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• Green screen writing FLV file libavformat

  5 mai 2013, par YYZ

  I've written a piece of C++ code that can capture webcam video frame, decode them, convert them to YUV420P, encode them and then write them to a file. If I use the mpeg2 codec and write to a .mpg file, everything works perfectly. But, if I use flv, then the output produced is just a green screen. I'm not sure if there are different encoder settings I need to set for encoding flv video ? Here's my code(the relevant parts) :

  Encoder settings :

  c->codec_id = codec_id;
  
  c->bit_rate = 400000;
  
  // Resolution must be a multiple of two.
  
  c->width    = 352;
  
  c->height   = 288;
  
  c->time_base.den = STREAM_FRAME_RATE;
  
  c->time_base.num = 1;
  
  //emit one intra frame every twelve frames at most
  
  c->gop_size      = 12;
  
  c->pix_fmt       = STREAM_PIX_FMT;

  Write the frames

  int ret;
  
  uint8_t *buffer = NULL;
  
  static struct SwsContext *sws_ctx;
  
  
  
  //Setup the codec context, and set its width and height to be equal to the input video width and height
  
  AVCodecContext *c = st->codec;
  
  c->width = pCodecCtx->width;
  
  c->height = pCodecCtx->height;
  
  
  
  av_init_packet(&packet);
  
  frameYUV = avcodec_alloc_frame();
  
  
  
  //Determine how big the buffer will need to be to store the captured frame
  
  numBytes = avpicture_get_size(STREAM_PIX_FMT,pCodecCtx->width,pCodecCtx->height);
  
  
  
  //Allocate the needed buffer size
  
  buffer = new uint8_t[numBytes];
  
  sws_ctx = sws_getContext(pCodecCtx->width,pCodecCtx->height,pCodecCtx->pix_fmt,
  
                          pCodecCtx->width,pCodecCtx->height,
  
                          STREAM_PIX_FMT,SWS_BICUBIC,NULL,NULL,NULL);
  
  
  
  //Fill the output frame
  
  avpicture_fill((AVPicture *)frameYUV,buffer,STREAM_PIX_FMT,pCodecCtx->width,pCodecCtx->height);
  
  
  
  //Read a video frame in
  
  av_read_frame(pFormatCtx,&packet);
  
  
  
  //Decode the contents of packet into pFrame
  
  avcodec_decode_video2(pCodecCtx,pFrame,&frameFinished,&packet);
  
  
  
  //Scale pFrame into frameYUV, and convert to PIXFMTYUV420P
  
  sws_scale
  
  (
  
      sws_ctx,
  
      (uint8_t const * const *)pFrame->data,
  
      pFrame->linesize,
  
      0,
  
      pCodecCtx->height,
  
      frameYUV->data,
  
      frameYUV->linesize
  
  );
  
  av_init_packet(&samsPacket);
  
  //Encode frameYUV
  
  avcodec_encode_video2(c, &samsPacket, frameYUV, &gotSamsPacket);
  
  
  
  AVPacket pkt = { 0 };
  
  int got_packet;
  
  av_init_packet(&pkt);
  
  // encode the image
  
  ret = avcodec_encode_video2(c, &pkt, frame, &got_packet);
  
  if (ret < 0){
  
      debugLogStreamSave->debug("Error encoding video frame");
  
      exit(1);
  
  }
  
  if (!ret && got_packet && pkt.size){
  
      pkt.stream_index = st->index;
  
  
  
      // Write the compressed frame to our output
  
      ret = av_interleaved_write_frame(oc, &pkt);

  Any help would be appreciated !

• C Code Using FFmpeg libraries - Compilation Error

  17 août 2013, par Ardatr

  I've been recently trying to compile C code that uses the FFmpeg libraries ; however, the compilation fails due to a reason that I cannot determine.

  The code that I've been trying to compile is the filtering_audio.c file on the Doxygen Documentation website for FFmpeg (I'll provide the link, since the code is too long to quote here) : http://ffmpeg.org/doxygen/trunk/doc_2examples_2filtering_audio_8c-example.html

  I use gcc to compile the code :

      gcc filter.c -lavformat -lavcodec -lavfilter  -lavutil

  And I get the following undefined reference errors :

      /tmp/cc90K2S5.o: In function `init_filters':
  
      filter.c:(.text+0x3e5): undefined reference to `av_int_list_length_for_size'
  
      filter.c:(.text+0x407): undefined reference to `av_int_list_length_for_size'
  
      filter.c:(.text+0x42d): undefined reference to `av_opt_set_bin'
  
      filter.c:(.text+0x482): undefined reference to `av_int_list_length_for_size'
  
      filter.c:(.text+0x4a4): undefined reference to `av_int_list_length_for_size'
  
      filter.c:(.text+0x4ca): undefined reference to `av_opt_set_bin'
  
      filter.c:(.text+0x51f): undefined reference to `av_int_list_length_for_size'
  
      filter.c:(.text+0x541): undefined reference to `av_int_list_length_for_size'
  
      filter.c:(.text+0x567): undefined reference to `av_opt_set_bin'
  
      /tmp/cc90K2S5.o: In function `print_frame':
  
      filter.c:(.text+0x76b): undefined reference to `av_frame_get_channel_layout'
  
      /tmp/cc90K2S5.o: In function `main':
  
      filter.c:(.text+0x831): undefined reference to `av_frame_alloc'
  
      filter.c:(.text+0x83d): undefined reference to `av_frame_alloc'
  
      filter.c:(.text+0x9de): undefined reference to `av_buffersrc_add_frame_flags'
  
      filter.c:(.text+0xa16): undefined reference to `av_buffersink_get_frame'
  
      filter.c:(.text+0xa58): undefined reference to `av_frame_unref'
  
      filter.c:(.text+0xab6): undefined reference to `av_frame_free'
  
      filter.c:(.text+0xac5): undefined reference to `av_frame_free'
  
      collect2: error: ld returned 1 exit status

  I understand that the undefined reference error indicates that it wasn't able to find the functions referenced from filtering_audio.c, but this doesn't make sense, since these functions should exist in the FFmpeg libraries.

  Any help is appreciated, thank you !

• Detect the type of an AVFrame instance

  23 octobre 2017, par stefk0

  Given a pointer pFrame to a structure of type AVFrame, is there a proper way to determine the type of the structure, i.e. either audio or video ?

  One way might be to check whether pFrame->nb_samples == 0 or not, but this doesn’t look right.