This adds several new options to the SVT-AV1 standalone encoder (--enable-variance-boost, --variance-boost-strength). I notice that, by default, variance boost appears to be turned off.

To use it with ffmpeg, you'd likely need to explicitly enable variance boost in the source code where ffmpeg applies the config options to the svtav1 encoder.

For testing, it's probably easier to run the standalone SvtAv1EncApp with aforementioned parameters and pipe ffmpeg's rawvideo output to it. Either way, I assume this functionality will eventually become an option for ffmpeg's -svtav1-params, now that it's in the SVT-AV1 main tree.

Disclaimer: this is just what I concluded from a quick check of the patch and ffmpeg code.