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• How do I pre-allocate the memory for libavcodec to write decoded frame data ?

  18 décembre 2018, par codemonkey

  I am trying to decode a video with libav by following the demo code : here

  I need to be able to control where the frame data in pFrame->data[0] is stored. I have tried setting pFrame->data to my own buffer as follows :

  // Determine required buffer size and allocate buffer 
  
  int numBytes = av_image_get_buffer_size(pixFmt, width, height, 1); 
  
  (uint8_t*) dataBuff = (uint8_t*) malloc (numBytes * sizeof(uint8_t)); 
  
  
  
  // Assign buffer to image planes in pFrame
  
  av_image_fill_arrays(frame->data, frame->linesize, dataBuff, pixFmt, width,
  
  height, 1);

  While this does set pFrame->data to be dataBuff (if I print their addresses, they are the same), this call ret = avcodec_receive_frame(pCodecContext, pFrame) to receive the decoded data always writes the data to a different address. It seems to manage its own memory somewhere in the underlying API and ignores the dataBuff that I assigned to pFrame right before.

  So I’m stuck—how can I tell libav to write decoded frame data to memory that I pre-allocate ? I’ve seen people ask similar questions online and in the libav forum but haven’t been able to find an answer.

  Many thanks 

• vc2enc : prevent random data

  5 mai 2016, par Christophe Gisquet

  vc2enc : prevent random data
  The slice prefix is 0 in the reference encoder and the decoder ignores it.
  
  Writing 0 there seems like the best temporary solution.
  The padding could have contained uninitialized data, but reference VC2
  
  encoders put 0xFF there, hence the memset value.
  Overall this allows producing bistreams with no random data for use by fate.
  

  • [DH] libavcodec/vc2enc.c

• avformat/oggdec : Don't skip over data whose checksum is used

  11 juillet, par Andreas Rheinhardt

  avformat/oggdec : Don't skip over data whose checksum is used
  The behavior of the ffio_*_checksum feature is not well defined
  
  when using avio_skip(). The code in oggdec.c relied on the skipped
  
  data (four bytes) to be checksummed, which is mostly true
  
  because short_seek_threshold is 32768 by default, so that
  
  avio_seek() will normally read data instead of calling the
  
  underlying seek function. Yet this has two problems :
  a) It relies on implementation details of avio_seek().
  
  b) There is an exception, namely if the AVIO_FLAG_DIRECT is set.
  
  In this case the underlying seek function (if set) is always
  
  called and the data is skipped, leading to CRC errors.
  So don't skip the data.
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
  

  • [DH] libavformat/oggdec.c