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• Fast movie creation using MATLAB and ffmpeg

  24 février 2018, par hyiltiz

  I have some time series data that I would like to create into movies. The data could be 2D (about 500x10000) or 3D (500x500x10000). For 2D data, the movie frames are simply line plot using plot, and for 3D data, we can use surf, imagesc, contour etc. Then we create a video file using these frames in MATLAB, then compress the video file using ffmpeg.

  To do it fast, one would try not to render all the images to display, nor save the data to disk then read it back again during the process. Usually, one would use getframe or VideoWriter to create movie in MATLAB, but they seem to easily get tricky if one tries not to display the figures to screen. Some even suggest plotting in hidden figures, then saving them as images to disk as .png files, then compress them using ffmpeg (e.g. with x265 encoder into .mp4). However, saving the output of imagesc in my iMac took 3.5s the first time, then 0.5s after. I also find it not fast enough to save so many files to disk only to ask ffmpeg to read them again. One could hardcopy the data as this suggests, but I am not sure whether it works regardless of the plotting method (e.g. plot, surf etc.), and how one would transfer data over to ffmpeg with minimal disk access.

  This is similiar to this, but immovie is too slow. This post 3 is similar, but advocates writing images to disk then reading them (slow IO).

• ffmpeg make a movie with multiple numbers in filename [closed]

  22 juillet 2024, par NNN

  I have the following four files

  


  mov_00_00.png
mov_00_01.png
mov_01_00.png
mov_01_01.png


  


  I can make a movie using

  


  ffmpeg.exe -i mov_%02d_00.png -filter:v "setpts=10*PTS" out.mp4

  


  But this does not consider the files ending with 01.png.

  


  I tried

  


  ffmpeg.exe -i mov_%02d_%02d.png -filter:v "setpts=10*PTS" out.mp4


  


  But it says

  


  [in#0 @ 0000021982d69000] Error opening input: No such file or directory
Error opening input file mov_%02d_%02d.png.
Error opening input files: No such file or directory


  


  What command line magic do I need to use to make this work ? Thanks.

  


  Edit : I'm using the Windows version under WSL and the glob option is not supported.

  


• The duration of movie file recorded by AVAssetWriter is not correct

  1er mars 2016, par ideawu

  I am recording video files with AVAssetWriter combining with AVCaptureVideoDataOutputSampleBufferDelegate. So I can take control over every frame the camara gives in the method :

  - (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection{
  
      double time = CMTimeGetSeconds(CMSampleBufferGetPresentationTimeStamp(sampleBuffer));
  
      NSLog(@"%f", time);
  
      [_videoInput appendSampleBuffer:sampleBuffer];
  
  }

  So I am so sure how many samples have been written to AVAssetWriter via AVAssetWriterInput, and I know exactly the start time and end time of the samples being written. The duration is calculated by end_time - start_time. Say the duration calculated in the video capture programm is 0.5 second.

  I get the .mov/.mp4 file on disk, inspected with ffmpeg -i, it shows a very different duration.

  ffmpeg -i m003.mp4 2>&1 | grep Dura
  
    Duration: 00:00:01.37, start: 0.836667, bitrate: 95 kb/s

  The movie file show a duration of 1.37 seconds, which it’s quite different with the EXACT duration 0.5.

  Does any one knows the reason ?