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• how av_freep in ffmpeg works ?

  11 août 2016, par Billy Bob

  I have some questions regarding void pointer in C. I found out the following code, part of ffmpeg that I couldn’t understand... Could someone explain to me how it works ?

  void av_freep(void *arg)
  
  {
  
          void *val;
  
  
  
          memcpy(&val, arg, sizeof(val));
  
          memcpy(arg, &(void *){ NULL }, sizeof(val));
  
          av_free(val);
  
  }

  and later it’s called like this :

  char *str;
  
  av_freep(&str);

  my questions :

  1. how passing address of str (&str) doesn’t trigger "incompatible type" warning when it’s compiled ? Shouldn’t &str type is become char** ?
  2. what’s the meaning of &(void *) NULL in one of memcpy parameter ?
  3. why it’s trying to free "val" which is not allocated ?

  Thanks !

• FFMPEG API real-time encoding, skipped frames

  24 octobre 2015, par Paul Knopf

  I am using FFMPEG to encode live video. If I can’t keep up with the live encoding, I need to drop frames. Currently, I just skip the encoding of a raw frame, and the PTS for the next frame is the same. Meaning, even if there is a skipped frame, the pts is always 1,2,3,4,5,6....

  This causes the playback of the file to skip ahead in time. Instead, I would like to add blank frames to the muxed file. I tried making PTS indicate a skipped frame by setting it to 1,2,4,5,8,...., but the FFMPEG h264 encoders depend on PTS being consecutive.

  How do I pass empty data to my mp4/container, indicating a blank space in time, so that decoders won’t appear to jump ahead in time, but instead just see a delay/freeze.

• FFMPEG : sws_scale does not produce any data in target frame

  7 juin 2019, par Vladimir Glushkov

  I use FFMPEG to decode H264 stream. After I get decoded YUV420 frames I want to convert them into RGB24.

  struct SwsContext * ctx = NULL;
  
  
  
  // frame is AVFrame in YUV420 obtained from decoder. It has all three strides and seem to be valid.
  
  
  
  if (ctx == NULL)
  
                  {
  
                      ctx = sws_getContext(frame->width, frame->height, frame->format, frame->width, frame->height,
  
                          AV_PIX_FMT_RGB24, SWS_BICUBIC, 0, 0, 0);
  
                  }
  
  
  
              AVFrame* frame2 = av_frame_alloc();
  
  
  
              int num_bytes = av_image_get_buffer_size(AV_PIX_FMT_RGB24, frame->width, frame->height, 32);
  
  
  
              uint8_t* frame2_buffer = (uint8_t *)av_malloc(num_bytes * sizeof(uint8_t));
  
  
  
              int size = av_image_fill_arrays(frame2->data, frame2->linesize, frame2_buffer, AV_PIX_FMT_RGB24, frame->width, frame->height, 32);
  
  
  
              int height_of_output = sws_scale(ctx, frame->data, frame->linesize, 0, frame->height, frame2->data, frame2->linesize);
  
  
  
              callbackFullRGB(state, frameIndex, 0, frame2->data[0], num_bytes, (__int32)frame2->format, (__int32)frame2->width, (__int32)frame2->height);
  
  
  
              av_frame_free(&frame2);

  However frame2 has no resolution set, pixel format is -1 and data buffer is empty. I have 1280x720 input, stride length is set to 3840 for output frame which is correct. sws_scale also returns 720 as a result - no errors, no exceptions.

  What might be wrong ?