How to do 2-pass VP9 encoding in PyAV?

[chaserhkj]

I wanted to do 2-pass VP9 encoding, as described by ffmpeg doc in PyAV.

However I discovered that we did have pass1, pass2 flags to enable the mode, but I couldn't find a way to assign the twopass_stat buffer for the underlying libavencode call

Is this something we don't support yet? If yes I'd love to work on a PR to support it, but I don't see an obvious existing way for PyAV to access the fields of codec-specific contexts like VPxEncoderContext so I am not sure if this is even doable.

[chaserhkj]

Ok just did a few dig and find out the libavcodec's standard way of doing 2-pass is not twopass_stats buffer as I have referred to above but stats_in and stats_out on the generic AVCodecContext struct.

And we have #791 #277 that I missed before since I was searching for "two pass" keyword while they are using the term "multi-pass"

I think this is still relevant feature since as quoted in ffmpeg VP9 doc:

Two-pass is the recommended encoding method for libvpx-vp9 as some quality-enhancing encoder features are only available in 2-pass mode.

I will try to proceed and see if #791 works, then re-submit the PR on behalf of original author @inrustwetrust with any needed edits since their account seems to be dormant .