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• pcx : read sample aspect ratio

  11 octobre 2012, par Paul B Mahol

  pcx : read sample aspect ratio

• Maintaining exact aspect ratio when scaling videos using ffmpeg

  31 mai 2012, par Skkard

  I have a mkv video, which is a mix of multiple resolution recordings, e.g. I have the first few seconds of widescreen 16:9 (1024x576) resolution, and the rest of the video if 4:3 (768x576) resolution. I want to scale this video down 3 times, while copying all the other attributes (audio codec, subtitles etc.). I use ffmpeg -i  -vf scale=iw/2:-1 -acodec copy . Also, VLC detects it's resolution as 720x576.

  The problem is that after the scaling, the resolution constantly becomes 4:3 (360x288). How can I maintain the dynamic aspect ratio of the input video file i.e. the 16:9 parts to scale to 16:9, while the 4:3 parts scale to 4:3 ?

  update

  The player size actually changes, atleast in mplayer, when the resolution is switched. I figured out the main problem. It seems each frame is tagged with a Sample Aspect Ratio (SAR), so when the player plays it, it can find the display aspect ratio. This SAR value isn't getting copied over when encoding to MKV. When encoding to MPG, it does get copied over and I get an exact copy, with the player switching sizes, but not with MKV.

  Output of ffprobe -show_streams filename :

  ffprobe version 0.10.3 Copyright (c) 2007-2012 the FFmpeg developers
  
    built on May  9 2012 17:51:07 with gcc 4.7.0 20120505 (prerelease)
  
    configuration: --prefix=/usr --enable-libmp3lame --enable-libvorbis --enable-libxvid --enable-libx264 --enable-libvpx --enable-libtheora --enable-libgsm --enable-libspeex --enable-postproc --enable-shared --enable-x11grab --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libschroedinger --enable-libopenjpeg --enable-librtmp --enable-libpulse --enable-gpl --enable-version3 --enable-runtime-cpudetect --disable-debug --disable-static
  
    libavutil      51. 35.100 / 51. 35.100
  
    libavcodec     53. 61.100 / 53. 61.100
  
    libavformat    53. 32.100 / 53. 32.100
  
    libavdevice    53.  4.100 / 53.  4.100
  
    libavfilter     2. 61.100 /  2. 61.100
  
    libswscale      2.  1.100 /  2.  1.100
  
    libswresample   0.  6.100 /  0.  6.100
  
    libpostproc    52.  0.100 / 52.  0.100
  
  Input #0, matroska,webm, from 'sample.mkv':
  
    Metadata:
  
      title           : Pan prstenu. Dve veze
  
    Duration: 00:00:29.80, start: 0.000000, bitrate: 3124 kb/s
  
      Stream #0:0(eng): Video: mpeg2video (Main), yuv420p, 720x576 [SAR 64:45 DAR 16:9], 15000 kb/s, 25 fps, 25 tbr, 1k tbn, 50 tbc (default)
  
      Stream #0:1(cze): Audio: mp2, 48000 Hz, stereo, s16, 128 kb/s (default)
  
  [STREAM]
  
  index=0
  
  codec_name=mpeg2video
  
  codec_long_name=MPEG-2 video
  
  codec_type=video
  
  codec_time_base=1/50
  
  codec_tag_string=[0][0][0][0]
  
  codec_tag=0x0000
  
  width=720
  
  height=576
  
  has_b_frames=1
  
  sample_aspect_ratio=64:45
  
  display_aspect_ratio=16:9
  
  pix_fmt=yuv420p
  
  level=8
  
  timecode=16:35:19:10
  
  id=N/A
  
  r_frame_rate=25/1
  
  avg_frame_rate=25/1
  
  time_base=1/1000
  
  start_time=0.000000
  
  duration=N/A
  
  nb_frames=N/A
  
  TAG:language=eng
  
  [/STREAM]
  
  [STREAM]
  
  index=1
  
  codec_name=mp2
  
  codec_long_name=MP2 (MPEG audio layer 2)
  
  codec_type=audio
  
  codec_time_base=1/48000
  
  codec_tag_string=[0][0][0][0]
  
  codec_tag=0x0000
  
  sample_fmt=s16
  
  sample_rate=48000
  
  channels=2
  
  bits_per_sample=0
  
  id=N/A
  
  r_frame_rate=0/0
  
  avg_frame_rate=125/3
  
  time_base=1/1000
  
  start_time=0.000000
  
  duration=N/A
  
  nb_frames=N/A
  
  TAG:language=cze
  
  [/STREAM]

• understand what is the GOP size (i-frame/p-frame ratio) in a video

  29 avril 2012, par nkint

  i am doing stuffs with a video in action script and i need to seek a video to acertain frame. according to the reference, using NetStream.seek i can seek only to the nearest k-frame :

  Seeks the keyframe (also called an I-frame in the video industry) closest to the specified location. The keyframe is placed at an offset, in seconds, from the beginning of the stream.
  
  
  
  Video streams are usually encoded with two types of frames, keyframes (or I-frames) and P-frames. A keyframe contains an entire image, while a P-frame is an interim frame that provides additional video information between keyframes. A video stream typically has a keyframe every 10-50 frames.

  and so i need to understand how many p-frame and i-frame there are in a video encoded in .f4v format.

  i am on mac and on ubuntu, i don't care about the tool to use (mayebe some ffmpeg/mencoder call should be perfect), any advice ?