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• How to capture camera devices on Windows using Libav ?

  22 septembre 2011, par Occulta

  Is there any way to capture frames from as many camera types as DirectShow do on Windows platform using Libav ? I need to capture a camera output without using DirectShow filters and I want my application to work with many camera devices types.

  I have searched the Internet about this capability of libav and found that it can be done via libav using special input format "vfwcap". Something like that (don't sure about code correctness - I wrote it by myself) :

  AVFormatParameters formatParams = NULL;
  
  AVInputFormat* pInfmt = NULL;
  
  pInFormatCtx*  pInFormatCtx = NULL;
  
  
  
  av_register_all();
  
  
  
  //formatParams.device = NULL; //this was probably deprecated and then removed
  
  formatParams.channel = 0;
  
  formatParams.standard = "ntsc"; //deprecated too but still available
  
  formatParams.width = 640;
  
  formatParams.height = 480;
  
  formatParams.time_base.num = 1000;
  
  formatParams.time_base.den = 30000; //so we want 30000/1000 = 30 frames per second
  
  formatParams.prealloced_context = 0;
  
  
  
  
  
  pInfmt = av_find_input_format("vfwcap");
  
  if( !pInfmt )
  
  {
  
    fprintf(stderr,"Unknown input format\n");
  
    return -1;
  
  }
  
  
  
  // Open video file (formatParams can be NULL for autodetecting probably)
  
  if (av_open_input_file(&pInFormatCtx, 0, pInfmt, 0, formatParams) < 0)
  
     return -1; // Couldn't open device
  
  
  
  /* Same as video4linux code*/

  So another question is : how many devices are supported by Libav ? All I have found about capture cameras output with libav on windows is advice to use DirectShow for this purpose because libav supports too few devices. Maybe situation has already changed now and it does support enough devices to use it in production applications ?

  If this isn't possible.. Well I hope my question won't be useless and this composed from different sources piece of code will help someone interested in this theme 'coz there are really too few information about it in the whole internet.

• During transcoding, does output quality of a video improve when i give output bitrate more than input video's bitrate ?

  19 septembre 2013, par Jobin Jose

  I use ffmpeg for converting videos.
  As i understand, the bitrate of a video stream is the number of bits which constitute the video over 1 second of time.
  What happens when i specify the output video bitrate to be more than the input video's bitrate ?
  For example :
  If bitrate of "Input.mp4" is 2000KBps and i want to convert it to "Output.mp4" with output bitrate set to 3000KBps.
  How will the converter create the extra 1000 bits(3000-2000) for every second of video ?

• How to concat mp4 files using libffmpeg in c program ?

  1er août 2013, par chichien

  I know ffmpeg command line is easy, but how to programmatically implement？ I'm not good at this，here is some code from internet, it is used to convert .mp4 to .ts，and i made some changes，but the audio stream problem persists：

  #include 
  
  #include 
  
  #include 
  
  #include 
  
  
  
  #include "libavformat/avformat.h"
  
  #include "libavcodec/avcodec.h"
  
  #include "libavutil/avutil.h"
  
  #include "libavutil/rational.h"
  
  #include "libavdevice/avdevice.h"
  
  #include "libavutil/mathematics.h"
  
  #include "libswscale/swscale.h"
  
  
  
  static AVStream* add_output_stream(AVFormatContext* output_format_context, AVStream* input_stream)
  
  {
  
  
  
      AVCodecContext* input_codec_context = NULL;
  
      AVCodecContext* output_codec_context = NULL;
  
  
  
      AVStream* output_stream = NULL;
  
      output_stream = av_new_stream(output_format_context, 0);
  
      if (!output_stream)
  
      {
  
          printf("Call av_new_stream function failed\n");
  
          return NULL;
  
      }
  
  
  
      input_codec_context = input_stream->codec;
  
      output_codec_context = output_stream->codec;
  
