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So what is designed for that? Generally as an encoder you want to target a certain (perceptual) quality level and when testing different settings you're aiming to hold that constant and see whether you achieve a lower bitrate. There should be a way to do that.

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You plot two RD curves with whatever quality metric you care about, then calculate the distance between them. Also known as the Bjontegaard metric.

And the issue with that is that they're using PSNR and SSIM as quality metrics, which are useful for images, but haven't been designed for video, so they cannot take into account temporal factors and how motion masks distortion, etc.

I think for AV1 they're using VMAF too, which is an actual video quality metric (https://github.com/Netflix/vmaf).

Sure you can use VMAF as the quality metric for BD-rate, that’s what BD-VMAF is

But even after choosing a method, I want to be able to set target quality in a meaningful way. Having a parameter that I can set to specify quality on a method-agnostic scale seems like a basic usability feature.

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