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Originally Posted by Boulder
What does the "auto variance" actually mean? I've always thought that aq-mode 1 is already some kind of automatically varying mode based on qg-size.
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The aq-mode options are all inherited from x264, which didn't have qg-size. I don't recall the specific differences in the algorithms. I imagine they've evolved some in x265 in any case. Modes 2/3 are more adaptive, which are generally better. But 2 was experimental in x264 for a long time, IIRC because some content it wouldn't adapt optimally.
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On HDR sources, mode 2 seems to produce noticably smaller files for the same CRF than mode 1. This would somehow point to the problem with the flat, non-tonemapped image x265 seems to use while analyzing things. Using aq-strength 1.8 for mode 1 makes the bitrate shoot through the roof compared to strength 1.0. I'm going to do some visual compares as soon as I have the time, but it's difficult because a 2-pass encode is a no-no since I'm not hitting a specific size. Comparing those two strengths at the bitrate that strength 1.0 produces at CRF 15 or so is not fair because strength 1.8 would require so much more bits.
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I generally recommend using 2-pass VBR when doing comparisons of features like this. With CRF you are changing bitrate AND quality together, so it's hard to tease out any actual encoding efficiency improvements. Going to a fixed file size at a reasonably challenging bitrate (so you're going to see some artifacts) is what I've found as the most efficient way to do these comparisons.