[FFmpeg-devel] [PATCH 1/3] avcodec/libwebpenc_animencoder: Don't return pkt without data/side-data

[Andreas Rheinhardt]

James Almer:
> On 4/10/2021 9:39 PM, Andreas Rheinhardt wrote:
>> They are essentially forbidden in our encoding API as they are
>> considered empty. So just set the data, but leave the size at zero.
> 
> This doesn't seem like a good solution. You're propagating dummy packets
> when the encoder didn't produce any. It's an ugly hack to workaround a
> misbehaving muxer.
> As this is a AV_CODEC_CAP_DELAY encoder, you're supposed to set
> got_packet to 0 in this scenario.
> 
> The muxer should then not expect this encoder to return more than one
> packet, which is all it ever truly produces, to assume it effectively
> came from it.
> 

Then feel free to fix this yourself. All I know is that the muxer
explicitly checks for the number of frames to be > 1. Not that I know
why. (And I also presume the "Note: without the following, avio only
writes 8 bytes to the file" to be outdated.)

>>
>> (The old encoding API allowed such packets: It used buffer_pkt_valid
>> to record whether the packet is empty or not. This has been changed
>> in 827d6fe73d2f5472c1c2128eb14fab6a4db29032 which broke said encoder.
>> Said regression has been reported in #9179 which this commit partially
>> fixes.)
>>
>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>
>> ---
>> Â  libavcodec/libwebpenc_animencoder.c | 3 +++
>> Â  1 file changed, 3 insertions(+)
>>
>> diff --git a/libavcodec/libwebpenc_animencoder.c
>> b/libavcodec/libwebpenc_animencoder.c
>> index 7f35a0b939..d871e85d43 100644
>> --- a/libavcodec/libwebpenc_animencoder.c
>> +++ b/libavcodec/libwebpenc_animencoder.c
>> @@ -34,6 +34,7 @@ typedef struct LibWebPAnimContext {
>> Â Â Â Â Â  WebPAnimEncoder *enc;Â Â Â Â  // the main AnimEncoder object
>> Â Â Â Â Â  int64_t prev_frame_pts;Â Â  // pts of the previously encoded frame.
>> Â Â Â Â Â  int done;Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â  // If true, we have assembled the
>> bitstream already
>> +Â Â Â  uint8_t padding_buf[AV_INPUT_BUFFER_PADDING_SIZE];
>> Â  } LibWebPAnimContext;
>> Â  Â  static av_cold int libwebp_anim_encode_init(AVCodecContext *avctx)
>> @@ -103,6 +104,8 @@ static int
>> libwebp_anim_encode_frame(AVCodecContext *avctx, AVPacket *pkt,
>> Â Â Â Â Â Â Â Â Â  }
>> Â  Â Â Â Â Â Â Â Â Â  pkt->pts = pkt->dts = frame->pts;
>> +Â Â Â Â Â Â Â  // Packets without data and side-data are not supported by
>> the API
>> +Â Â Â Â Â Â Â  pkt->data = s->padding_buf;
>> Â Â Â Â Â Â Â Â Â  s->prev_frame_pts = frame->pts;Â  // Save for next frame.
>> Â Â Â Â Â Â Â Â Â  ret = 0;
>> Â Â Â Â Â Â Â Â Â  *got_packet = 1;
>>
> 
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