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• Can I know which byte range to read from a remote mp4 file for FFMpeg to decode a keyframe ?

  12 octobre 2023, par db9117

  I need to decode a of keyframe of a video file (mp4, h264 encoded). I know the timestamp of the keyframe I want to extract/decode. I want to minimize amount of data being read in memory. For this, I need to know beforehand exactly the minimal byte range I would require that encompasses this keyframe. How do I know what is the minimal byte range in the whole mp4 byte stream I need to read in order to be able to decode the keyframe ?

  


  I currently find the appropriate keyframe in the index_entries contained in the header. I get its byte position (pos attribute) and timestamp (timestamp attribute). I calculate the range as follows :

  


  startBytes : minimum of :

  


  

  1. the pos of the keyframe

  


  2. the pos of the nearest index entry in the audio stream happening at or before the keyframe's timestamp.

  


  


  This way when it's decoding the frame, if it also needs the audio content for demuxing, it would have it.

  


  endBytes : maximum of :

  


  

  1. the pos of the next frame in the video stream's index, after the keyframe

  


  2. the pos of the next frame in the audio stream's index after the timestamp of the wished keyframe.

  


  


  This way I know that I have everything up until the next frame in the index, which theoretically should be enough to decode the keyframe only.

  


  I then read the appropriate byte range.

  


  When I try to decode the frame, I run in a loop until I succeed :

  


  

  • avcodec_read_frame

  


  • avcodec_send_packet

  


  • avcodec_receive_frame

  


  


  I ignore AVERROR(EAGAIN) errors.

  


  avcodec_receive_frame fails multiple times with error AVERROR(EAGAIN) which I ignore, until it fails saying that the memory it wants to read isn't available (wants to read after endBytes). I explicitly tell it to fail if it wants to read more than it has already read.

  


  Note : for other keyframes at other positions in other videos, it sometimes succeeds (probably because the range is big enough by chance), but it fails more often than not.

  


  My question is : Why is the end of the range not enough to be able to decode only the one keyframe ? Is there any way to more precisely calculate the exact range in bytes I would need in order to decode a particular keyframe ?

  


• generating a video mosaic in client-side js

  1er novembre 2020, par redbrain

  I'm writing a web app for the creation of mosaic videos, where multiple videos can tiled next to each other, like a virtual meeting or grid of surveillance cameras. I'd like the user to be able to download their creation as an video file.

  


  I've currently been accomplishing this using a Node.js server to serve the rest of the site (which is static html/js) and then having one route execute ffmpeg and return the result. However, if this can be accomplished on the client side, the app would be easier to scale since I'd only have to serve static files, and nicer to write with only the client-side in mind.

  


  Does anyone know of a proper or clean way to do this ? I've looked at the following methods so far :

  


  

  • using ffmpeg ported to webassembly - this seems to be used constantly for any sort of client-side video editing ; however it seems overkill and clunky for this purpose

  


  • tiling all the videos in a <canvas></canvas> and rendering its output to an mp4 via the MediaStreamAPI - could be the best way, but it's not well documented

  


  


  If I'm missing something obvious, please let me know. I figured I would look further for the best method before writing further. Thanks in advance.

  


• Is it possible .FLV to .MP4 conversion on client side by ffmpeg-asm.js ?

  24 mars 2016, par Egor M.

  Is it possible to do .FLV to .MP4 conversion on client side by ffmpeg-asm.js ?
  The main issue lot’s of mobile video players didn’t support flash, but .FLV still popular at the internet ;(

  Have no idea how to avoid server-side conversion and storing...