  
  
      output_codec_context->codec_id = input_codec_context->codec_id;
  
      output_codec_context->codec_type = input_codec_context->codec_type;
  
      output_codec_context->codec_tag = input_codec_context->codec_tag;
  
      output_codec_context->bit_rate = input_codec_context->bit_rate;
  
      output_codec_context->extradata = input_codec_context->extradata;
  
      output_codec_context->extradata_size = input_codec_context->extradata_size;
  
  
  
      if (av_q2d(input_codec_context->time_base) * input_codec_context->ticks_per_frame > av_q2d(input_stream->time_base) && av_q2d(input_stream->time_base) < 1.0 / 1000)
  
      {
  
          output_codec_context->time_base = input_codec_context->time_base;
  
          output_codec_context->time_base.num *= input_codec_context->ticks_per_frame;
  
      }
  
      else
  
      {
  
          output_codec_context->time_base = input_stream->time_base;
  
      }
  
      switch (input_codec_context->codec_type)
  
      {
  
      case AVMEDIA_TYPE_AUDIO:
  
          output_codec_context->channel_layout = input_codec_context->channel_layout;
  
          output_codec_context->sample_rate = input_codec_context->sample_rate;
  
          output_codec_context->channels = input_codec_context->channels;
  
          output_codec_context->frame_size = input_codec_context->frame_size;
  
          if ((input_codec_context->block_align == 1 && input_codec_context->codec_id == CODEC_ID_MP3) || input_codec_context->codec_id == CODEC_ID_AC3)
  
          {
  
              output_codec_context->block_align = 0;
  
          }
  
          else
  
          {
  
              output_codec_context->block_align = input_codec_context->block_align;
  
          }
  
          break;
  
      case AVMEDIA_TYPE_VIDEO:
  
          output_codec_context->pix_fmt = input_codec_context->pix_fmt;
  
          output_codec_context->width = input_codec_context->width;
  
          output_codec_context->height = input_codec_context->height;
  
          output_codec_context->has_b_frames = input_codec_context->has_b_frames;
  
          if (output_format_context->oformat->flags & AVFMT_GLOBALHEADER)
  
          {
  
              output_codec_context->flags |= CODEC_FLAG_GLOBAL_HEADER;
  
          }
  
          break;
  
      default:
  
          break;
  
      }
  
  
  
      return output_stream;
  
  }
  
  
  
  //[[** from ffmpeg.c
  
  static void write_frame(AVFormatContext *s, AVPacket *pkt, AVCodecContext *avctx, AVBitStreamFilterContext *bsfc){
  
      int ret;
  
  
  
      while(bsfc){
  
          AVPacket new_pkt= *pkt;
  
          int a= av_bitstream_filter_filter(bsfc, avctx, NULL,
  
                                            &new_pkt.data, &new_pkt.size,
  
                                            pkt->data, pkt->size,
  
                                            pkt->flags & AV_PKT_FLAG_KEY);
  
          if(a>0){
  
              av_free_packet(pkt);
  
              new_pkt.destruct= av_destruct_packet;
  
          } else if(a<0){
  
              fprintf(stderr, "%s failed for stream %d, codec %s\n",
  
                      bsfc->filter->name, pkt->stream_index,
  
                      avctx->codec ? avctx->codec->name : "copy");
  
              //print_error("", a);
  
              //if (exit_on_error)
  
              //    ffmpeg_exit(1);
  
          }
  
          *pkt= new_pkt;
  
  
  
          bsfc= bsfc->next;
  
      }
  
  
  
      ret= av_interleaved_write_frame(s, pkt);
  
      if(ret < 0){
  
          //print_error("av_interleaved_write_frame()", ret);
  
          fprintf(stderr, "av_interleaved_write_frame(%d)\n", ret);
  
          exit(1);
  
      }
  
  }
  
  //]]**
  
  
  
  int main(int argc, char* argv[])
  
  {
  
  
  
      const char* input;
  
      const char* output;
  
      const char* output_prefix = NULL;
  
      char* segment_duration_check = 0;
  
      const char* index = NULL;
  
      char* tmp_index = NULL;
  
      const char* http_prefix = NULL;
  
      long max_tsfiles = NULL;
  
      double prev_segment_time = 0;
  
      double segment_duration = 0;
  
