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• ffmpeg decoding yuv420p to rgb shifts luminance/brightness down

  6 avril 2013, par glopes

  I'm recording video from a grayscale camera into an MPEG4 video file using FFMPEG. Recently I noticed a weird effect : if I playback the file using FFMPEG or windows media player, the output frames are noticeably darker (by about 10 brightness values) than the original source.

  I thought the encoding step was doing this until I opened the same file in VLC and it gave me back the correct result. I played around with FFMPEG command lines to decode a single frame and realized that if I decode a frame into GRAY8 pixel format, the brightness/luminance values are preserved. Here's the command and ffmpeg output :

  ffmpeg -ss 0.5 -i video.avi -vframes 1 -t 1 -s 1280x680 -pix_fmt gray gray.bmp
  
  
  
  ffmpeg version 1.2 Copyright (c) 2000-2013 the FFmpeg developers
  
    built on Apr  4 2013 12:40:58 with gcc 4.6.2 (GCC)
  
    configuration: --enable-w32threads
  
    libavutil      52. 18.100 / 52. 18.100
  
    libavcodec     54. 92.100 / 54. 92.100
  
    libavformat    54. 63.104 / 54. 63.104
  
    libavdevice    54.  3.103 / 54.  3.103
  
    libavfilter     3. 42.103 /  3. 42.103
  
    libswscale      2.  2.100 /  2.  2.100
  
    libswresample   0. 17.102 /  0. 17.102
  
  Input #0, avi, from 'video.avi':
  
    Metadata:
  
      encoder         : Lavf53.32.100
  
    Duration: 00:00:07.47, start: 0.000000, bitrate: 16532 kb/s
  
      Stream #0:0: Video: mpeg4 (Simple Profile) (FMP4 / 0x34504D46), yuv420p, 128
  
  0x720 [SAR 1:1 DAR 16:9], 30 tbr, 30 tbn, 30 tbc
  
  Output #0, image2, to 'gray.bmp':
  
    Metadata:
  
      encoder         : Lavf54.63.104
  
      Stream #0:0: Video: bmp, gray, 1280x680 [SAR 17:18 DAR 16:9], q=2-31, 200 kb
  
  /s, 90k tbn, 30 tbc
  
  Stream mapping:
  
    Stream #0:0 -> #0:0 (mpeg4 -> bmp)
  
  Press [q] to stop, [?] for help
  
  frame=    1 fps=0.0 q=0.0 Lsize=N/A time=00:00:00.03 bitrate=N/A
  
  video:851kB audio:0kB subtitle:0 global headers:0kB muxing overhead -100.002524%

  However, if I decode the same frame into an rgb color pixel format, I end up again with the darker frames :

  ffmpeg -ss 0.5 -i video.avi -vframes 1 -t 1 -s 1280x680 -pix_fmt bgr24 rgb.bmp
  
  
  
  ffmpeg version 1.2 Copyright (c) 2000-2013 the FFmpeg developers
  
    built on Apr  4 2013 12:40:58 with gcc 4.6.2 (GCC)
  
    configuration: --enable-w32threads
  
    libavutil      52. 18.100 / 52. 18.100
  
    libavcodec     54. 92.100 / 54. 92.100
  
    libavformat    54. 63.104 / 54. 63.104
  
    libavdevice    54.  3.103 / 54.  3.103
  
    libavfilter     3. 42.103 /  3. 42.103
  
    libswscale      2.  2.100 /  2.  2.100
  
    libswresample   0. 17.102 /  0. 17.102
  
  Input #0, avi, from 'video.avi':
  
    Metadata:
  
      encoder         : Lavf53.32.100
  
    Duration: 00:00:07.47, start: 0.000000, bitrate: 16532 kb/s
  
      Stream #0:0: Video: mpeg4 (Simple Profile) (FMP4 / 0x34504D46), yuv420p, 128
  
  0x720 [SAR 1:1 DAR 16:9], 30 tbr, 30 tbn, 30 tbc
  
  Output #0, image2, to 'rgb.bmp':
  
    Metadata:
  
      encoder         : Lavf54.63.104
  
      Stream #0:0: Video: bmp, bgr24, 1280x680 [SAR 17:18 DAR 16:9], q=2-31, 200 k
  
  b/s, 90k tbn, 30 tbc
  
  Stream mapping:
  
    Stream #0:0 -> #0:0 (mpeg4 -> bmp)
  
  Press [q] to stop, [?] for help
  
  frame=    1 fps=0.0 q=0.0 Lsize=N/A time=00:00:00.03 bitrate=N/A
  
  video:2550kB audio:0kB subtitle:0 global headers:0kB muxing overhead -100.000843%

  This happens with every single grayscale video that I encode to MPEG4 with FFMPEG. My best guess so far is it has to do with how the container pixel format gets converted to/from. Since I'm using MPEG4, the file pixel format is YUV420P. I have no idea how ffmpeg encodes from GRAY8 to YUV420P, but maybe it stores just the luminance values in Y... if this happens, then decoding from this to RGB could produce darker pixels by the scaling factor that is applied to the luminance matrix ?

  To sum it up :

  1) How can encoding a grayscale video to YUV420P with FFMPEG and decoding back produce wrong (darker) brightness values when decoding to RGB versus GRAY8 ? Presumably once the frames are in YUV420P format it shouldn't matter whether the source is actually grayscale or not so the result should be equivalent, no ?

  2) How does VLC avoid this situation ? I was under the impression that VLC used FFMPEG as well for video decoding, but somehow they managed to figure out how to produce the correct values without requiring me to indicate explicitly that the video was grayscale.

• FFMPEG+video+video.js dont show up the video

  18 avril 2013, par user1843970

  I have a video content type where I want to show the video to the user. I downloaded video module and video.js for this purpose. I have successfully installed ffmpeg transcoder in apache serv

  Now I go to admin/config/media/video. Withing players tab I select HTML5 player for mp4 video and video.js to run HTML5 video like below :-

  

  Now I go to Video Content Type and add Video field with Video Upload as its type and save it.The video is successfully converted but after clicking on play button nothing happens.

  

  Need your help in this.

• how to save h264 stream into .264 file

  6 mars 2015, par user3547405

  I’m using nginx-rtmp-module build hls server, when I play it on safari, it does not shows image,then i use this command ffmpeg -i rtmp://192.168.4.213/hls/testH264 -c copy -map 0 -f segment -segment_list playlist.m3u8 -segment_time 5 output%03d.ts, it shows missing picture in access unit with size 28184,so i think maybe the raw h264 stream has some problem,then i directly save the h264 stream into .264 file,but when i play the .264 file with vlc,the screen is fuzzy,help