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• Can I know which byte range to read from a remote mp4 file for FFMpeg to decode a keyframe ?

  12 octobre 2023, par db9117

  I need to decode a of keyframe of a video file (mp4, h264 encoded). I know the timestamp of the keyframe I want to extract/decode. I want to minimize amount of data being read in memory. For this, I need to know beforehand exactly the minimal byte range I would require that encompasses this keyframe. How do I know what is the minimal byte range in the whole mp4 byte stream I need to read in order to be able to decode the keyframe ?

  


  I currently find the appropriate keyframe in the index_entries contained in the header. I get its byte position (pos attribute) and timestamp (timestamp attribute). I calculate the range as follows :

  


  startBytes : minimum of :

  


  

  1. the pos of the keyframe

  


  2. the pos of the nearest index entry in the audio stream happening at or before the keyframe's timestamp.

  


  


  This way when it's decoding the frame, if it also needs the audio content for demuxing, it would have it.

  


  endBytes : maximum of :

  


  

  1. the pos of the next frame in the video stream's index, after the keyframe

  


  2. the pos of the next frame in the audio stream's index after the timestamp of the wished keyframe.

  


  


  This way I know that I have everything up until the next frame in the index, which theoretically should be enough to decode the keyframe only.

  


  I then read the appropriate byte range.

  


  When I try to decode the frame, I run in a loop until I succeed :

  


  

  • avcodec_read_frame

  


  • avcodec_send_packet

  


  • avcodec_receive_frame

  


  


  I ignore AVERROR(EAGAIN) errors.

  


  avcodec_receive_frame fails multiple times with error AVERROR(EAGAIN) which I ignore, until it fails saying that the memory it wants to read isn't available (wants to read after endBytes). I explicitly tell it to fail if it wants to read more than it has already read.

  


  Note : for other keyframes at other positions in other videos, it sometimes succeeds (probably because the range is big enough by chance), but it fails more often than not.

  


  My question is : Why is the end of the range not enough to be able to decode only the one keyframe ? Is there any way to more precisely calculate the exact range in bytes I would need in order to decode a particular keyframe ?

  


• I ues FFMPEG to convert bitmap to yuv,and try to save it,but failed

  20 avril 2013, par liaoyuandeyehuo

  I use this way to convert bitmap to yuv
  Save bitmap to video (libavcodec ffmpeg)
  and then save the buffer like this :

  FILE* fp=fopen("aha.yuv","w");
  
  if(!fp) return NULL;
  
  fwrite(outPic->data[0],nbytes,1,fp);
  
  fwrite(outBuffer,nbytes,1,fp);
  
  fclose(fp);

  but it has only several lines. All most all of the .yuv file is gray.
  I am sure the inbuffer points to the input BGR32 data. So where is wrong ?
  Is The way to save the outBuffer right ?

• How to view the commands while making FFmpeg ?

  28 décembre 2011, par Saptarshi Biswas

  When I make ffmpeg, I can see following lines

  CC  ffmpeg.o
  
  LD  ffmpeg_g
  
  CP  ffmpeg
  
  STRIP   ffmpeg

  What is a quick way to find the commands for CC, LD etc. with as less modification to the Makefile ?