI bits

[James Almer]

On 4/22/2020 9:17 PM, Andreas Rheinhardt wrote:
> James Almer:
>> Fixes ticket #8622
>>
>> Signed-off-by: James Almer <jamrial at gmail.com>
>> ---
>> It fixes it assuming the Picture Timing in that sample is not being misparsed
>> by cbs_h265.
>> We're compliant with the latest revision of the spec, so any
>> reserved_payload_extension_data found in a sample created today is almost
>> guaranteed to be bogus. But the spec states that they should be skipped if
>> found, for forward compatibility reasons.
>> Would be worth checking if the nvenc encoder is at fault, writing faulty SEI
>> messages.
>>
>> This patch could for that matter make many potential cases of undiscovered
>> cbs_h265 SEI misparsing be handled as if the remaining bits were these reserved
>> bits.
> 
> That's unfortunately true. I don't see a way to avoid that.
> 
>>
>>  libavcodec/cbs_h265.h                 |  1 +
>>  libavcodec/cbs_h265_syntax_template.c | 44 +++++++++++++++++++++++++--
>>  2 files changed, 43 insertions(+), 2 deletions(-)
>>
>> diff --git a/libavcodec/cbs_h265.h b/libavcodec/cbs_h265.h
>> index 2c1e153ad9..73897f77a4 100644
>> --- a/libavcodec/cbs_h265.h
>> +++ b/libavcodec/cbs_h265.h
>> @@ -715,6 +715,7 @@ typedef struct H265RawSEIPayload {
>>              AVBufferRef *data_ref;
>>          } other;
>>      } payload;
>> +    H265RawExtensionData extension_data;
>>  } H265RawSEIPayload;
>>  
>>  typedef struct H265RawSEI {
>> diff --git a/libavcodec/cbs_h265_syntax_template.c b/libavcodec/cbs_h265_syntax_template.c
>> index f978e16549..ef3254d27f 100644
>> --- a/libavcodec/cbs_h265_syntax_template.c
>> +++ b/libavcodec/cbs_h265_syntax_template.c
>> @@ -2054,11 +2054,43 @@ static int FUNC(sei_alpha_channel_info)(CodedBitstreamContext *ctx,
>>      return 0;
>>  }
>>  
>> +static int FUNC(payload_extension)(CodedBitstreamContext *ctx, RWContext *rw,
>> +                                   H265RawExtensionData *current, uint32_t payload_size,
>> +                                   int cur_pos)
>> +{
>> +    int err, i;
>> +
>> +#ifdef READ
>> +    if (cbs_h265_payload_extension_present(rw, payload_size, cur_pos)) {
>> +        GetBitContext tmp = *rw;
>> +        int payload_zero_bits, bits_left = 8 * payload_size - cur_pos;
>> +        if (bits_left > 8)
>> +            skip_bits_long(&tmp, bits_left - 8);
>> +        payload_zero_bits = ff_ctz(get_bits(&tmp, FFMIN(bits_left, 8)));
>> +        if (payload_zero_bits >= bits_left - 1)
>> +            return AVERROR_INVALIDDATA;
>> +        current->bit_length = bits_left - payload_zero_bits - 1;
>> +        allocate(current->data, (current->bit_length + 7) / 8);
>> +        for (i = 0; i < current->bit_length; i++) {
>> +            uint8_t bit;
>> +            xu(1, reserved_payload_extension_data, bit, 0, 1, 0);
>> +            current->data[i / 8] |= bit << (7 - i % 8);
>> +        }
>> +    }
>> +#else
>> +    for (i = 0; i < current->bit_length; i++)
>> +        xu(1, reserved_payload_extension_data,
>> +           current->data[i / 8] >> (7 - i % 8) & 1, 0, 1, 0);
> 
> Do we really need to write one bit at a time? Why not simply one byte at
> a time and then output the rest in one go? The specs treat it as a
> bitfield and not as individual bits, so I think we are free to group it
> as we please.

Mmh, ok.

> 
> The same goes for the other extension_data() function, of course. And
> for reading.

Doing it for the other function will affect the existing trace output,
so I'm inclined to leave it as is.

> 
> And shouldn't i be better size_t?

The correct type would be ptrdiff_t, but we never bothered with it and
always used int.

> 
>> +#endif
>> +
>> +    return 0;
>> +}
>> +
>>  static int FUNC(sei_payload)(CodedBitstreamContext *ctx, RWContext *rw,
>>                               H265RawSEIPayload *current, int prefix)
>>  {
>>      int err, i;
>> -    int start_position, end_position;
>> +    int start_position, current_position, end_position;
>>  
>>  #ifdef READ
>>      start_position = get_bits_count(rw);
>> @@ -2122,7 +2154,15 @@ static int FUNC(sei_payload)(CodedBitstreamContext *ctx, RWContext *rw,
>>          }
>>      }
>>  
>> -    if (byte_alignment(rw)) {
>> +#ifdef READ
>> +    current_position = get_bits_count(rw) - start_position;
>> +    if (current_position != 8 * current->payload_size) {
>> +#else
>> +    current_position = put_bits_count(rw) - start_position;
>> +    if (byte_alignment(rw) || current->extension_data.bit_length) {
>> +#endif
>> +        CHECK(FUNC(payload_extension)(ctx, rw, &current->extension_data,
>> +                                      current->payload_size, current_position));
>>          fixed(1, bit_equal_to_one, 1);
>>          while (byte_alignment(rw))
>>              fixed(1, bit_equal_to_zero, 0);
>>
> 
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