• Compression Techniques

  Decoding then Encoding video: very low quality output: why ??

  Posted by Dhirendra Ambani on June 21, 2011 at 7:25 am

  I have coded to decode a video and then to encode it back with most of the parameters similar to original one( so that I could interfere with the frames so produced). The method is successful, but the so produced video is of very low quality and size 1/4th of the original.
  I am pasting my code here.

  #include “stdafx.h”
  #include
  #include
  #include
  #ifdef __cplusplus
  extern “C” {
  #endif
  #include “libavformat/avformat.h”
  #include “libavcodec/avcodec.h”
  #include “libswscale/swscale.h”

  #ifdef __cplusplus
  }
  #endif

  AVCodecContext *pCodecCtx_decode;
  AVCodec *pCodec2;
  int outbuf_size=100000,frame_count=0;
  uint8_t *outbuffer;

  static AVStream *add_video_stream(AVFormatContext *oc, enum CodecID codec_id)
  {
  AVCodecContext *c;
  AVStream *st;

  st = av_new_stream(oc, 0);

  c=st->codec;
  c->codec_id=pCodecCtx_decode->codec_id;
  c->codec_type=AVMEDIA_TYPE_VIDEO;
  c->bit_rate = pCodecCtx_decode->bit_rate;
  c->width =pCodecCtx_decode->width;
  c->height = pCodecCtx_decode->height;
  c->time_base.num=pCodecCtx_decode->time_base.num;
  c->time_base.den=pCodecCtx_decode->time_base.den;
  c->gop_size = pCodecCtx_decode->gop_size;
  c->pix_fmt = pCodecCtx_decode->pix_fmt;
  c->bit_rate_tolerance=40000000;//pCodecCtx_decode->bit_rate_tolerance;

  if (c->codec_id == CODEC_ID_MPEG2VIDEO) {
  c->max_b_frames = 2;
  }
  if (c->codec_id == CODEC_ID_MPEG1VIDEO){
  c->mb_decision=2;
  }

  return st;
  }

  static void open_video(AVFormatContext *oc, AVStream *st)
  {
  AVCodec *codec;
  AVCodecContext *c;

  c = st->codec;

  codec = avcodec_find_encoder(c->codec_id);
  if (!codec) {
  fprintf(stderr, “codec not found\n”);
  exit(1);
  }

  /* open the codec */
  if (avcodec_open(c, codec) < 0) {
  fprintf(stderr, “could not open codec\n”);
  exit(1);
  }

  outbuffer = NULL;
  if (!(oc->oformat->flags & AVFMT_RAWPICTURE)) {
  outbuf_size = 200000;
  outbuffer = (uint8_t*)av_malloc(outbuf_size);
  }
  }

  static void write_video_frame(AVFormatContext *oc, AVStream *st, AVFrame *pFrame,AVPacket * avpkt_dec)
  {
  int out_size, ret;
  AVCodecContext *c;

  c = st->codec;

  out_size = avcodec_encode_video(c, outbuffer, outbuf_size, pFrame);
  if (out_size > 0) {
  AVPacket pkt;
  av_init_packet(&pkt);

  if (c->coded_frame->pts != AV_NOPTS_VALUE)
  pkt.pts= av_rescale_q(c->coded_frame->pts, c->time_base, st->time_base);
  if(c->coded_frame->key_frame)
  pkt.flags |= AV_PKT_FLAG_KEY;

  pkt.pts=avpkt_dec->pts;pkt.dts = pkt.pts;
  pkt.stream_index= st->index;
  pkt.data= outbuffer;
  pkt.size= out_size;
  printf(“encoding frame %3d (size=%5d)\n”, frame_count, out_size);
  ret=av_interleaved_write_frame(oc,&pkt);
  if(!ret)
  ret= url_ferror(oc->pb);

  av_free_packet(&pkt);
  } else {
  ret = 0;
  }

  if (ret != 0) {
  fprintf(stderr, “Error while writing video frame\n”);
  exit(1);
  }
  }
  int myconverter(const char * infile,const char * outfile)
  {
  AVFormatContext *pFormatCtx;
  int i, videoStream;

