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Yahweasel / libav.js

This is a compilation of the libraries associated with handling audio and video in ffmpeg—libavformat, libavcodec, libavfilter, libavutil, libswresample, and libswscale—for emscripten, and thus the web.

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Random access to packets

reinhrst opened this issue · comments

commented

I would like to use libav.js as demuxer to be used with WebCodecs to make a video player (thanks for all the amazing work here, making my life a lot easier :)).
The video files are drag-and-dropped into the webpage, and might be too large to fit in memory.

Making the system play a videofile from start to end is easy enough (using ReadAhead file, libav.ff_read_multi and https://github.com/Yahweasel/libavjs-webcodecs-bridge.

However it's unclear to me how I could support random access in this video file. I could get/scan all packets when the video file is received (this takes about 30 seconds for a 4GB file-- not ideal, but acceptable), I don't want to store the packets in memory.

It seems to me that I could either save the packets to e.g. Origin Private File System, in order to access them later, or I could for every random access start parsing the video from the start until the requested frame is found.

Am I missing something? Is there any way to get quickly to a particular packet?

commented

Part of the issue seems to be that I thought only the functions in https://github.com/Yahweasel/libav.js/blob/master/docs/API.md are available; by now I realise that these functions are just javascript combining a bunch of lower level functions, which do seem to allow more granular access.

I will try to come up with a PoC and post it when I have something.

commented

What you want is to implement a block reader device that performs whichever query is needed on read. See https://github.com/Yahweasel/libav.js/blob/master/docs/API.md#mkblockreaderdev . Then, yes, API.md does not discuss the underlying libav API (see the first paragraph of API.md :) ), and seeking is not part of the high-level API. You want libav's seeking functions.

Some references you may want to look at are the demo (demo/demo.html), which can seek, and libavjs-webcodecs-bridge ( https://github.com/Yahweasel/libavjs-webcodecs-bridge ), which bridges libav.js and WebCodecs.

commented

Thanks! I should be able to find my way from here; if I run into something, I'd be happy to make another issue.