Thursday, February 11, 2021

Dirac video codec authors on Rate-Distortion Optimization

"This description makes RDO sound like a science: in fact it isn't and the reader will be pleased to learn that there is plenty of scope for engineering ad-hoc-ery of all kinds. This is because there are some practical problems in applying the procedure:"

"Perceptual fudge factors are therefore necessary in RDO in all types of coders." "There may be no common measure of distortion. For example: quantising a high-frequency subband is less visually objectionable than quantising a low-frequency subband, in general. So there is no direct comparison with the significance of the distortion produced in one subband with that produced in another. This can be overcome by perceptual weighting.."

In other words: yea it's a bit of a hack.

This is what I've found with RDO texture encoding. If you use the commonly talked about formula (j=D+l*R, optimize for min j) it's totally unusable (you'll get horrible distortion on flat/smooth blocks, which is like 80% of the blocks on some images/textures). Stock MSE doesn't work. You need something else. 

Adding a linear scale to MSE kinda works (that's what bc7enc_rdo does) but you need ridiculous scales for some textures, which ruins R-D performance.

So if you take two RDO texture encoders, benchmark them, and look at just their PSNR's, you are possibly fooling yourself and others. One encoder with higher PSNR (and better R-D performance) may visually look worse than the other. It's part art, not all science.

With bc7enc_rdo, I wanted to open source *something* usable for most textures with out of the box settings, even though I knew that its smooth block handling needed work. Textures with skies like kodim03 are challenging to compress without manually increasing the smooth block factor. kodim23 is less challenging because its background has some noise.

Releasing something open source with decent performance that works OK on most textures is more important than perfection. 

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