Seeking backwards is very slow with VP9 or AV1

[magiblot]

Hi,

To reproduce this issue you can use any high resolution video from YouTube. For example:

yt-dlp -f 308 https://www.youtube.com/watch?v=LXb3EKWsInQ (webm, VP9, 1440p)

Then load it into AviSynth and play it backwards:

LwLibavVideoSource("test.webm")
Reverse

For me, it runs painfully slow.

Apparently, the filter decodes every frame since the last keyframe (k) to the current frame (i). But it caches none of these intermediate frames, so when asking for the previous frame (i - 1) it decodes all frames from k to i - 1 again. And on top of that, it doesn't seem to use more than 2 threads, so it doesn't take advantage of the hardware concurrency.

It would be great if something could be done about this.

Cheers.

[AkarinVS]

Regarding the 2 cpu limit, it seems ffmpeg's libavcodec vp9 decoder could only use at most 2 cpus, even if playing forward. Caching previous decoded frames when seeking could be done, but it will only help your specific case of playing backwards, and it will actually be detrimental to the most common use case, i.e. playing forwards after the seek. In fact, I believe this is better addressed at the AVS/VS level. It could detect that you're going to request frames from a source filter backwards and reorder the requests and cache them accordingly.