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• Why does FFMPEG adds up extra bit rate to video ? [migrated]

  5 juin 2012, par user735647

  I use FFMPEG (command line Input) to convert my videos to a specific output format. The problem I am facing is when I try to pass a constant bit rate(700 kbps) to FFMPEG, the result is an output video with a different bit rate(say 1000 kbps). This phenomenon occurs invariably for all videos.Why is this happening ? I need to maintain a constant bit rate. Can anyone help me out.

  My FFMPEG version is 0.5

  The command line parameter which I am passing to FFMPEG is,

  -i {inputfile}
  
  -b 700k -ab 64k
  
  -vcodec libx264
  
  -acodec libfaac -ac 2 -ar 44100
  
  -y -s 320x240 
  
  {outputfile}

  EDIT :

  I was able to force CBR with a fluctuation of 3% when I used the following parameters.

   ffmpeg -i myfile.avi
  
   -b 4000k -minrate 4000k 
  
   -maxrate 4000k -bufsize 1835k   out.m2v

  But when I used -maxrate and - minrate along with my parameter set I was not able to force CBR. My parameter set is as follows,

  -i {inputfile}
  
  -b 1200k -minrate 1200k 
  
   -maxrate 1200k -bufsize 1200k 
  
  -ab 64k -vcodec libx264
  
  -acodec libfaac -ac 2 -ar 44100
  
  -y -s 320x240 
  
  {outputfile}

  Why is this happening ?

• How to set time_base when muxing AVI with libavformat ?

  18 mai 2012, par Anastasia

  In my application I receive from a remote server synchronized video
  (mpeg4) and audio (mp3) and mux them to avi file. The video comes
  frame-by-frame, and the audio comes in small chunks, say 200-250 ms.
  Both video frames and audio chunks have timestamps in milliseconds.

  My question is how to set audio time_base and pts correctly ?
  For video I set time_base.num = framerate, time_base.den = 1 ; and
  calculate pts as follows :

    AVRational time_base_1kHz; 
  
    time_base_1kHz.num = 1; 
  
    time_base_1kHz.den = 1000; 
  
    packet.pts = av_rescale_q(timeStamp - baseTimeStamp_, time_base_1kHz, videoStream_->time_base); 

  where baseTimeStamp_ is the 1st timestamp of the stream.

  But if I try do similar calculation for audio, I don't get playable
  avi. If I always set to audio AV_NOPTS_VALUE, then avi has playable
  video, but no audio.

  So what are the correct values for audio time_base and pts's ?
  Do I set time_base and pts's for video correctly ?

• Not able to encode image with ffmpeg

  29 juin 2012, par user1310596

  I am trying to encode one image with the help of ffmpeg library.. Actually i want to encode live video but for now i am starting with encoding an image..
  Here is my code for that..

      av_register_all();
  
      avcodec_init();
  
      avcodec_register_all();
  
      avformat_alloc_context();
  
  
  
      AVCodec *codec;
  
      AVCodecContext *ctx= NULL;
  
      int out_size, size, outbuf_size;
  
      AVFrame *picture;
  
      uint8_t *outbuf;
  
      unsigned char *flvdata = malloc(sizeof(unsigned char) * 30);
  
  
  
  
  
      outbuf_size = 100000;
  
      outbuf = malloc(outbuf_size);
  
  
  
  
  
      printf("Video encoding\n");
  
  
  
      codec = avcodec_find_encoder(CODEC_ID_FLV1);
  
      if (!codec) {
  
              fprintf(stderr, "codec not found\n");
  
              exit(1);
  
      }
  
  
  
      ctx= avcodec_alloc_context();
  
      picture= avcodec_alloc_frame();
  
  
  
  
  
      ctx->width = 320;
  
      ctx->height = 240;
  
      ctx -> sample_rate = 11025;
  
      ctx -> time_base.den = 1000;
  
      ctx -> time_base.num = 23976;
  
      ctx -> codec_id = CODEC_ID_FLV1;
  
      ctx -> codec_type = CODEC_TYPE_VIDEO;
  
      ctx->pix_fmt = PIX_FMT_YUV420P;
  
  
  
      if (avcodec_open(ctx, codec) < 0) {
  
              fprintf(stderr, "could not open codec\n");
  
              exit(1);
  
      }
  
  
  
      outbuf_size = 100000;
  
      outbuf = malloc(outbuf_size);
  
      size = ctx->width * ctx->height;
  
  
  
      AVFrame* outpic = avcodec_alloc_frame();
  
      int nbytes = avpicture_get_size(PIX_FMT_YUV420P, ctx->width, ctx->height);
  
  
  
      uint8_t* outbuffer = (uint8_t*)av_malloc(nbytes);
  
  
  
      fflush(stdout);
  
  
  
      int numBytes = avpicture_get_size(PIX_FMT_YUV420P, ctx->width, ctx->height);
  
  
  
      UIImage *image = [UIImage imageNamed:[NSString stringWithFormat:@"0.jpg"]];
  
      CGImageRef newCgImage = [image CGImage];
  
  
  
      CGDataProviderRef dataProvider = CGImageGetDataProvider(newCgImage);
  
      CFDataRef bitmapData = CGDataProviderCopyData(dataProvider);
  
      long dataLength = CFDataGetLength(bitmapData);
  
  
  
      uint8_t *buffer = (uint8_t *)av_malloc(dataLength);
  
      buffer = (uint8_t *)CFDataGetBytePtr(bitmapData);
  
  
  
      for(int i = 0; i < dataLength; i++)
  
      {
  
              if((i + 1) % 16 == 1 && i != 1)
  
                      printf("\n");
  
              printf("%X\t",buffer[i]); // getting something different than the     actual hex value of the image
  
      }
  
  
  
  
  
  
  
      outpic -> pts = 0;        
  
  
  
      avpicture_fill((AVPicture*)picture, buffer, PIX_FMT_RGB8, ctx->width, ctx->height);
  
  
  
      avpicture_fill((AVPicture*)outpic, outbuffer, PIX_FMT_YUV420P, ctx->width, ctx->height);
  
  
  
      struct SwsContext* fooContext = sws_getContext(ctx->width, ctx->height, 
  
                                                             PIX_FMT_RGB8, 
  
                                                             ctx->width, ctx->height, 
  
                                                             PIX_FMT_YUV420P, 
  
                                                             SWS_FAST_BILINEAR, NULL, NULL, NULL);
  
  
  
      sws_scale(fooContext, picture->data, picture->linesize, 0, ctx->height, outpic->data, outpic->linesize);
  
  
  
      printf("abcdefghijklmnop");
  
      out_size = avcodec_encode_video(ctx, outbuf, outbuf_size, outpic);
  
      printf("\n\n out_size %d   outbuf_size %d",out_size,outbuf_size);

  so the size of image is 29 kb and the avcodec_encode_video is returning 20143.. So is it correct ? I mean i am encoding so its size should also decrease... And i have opened that image in hex mode and i am getting different data than that is in buffer (as shown in the code). so i think that buffer is not getting correct data. Right ?
  Can anyone please help me with my code ?
  Thank you in advance...