  
  
      AVInputFormat* ifmt = NULL;
  
      AVOutputFormat* ofmt = NULL;
  
      AVFormatContext* ic = NULL;
  
      AVFormatContext* oc = NULL;
  
      AVStream* video_st = NULL;
  
      AVStream* audio_st = NULL;
  
      AVCodec* codec = NULL;
  
      AVDictionary* pAVDictionary = NULL;
  
  
  
      long frame_count = 0;
  
  
  
      if (argc != 3) {
  
          fprintf(stderr, "Usage: %s inputfile outputfile\n", argv[0]);
  
          exit(1);
  
      }
  
  
  
      input = argv[1];
  
      output = argv[2];
  
  
  
      av_register_all();
  
  
  
      char szError[256] = {0};
  
      int nRet = avformat_open_input(&ic, input, ifmt, &pAVDictionary);
  
      if (nRet != 0)
  
      {
  
          av_strerror(nRet, szError, 256);
  
          printf(szError);
  
          printf("\n");
  
          printf("Call avformat_open_input function failed!\n");
  
          return 0;
  
      }
  
  
  
      if (av_find_stream_info(ic) < 0)
  
      {
  
          printf("Call av_find_stream_info function failed!\n");
  
          return 0;
  
      }
  
  
  
      ofmt = av_guess_format("mpegts", NULL, NULL);
  
      if (!ofmt)
  
      {
  
          printf("Call av_guess_format function failed!\n");
  
          return 0;
  
      }
  
  
  
      oc = avformat_alloc_context();
  
      if (!oc)
  
      {
  
          printf("Call av_guess_format function failed!\n");
  
          return 0;
  
      }
  
      oc->oformat = ofmt;
  
  
  
      int video_index = -1, audio_index = -1;
  
      for (unsigned int i = 0; i < ic->nb_streams && (video_index < 0 || audio_index < 0); i++)
  
      {
  
          switch (ic->streams[i]->codec->codec_type)
  
          {
  
          case AVMEDIA_TYPE_VIDEO:
  
              video_index = i;
  
              ic->streams[i]->discard = AVDISCARD_NONE;
  
              video_st = add_output_stream(oc, ic->streams[i]);
  
              break;
  
          case AVMEDIA_TYPE_AUDIO:
  
              audio_index = i;
  
              ic->streams[i]->discard = AVDISCARD_NONE;
  
              audio_st = add_output_stream(oc, ic->streams[i]);
  
              break;
  
          default:
  
              ic->streams[i]->discard = AVDISCARD_ALL;
  
              break;
  
          }
  
      }
  
      codec = avcodec_find_decoder(video_st->codec->codec_id);
  
      if (codec == NULL)
  
      {
  
          printf("Call avcodec_find_decoder function failed!\n");
  
          return 0;
  
      }
  
  
  
      if (avcodec_open(video_st->codec, codec) < 0)
  
      {
  
          printf("Call avcodec_open function failed !\n");
  
          return 0;
  
      }
  
  
  
      if (avio_open(&oc->pb, output, AVIO_FLAG_WRITE) < 0)
  
      {
  
          return 0;
  
      }
  
  
  
      if (avformat_write_header(oc, &pAVDictionary))
  
      {
  
          printf("Call avformat_write_header function failed.\n");
  
          return 0;
  
      }
  
  
  
      //[[++
  
      AVBitStreamFilterContext *bsfc = av_bitstream_filter_init("h264_mp4toannexb");
  
      //AVBitStreamFilterContext *absfc = av_bitstream_filter_init("aac_adtstoasc");
  
      if (!bsfc) {
  
          fprintf(stderr, "bsf init error!\n");
  
          return -1;
  
      }
  
      //]]++
  
  
  
      int decode_done = 0;
  
      do
  
      {
  
          double segment_time = 0;
  
          AVPacket packet;
  
  
  
          decode_done = av_read_frame(ic, &packet);
  
          if (decode_done < 0)
  
              break;
  
