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• FFMPEG Understanding AVFrame::linesize (Audio)

  4 décembre 2015, par user3584691

  As per the doucmentation of AVFrame, for audio, lineSize is size in bytes of each plane and only linesize[0] may be set. But however, am unsure whether lineszie[0] is holding per plane buffer size or is it the complete buffer size and we have to divide it by no of channels to get per plane buffer size.

  For Example, when I call
  int data_size = av_samples_get_buffer_size(NULL, iDesiredNoOfChannels, iAudioSamples, (AVSampleFormat)iDesiredFormat, 0) ; For iDesiredNoOfChannels = 2, iAudioSamples = 1024 & iDesiredFormat = AV_SAMPLE_FMT_FLTP data_size=8192. Pretty straightforward, as each sample is 4 bytes and since there are 2 channels total memory will be (1024 * 4 * 2) bytes. As such lineSize[0] should be 4096 for planar audio. data[0] & data[1] should be each of size 4096. However, pFrame->lineSize[0] is giving 8192. So to get the size per plane, I have to do pFrame->lineSize[0] / pFrame->channels. Isn’t this behaviour different from what the documentation suggests or is my understanding of the documentaion wrong.

• avutil : add P010 pixel format

  8 décembre 2015, par Hendrik Leppkes

  avutil : add P010 pixel format
  P010 is the 10-bit variant of NV12 (planar luma, packed chroma), using two
  
  bytes per component to store 10-bit data plus 6-bit zeroes in the LSBs.
  

  • [DH] libavutil/pixdesc.c
  • [DH] libavutil/pixfmt.h
  • [DH] libavutil/version.h

• Saving raw YUV420P frame FFmpeg/Libav

  14 mars 2016, par Sir DrinksCoffeeALot

  Previously i successfully opened, decoded and saved frames from .mp4 file. Raw frames were in YUV420P format which i converted to RGB24 using sws_scale() function and saved them into a .ppm file. What i’m trying to do now is to keep raw decoded frames in YUV420P, or convert frames that i get to YUV420P just to make sure they are in YUV420P. But the problem is that i don’t know which type of file should i use, i’m guessing .yuv ? And another problem is that i also don’t know how to save YUV420P data.

  I used to save RGB24 in .ppmlike this :

  for (y = 0; y < height; y++)
  
  {
  
  fwrite(frame->data[0] + y*frame->linesize[0], 1, width * 3, pf);
  
  }

  which worked fine since i was using only 1 plane. But YUV420P uses 3 planes (Y,Cb,Cr). What i tried to do is to save Y component first, and Cb/Cr after that.

  for (y = 0; y < height; y++)
  
  {
  
      fwrite(frame->data[0] + y*frame->linesize[0], 1, width, pf);
  
  }
  
  
  
  for (y = 0; y < height / 2; y++)
  
  {
  
      fwrite(frame->data[1] + y*frame->linesize[1], 1, width, pf);
  
      fwrite(frame->data[2] + y*frame->linesize[2], 1, width, pf);
  
  }

  But as you can guess that is not working at all. Actually it does, but it saves only Y component. Could anyone guide me to right direction please ?

  EDIT
  I’ve changed mine saveFrame() function like you said, but instead of interleaved chroma writing i used planar. But, i’m still getting only Y component picture (black-white).

  fprintf(pf, "P5\n%d %d\n255\n", width, height);
  
  
  
  for (y = 0; y < height; y++)
  
  {
  
      fwrite(frame->data[0] + y*frame->linesize[0], 1, width, pf);
  
  }
  
  
  
  for (y = 0; y < height / 2; y++)
  
  {
  
      fwrite(frame->data[1] + y*frame->linesize[1], 1, width / 2, pf);
  
  }
  
  
  
  for (y = 0; y < height / 2; y++)
  
  {
  
      fwrite(frame->data[2] + y*frame->linesize[2], 1, width / 2, pf);
  
  }