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• How to encode a mp4 which has high quality,fast encode speed and low bitrate using FFmpeg API？

  8 septembre 2017, par OnlYet

  I have lots of ARGB binary data and need to encode them to a mp4 file using FFmpeg API.The most difficult things is to deal with the balance between quality,speed and bitrate.

  Below is two download links for mp4 files generated from the same ARGB data.

  • Generated through FFmpeg API : MP4 #1

  • Generated through FFmpeg command line : MP4 #2

  The mp4 generated through command line has high definition and low bitrate(745kbps) while the other has not enough high quality but high bitrate(1009kbps) I don’t know the parameters of the mp4 generated through command line but a part of my code is below :

   codec_ctx->codec_type = AVMEDIA_TYPE_VIDEO;
  
   codec_ctx->width = width;
  
   codec_ctx->height = height;
  
   codec_ctx->time_base.num = 1;
  
   codec_ctx->time_base.den = fps;
  
   stream->time_base = { 1, fps };
  
  if (!strcmp(out_fmt, "mp4"))
  
      {
  
          codec_ctx->codec_id = AV_CODEC_ID_H264;
  
          codec_ctx->bit_rate = 800 * 1000;
  
          codec_ctx->rc_max_rate = 800 * 1000;
  
          //codec_ctx->rc_min_rate = 200 * 1000;
  
          codec_ctx->rc_buffer_size = 500 * 1000;
  
          /* More gop_size less file size */
  
          codec_ctx->gop_size = 30;
  
          codec_ctx->max_b_frames = 3;
  
          /* Set relevant parameters of H264 */
  
          codec_ctx->qmin = 10;   //default 2
  
          codec_ctx->qmax = 31;   //default 31
  
          codec_ctx->max_qdiff = 4;
  
          codec_ctx->me_range = 16;   //default 0 
  
          codec_ctx->max_qdiff = 4;   //default 3 
  
          codec_ctx->qcompress = 0.6; //default 0.5
  
          ret = av_dict_set(dict, "profile", "high", 0);
  
          // Keep a balance between speed and quality when encoding
  
          ret = av_dict_set(dict, "preset", "superfast", 0);
  
          ret = av_dict_set(dict, "threads", "0", 0);
  
          ret = av_dict_set(dict, "crf", "26", 0);    
  
          // set delay time
  
          ret = av_dict_set(dict, "tune", "zerolatency", 0);
  
          return;
  
      }

  I know the parameters that affect video quality are mainly these :

  bitrate, "qp", "crf", gop_size, qmin and "preset".

  I tried to increase gop_size to decrease bitrate but mp4 quality became lower. I also found that the less crf, the more quality and the more bitrate. The less qmin,the more quality and the more bitrate. And preset a higher encode speed, the more bitrate.

  In fact, I want to set "preset" to ultrafast and set "crf" to 20 but the output mp4 will be larger.So why the mp4 generated through command line can be high quality but low bitrate ? How to set the parameters of AVCodecContext to generate a mp4 with high quality,low bitrate and high speed ?

  Supplementary notes ：
  The mp4 generated through command line stay low bitrate most of the time and higher bitrate sometimes but my mp4 is always stay relatively high bitrate. I think the point is to control the bitrate.

• Burn in delayed subtitles

  11 février 2017, par sygi

  I would like to burn subtitles into a video file using a console program. Previously I used ffmpeg to do that :

  ffmpeg -i input.mkv -vf subtitles=sub.srt -strict -2 out.mp4

  However, I am having problems when my subtitles are delayed with regard to the movie. I looked into -ioffset option and setpts filter, but they allow me move the video, but I’d like the video timing to stay unchanged and only burn in the subtitles in a delayed manner.

  Is there a way to do it in ffmpeg (I suppose no, as burning subtitles is a filter and I don’t see a way to "change" a behavior of filters, but I haven’t looked at all of them) ? If not, is there an easy-to-use console program which would delay the subtitles before burning them in ?

• How can I apply zoom in/out filter on specific frame in video using ffmpeg ?

  4 novembre 2019, par Axita Kathiriya

  I want to apply zoom filter on specific frame on video using ffmpeg.

  for example, I want to make a slideshow of 5 images and each image will take 4-5 seconds for slide. I want zoom in/out effect on each image. So, i want to apply this effect at that time when particular image stay for 4-5 seconds. Please see this video link. I want same result like this using ffmpeg : https://youtu.be/nNTiyRDhlRY please refer first two slides of this link.