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• How do I use ffmpeg to merge all audio streams (in a video file) into one audio channel ?

  16 décembre 2019, par wkimbrough

  I am attempting to use ffmpeg for a number of files.
  The actual number of audio streams (there is usually one channel per stream) per file isn’t known until I’m using ffmpeg.
  The desired outcome is to somehow have ffmpeg get the count of audio channel, use the number in the command line to amerge those into one single audio channel.
  The goal is to create a preview version of the original video file for use in a simple HTML5 page.
  Is this possible in just one call to ffmpeg ?
  (Also, apologies as some parts of this problem I’m still learning about)

  Edit :
  Dumas stackoverflow asker here.
  Yes, I’ve been trying multiple combinations of ffmpeg args.
  To answer the other question, we have video files that have multiple streams, usually with single channels.
  I’ll post some cmdline examples shortly.

  This cmdline example kind of does what I want ; there are 8 streams, and I’m able to combine all audio into one. THe issue is having to know the number before running ffmpeg :

  ffmpeg -i EXAMPLE.MOV -filter_complex "[0:v]scale=-2:720,format=yuv420p[v];[0:a]amerge=inputs=8[a]" -map "[v]" -map "[a]" -c:v libx264 -crf 23 -preset medium -c:a libmp3lame -ar 44100 -ac 2 OUTPUT.mov

• FFmpeg using avcodec_decode_audio4 to decode an audio frame failed

  14 octobre 2014, par Jeff Chen

  well,I’m learning FFmpeg APIs recently.And I want to get a *.MP4 file’s audio frames but failed. My coding is :

  int pktSize = packet->size;  
  
  while (pktSize > 0) {  
  
      avcodec_get_frame_defaults(self.av_Context->pAudioFrame);  
  
      int gotframe = 0;   
  
      int len = avcodec_decode_audio4(self.av_Context->pAudioCodecCtx, self.av_Context->pAudioFrame, &gotframe, packet);  
  
      if (len < 0) {  
  
          char buffer[1024];  
  
          av_strerror(len, buffer, 1024);  
  
          NSLog(@"decode audio error,error:%d(%s)",len,buffer);  
  
          break;  
  
      }  
  
      if (gotframe) {  
  
          NSLog(@"got a audio frame");  
  
          KxAudioFrame *frame = [self handleAudioFrame:self.av_Context->pAudioFrame];  
  
          if (frame) {  
  
              [self.audioFrames addObject:frame];  
  
              NSLog(@"self.audioFrames.count = %d++",self.audioFrames.count);  
  
          }  
  
      }  
  
      if (len == 0) {  
  
          break;  
  
      }  
  
      pktSize -= len;
  
  }    

  the len always return -22,anyone knows why ?please tell me the answer,thanks very much !

• Does Buffer Source change the pixel format to `pix_fmt` or does it only read the input as if it was that format ?

  5 décembre 2022, par Wynell

  I am learning libav and related things.
  
I came across this example from the ffmpeg documentation :

  


       /* buffer video source: the decoded frames from the decoder will be inserted here. */
     snprintf(args, sizeof(args),
             "video_size=%dx%d:pix_fmt=%d:time_base=%d/%d:pixel_aspect=%d/%d",
             dec_ctx->width, dec_ctx->height, dec_ctx->pix_fmt,
             time_base.num, time_base.den,
             dec_ctx->sample_aspect_ratio.num, dec_ctx->sample_aspect_ratio.den);
  
     ret = avfilter_graph_create_filter(&buffersrc_ctx, buffersrc, "in",
                                        args, NULL, filter_graph);


  


  If I specify a pix_fmt other than dec_ctx->pix_fmt (for example, rgb24), will this buffer source convert the pixel format to rgb24, or will it instead read the source as if it was in rgb24 format, thereby causing incorrect decoding ?

  


  And what happens if I set a different video_size ?