Sorry, can't really answer that, but I'm curious about
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a. How do you define 'optimal bitrate'?
b. How do you get VP8 to archive an average bitrate that you specify? (last time I checked VP8&VP9 both weren't able to at least nearly hit the average bitrate I specified and thus specifying for example 1000kBit/s and ending up with a 200kBits or 3000kBit/s stream kind of killed the use of 2pass for VP8&VP9)
So that I didn't just post some questions:
In general, you could basically to the same thing all the other compression tests and their users do:
1st do a bunch of encodes and come up with a rule of the thumb about how many bits are needed per pixel so the output will look good for you
2nd do a bunch of encodes and come up with a correlation between the average quantizer used and your pixel goal
3rd encode a portion of your input encode it with a fixed quantizer, this will give you an approximated relation of quantizers and your pixel goal for your current source. Use that approximation and your pixel goal to calculate the average bitrate you want.
4th encode your input with the average bitrate (<- no clue how to do this with vp8 and vp9) you now aim for and assuming your previous approximations and rules are robust enough you archived your goal.
(I'm not aware of any one who did all this and implemented it into some tool)
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1st pass stats file in notepad, but it's all garbled.
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iirc those are embl encoded (vpxenc source code should contain methods to read, write serialize and de-serialize those; haven't seen a tool which allows one to read such a stats file in a decent way)
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that will help me determine the appropriate bitrate?
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encode a lot of content with different bitrates and remember which average bitrate archived a (for you) decent output
=> I had to use VP8, I would probably stick with constant quantizer encoding or (assuming I managed to find a way to produce clips with and average bitrate I specified) I would probably choose a bitrate which is roughly 1.5 times larger than the bitrate I would use for MPEG-4 ASP.
(btw. VP9 would be - a lot - slower encoding wise, normally not get near the bitrate you specified for 2pass encoding, but it would require less bitrate to archive the same quality that VP8 provided, so depending on your goals VP9 might be worth checking it out)
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