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• creation of video file with alpha channel/transparency

  3 décembre 2015, par boerfi

  I’m experimenting with a video file with alpha channel.
  Later on the file should be an overlay for another video. It will be merged with another file in real time and saved to another file. But that is not the important point. The problem starts earlier because I can’t even play it.

  The video is a png-encoded Quicktime-file which I cannot play with any video player. WMP, VLC, MPC and ffplay all show various problems which all lead to the problem that the images seem to get decoded too slowly.
  The strange thing is that neither cpu (i7) nor my ssd are on limit but the file isn’t played correctly.

  Since the problem appears with all players I think it’s based on using png in video. I googled but didn’t find a proper way to create a partly transparent video file.
  I tried various methods of exporting (with adobe media encoder) with no result : the file lost its transparency or is running too slow.

  The resulting file which I create with my SDK is correct (video and audio are synchronous and fluid) but it takes 1 minute to render a video of 40 seconds although it works in real time with files without transparency.

  Does anyone know what kind of file I should export that has a minimum resolution of 720p, that is partly transparent and that can be played with ffplay in real time ?
  I would also appreciate any experiences with partly transparent videos which could help me because I couldn’t find any helpful links ?

  Thanks,
  Marius

• ffmpeg does not recognize audio stream in video

  13 février 2021, par M Taher

  I have these flv files that exported from Adobe Connect and I want to extract their audio with ffmpeg. ffmpeg does not recognize the audio on some of them. This log is from one file that has issues :

  


  >ffmpeg.exe -i r7anupl75gl_0_1965937.flv test1.mp3
ffmpeg version N-76123-g002b049 Copyright (c) 2000-2015 the FFmpeg                                                                      
built with gcc 5.2.0 (GCC)
configuration: --enable-gpl --enable-version3 --disable-w32threads                    
libavutil      55.  4.100 / 55.  4.100
libavcodec     57.  7.100 / 57.  7.100
libavformat    57.  8.102 / 57.  8.102
libavdevice    57.  0.100 / 57.  0.100
libavfilter     6. 11.100 /  6. 11.100
libswscale      4.  0.100 /  4.  0.100
libswresample   2.  0.100 /  2.  0.100
libpostproc    54.  0.100 / 54.  0.100
[flv @ 0000000000522140] Could not find codec parameters for stream 1        

(Audio: none, 0 channels): unknown codec
Consider increasing the value for the 'analyzeduration' and 'probesize'   
options
Input #0, flv, from 'r7anupl75gl_0_1965937.flv':
Metadata:
lasttimestamp   : 5026
lastkeyframetimestamp: 5022
audiodelay      : 0
canSeekToEnd    : false
creationdate    : unknown
metadatacreator : Manitu Group FLV MetaData Injector 2
videosize       : 251586025
audiosize       : 55776880
datasize        : 516
Duration: 01:23:47.75, start: 0.000000, bitrate: 489 kb/s
Stream #0:0: Video: vp6f, yuv420p, 1184x704, 1k tbr, 1k tbn, 1k tbc
Stream #0:1: Audio: none, 0 channels
Output #0, mp3, to 'test1.mp3':
Output file #0 does not contain any stream


  


  Note : audio codec is Nellymoser Asao and this command works successfully for some files.

  


• HTML 5 currentTime accuracy

  1er septembre 2015, par Daniel

  I’m working on a project where we are using the value from the video element’s currentTime property to perform processing on the server backend using ffmpeg. I’ve come across an issue where the video element seems to report a time code that is slightly different from the time code ffmpeg needs to access the correct point in the video.

  So for instance in Firefox if the currentTime property reports that the current video time is 26.83 I might find that the frame I really want ended at 26.72 and so if I use the time to extract a frame using ffmpeg on the server I get the next frame instead of the current frame.

  The amount of offset seems to be slightly different in different parts of the video and in different videos. But the offset is usually close to one tenth of a second in Firefox. In chrome the currentTime actually seems to be ahead or the correct currentTime by about 5 hundredths of a second. It’s more difficult to figure out the offset in IE because the place where the frame shifts seems to change as I enter different time codes to look for the exact time code where the frame changes.

  I’m pretty sure the time as used by ffmpeg is the correct time. It seems to agree more closely with other video editing software such as adobe premier.

  Any ideas on what could be causing this behavior ?

  JS to get currentTime :

  AVideo.prototype.getCurrentTime = function()
  
  {
  
     return this.videoElement[0].currentTime;
  
  };

  Resulting ffmpeg command :

  ffmpeg -y -i '/tmp/myVideo.mov' -vframes 1 -ss 2.4871 -f image2   -y '/tmp/myFrame.jpg' 2>&1