  
  
          if (av_dup_packet(&packet) < 0)
  
          {
  
              printf("Call av_dup_packet function failed\n");
  
              av_free_packet(&packet);
  
              break;
  
          }
  
  
  
          //[[**
  
          if (packet.stream_index == audio_index) {
  
              segment_time = (double)audio_st->pts.val * audio_st->time_base.num / audio_st->time_base.den;
  
              nRet = av_interleaved_write_frame(oc, &packet);   
  
          } else if (packet.stream_index == video_index) {
  
              if (packet.flags & AV_PKT_FLAG_KEY) {
  
                  segment_time = (double)video_st->pts.val * video_st->time_base.num / video_st->time_base.den;
  
              } else {
  
                  segment_time = prev_segment_time;
  
              }
  
              //nRet = av_interleaved_write_frame(oc, &packet);  
  
              write_frame(oc, &packet, video_st->codec, bsfc);
  
          }
  
          //]]**
  
  
  
          if (nRet < 0)
  
          {
  
              printf("Call av_interleaved_write_frame function failed: %d\n", nRet);
  
          }
  
          else if (nRet > 0)
  
          {
  
              printf("End of stream requested\n");
  
              av_free_packet(&packet);
  
              break;
  
          }
  
          av_free_packet(&packet);
  
          frame_count++;
  
      }while(!decode_done);
  
  
  
      av_write_trailer(oc);
  
      printf("frame_count = %d\n", frame_count);
  
  
  
      av_bitstream_filter_close(bsfc);  
  
      avcodec_close(video_st->codec);
  
      for(unsigned int k = 0; k < oc->nb_streams; k++)
  
      {
  
          av_freep(&oc->streams[k]->codec);
  
          av_freep(&oc->streams[k]);
  
      }
  
      av_free(oc);
  
      //getchar();
  
      return 0;
  
  }

  Compile this code, to got an executable file named muxts, and then :

  $ ./muxts vid1.mp4 vid1.ts

  No error message printed，but the audio stream was unsynchronized and noise。Check the .ts file using ffmpeg :

  $ ffmpeg -i vid1.ts
  
  ffmpeg version 0.8.14-tessus, Copyright (c) 2000-2013 the FFmpeg developers
  
    built on Jul 29 2013 17:05:18 with llvm_gcc 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.1.00)
  
    configuration: --prefix=/usr/local --arch=x86_64 --as=yasm --extra-version=tessus --enable-gpl --enable-nonfree --enable-version3 --disable-ffplay --enable-libvorbis --enable-libmp3lame --enable-libx264 --enable-libxvid --enable-bzlib --enable-zlib --enable-postproc --enable-filters --enable-runtime-cpudetect --enable-debug=3 --disable-optimizations
  
    libavutil    51.  9. 1 / 51.  9. 1
  
    libavcodec   53.  8. 0 / 53.  8. 0
  
    libavformat  53.  5. 0 / 53.  5. 0
  
    libavdevice  53.  1. 1 / 53.  1. 1
  
    libavfilter   2. 23. 0 /  2. 23. 0
  
    libswscale    2.  0. 0 /  2.  0. 0
  
    libpostproc  51.  2. 0 / 51.  2. 0
  
  
  
  Seems stream 0 codec frame rate differs from container frame rate: 180000.00 (180000/1) -> 90000.00 (180000/2)
  
  Input #0, mpegts, from 'vid1.ts':
  
    Duration: 00:00:03.75, start: 0.000000, bitrate: 3656 kb/s
  
    Program 1
  
      Metadata:
  
        service_name    : Service01
  
        service_provider: FFmpeg
  
      Stream #0.0[0x100]: Video: h264 (Baseline), yuv420p, 640x480, 90k tbr, 90k tbn, 180k tbc
  
      Stream #0.1[0x101]: Audio: aac, 48000 Hz, mono, s16, 190 kb/s
  
  At least one output file must be specified

  What should i do ?

  If this issue fixed , how can i concat multi .ts files into single .mp4 file ?