  AVCodec *pCodec1;
  AVFrame *pFrame;
  AVPacket packet;
  int frameFinished;

  avcodec_init();
  av_register_all();
  if(av_open_input_file(&pFormatCtx,infile, NULL, 0, NULL)!=0)
  {fprintf(stderr,” Couldn’t open file %s “,infile);return -1; }

  if(av_find_stream_info(pFormatCtx)<0)
  {printf(“Couldn’t find stream information”);return -1;} //

  dump_format(pFormatCtx, 0, infile, 0);

  videoStream=-1;
  for(i=0; i<(pFormatCtx->nb_streams); i++)
  if(pFormatCtx->streams[i]->codec->codec_type==AVMEDIA_TYPE_VIDEO) {
  videoStream=i;
  break;
  }
  if(videoStream==-1)
  {printf(“Didn’t find a video stream”);return -1;}

  // Get a pointer to the codec context for the video stream
  pCodecCtx_decode=pFormatCtx->streams[videoStream]->codec;

  pCodec1=avcodec_find_decoder(pCodecCtx_decode->codec_id);
  if(pCodec1==NULL) {
  fprintf(stderr, “Unsupported codec!\n”);
  return -1;
  }
  if(avcodec_open(pCodecCtx_decode, pCodec1)<0)
  return -1;

  pFrame=avcodec_alloc_frame();

  AVOutputFormat *fmt;
  AVFormatContext *oc;
  int out_size;
  AVStream *video_st = NULL;

  oc=avformat_alloc_output_context(NULL,NULL,outfile);
  if (!oc) {
  printf(“Could not deduce output format from file extension: using MPEG.\n”);
  oc=avformat_alloc_output_context(“mpeg”, NULL, outfile);
  }
  if (!oc) {
  exit(1);
  }
  fmt=oc->oformat;

  if (fmt->video_codec != CODEC_ID_NONE) {
  video_st = add_video_stream(oc, fmt->video_codec);
  }

  av_dump_format(oc, 0, outfile, 1);

  if (video_st)
  open_video(oc, video_st);

  /* open the output file, if needed */
  if (!(fmt->flags & AVFMT_NOFILE)) {
  if (avio_open(&oc->pb, outfile, AVIO_FLAG_WRITE) < 0) {
  fprintf(stderr, “Could not open ‘%s’\n”, outfile);
  exit(1);
  }
  }

  av_write_header(oc);
  i=0;
  while(av_read_frame(pFormatCtx, &packet)>=0) {
  if(packet.stream_index==videoStream) {
  AVPacket avpkt_dec;
  av_init_packet(&avpkt_dec);
  avpkt_dec.data = packet.data;
  avpkt_dec.size = packet.size;
  avpkt_dec.flags = AV_PKT_FLAG_KEY;
  avcodec_decode_video2(pCodecCtx_decode, pFrame, &frameFinished,&avpkt_dec);

  if(pFrame->pict_type==AV_PICTURE_TYPE_I ||(frameFinished && ((frame_count++)%2==0))) {
  write_video_frame(oc, video_st, pFrame,&avpkt_dec);

  }
  }

  av_free_packet(&packet);
  }
  av_write_trailer(oc);

  /* free the streams */
  for(i = 0; i < oc->nb_streams; i++) {
  av_freep(&oc->streams[i]->codec);
  av_freep(&oc->streams[i]);
  }

  if(video_st){
  //avcodec_close(video_st->codec);
  av_free(outbuffer);
  }
  if (!(fmt->flags & AVFMT_NOFILE)) {
  avio_close(oc->pb);
  }
  av_free(oc);
  av_free(pFrame);
  avcodec_close(pCodecCtx_decode);
  av_close_input_file(pFormatCtx);
  return 0;
  }

  int main(int argc, char *argv[])
  {
  const char *infile,*outfile;

  infile = “D:\\Video\\crawling.avi”; //input file name
  outfile=”D:\\Video\\converted.avi”; // output file name
  myconverter(infile,outfile);
  return 0;
  }

  help of any kind is appreciated

  Dhirendra Ambani replied 15 years, 2 months ago 1 Member · 0 Replies
• 0 Replies

Reply to this Discussion! Login or Sign Up