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• lavfi/vf_libplacebo : determine PTS of next frame from any input

  15 juin 2023, par Niklas Haas

  lavfi/vf_libplacebo : determine PTS of next frame from any input
  When combining multiple inputs with different PTS and durations, in
  
  input-timed mode, we emit one output frame for every input frame PTS,
  
  from *any* input. So when combining a low FPS stream with a high FPS
  
  stream, the output framerate would match the higher FPS, independent of
  
  which order they are specified in.
  

  • [DH] libavfilter/vf_libplacebo.c

• Best logical formula to determine perceptual / "experienced" quality of a video, given resolution / fps and bitrate ?

  20 mars 2023, par JamesK

  I am looking for a formula that can provide me with a relatively decent approximation of a Video's playback quality that can be calculated based off of four metrics : width, height, fps, and bitrate (bits/sec). Alternatively, I can also use FFMPEG or similar tools to calculate a Video's playback quality, if any of those tools provide something like what I am looking for here.

  


  An example of what a Video might look like in my problem is as follows :

  


  interface Video {
  /** The width of the Video (in pixels). */
  width: number
  /** The height of the Video (in pixels). */
  height: number
  /** The frame rate of the Video (frames per second). */
  fps: number
  /** The bitrate of the video, in bits per second (e.g. 5_000_000 = 5Mbit/sec) */
  bitrate: number
}


  


  I came up with the following function to compute the average amount of bits available for any given pixel per second :

  


  const computeVideoQualityScalar = (video: Video): number => {
  // The amount of pixels pushed to the display, per frame.
  const pixelsPerFrame = video.width * video.height
  
  // The amount of pixels pushed to the display, per second.
  const pixelsPerSecond = pixelsPerFrame * video.fps
  
  // The average amount of bits used by each pixel, each second,
  // to convey all data relevant to that pixel (e.g. color data, etc)
  const bitsPerPixelPerSecond = video.bitrate / pixelsPerSecond
  
  return bitsPerPixelPerSecond
}


  


  While my formula does do a good job of providing a more-or-less "standardized" assessment of mathematical quality for any given video, it falls short when I try to use it to compare videos of different resolutions to one another. For example, a 1080p60fps video with a bitrate of 10Mbit/sec has a greater visual fidelity (at least, subjectively speaking, to my eyes) than a 720p30fps video with a bitrate of 9Mbit/sec, but my formula would score the 720p30fps video significantly higher than the 1080p60fps video because the 720p video has more bits available per pixel per second than the 1080p video.

  


  I am struggling to come up with ideas as to how to either come up with a different way to calculate the "subjective video quality" for a given video, or extend upon my existing idea here.

  


• How to determine if ffmpeg supports an extension for the output file ?

  9 février 2023, par Juribiyan

  When ffmpeg is given a command to output a file with a particular extension it presumably runs an internal algorithm do determine what encoder and/or muxer to use with it. Is there a command to find out whether a particular output extension is supported or not ? Like in imagemagick with a command magick identify -list format ?

  


  ffmpeg -h muxer=mp4 does not work with image formats like jpg and png even though ffmpeg is capable of outputting images of these formats.

  


  ffmpeg -formats does not seem reliable enough as well as instead of jpg it has singlejpg and does not include png (only apng and png_pipe).