[Optimization] VMAF has seemingly no acceleration

[jadenquinn]

I am aware that having VMAF is very cool for a program like this, however it doesn't go very f- it goes slow. About 4 FPS for 4K60 video, which would simply take too long. Programs like FFMetrics have hardware acceleration to some degree, which seems to boost the about of frames it can "process" about 3-4 fold. That's an open source project too, so my suggestion would be to go look at their code to see if you could implement that speed boost to make VMAF useful in Shutter Encoder for most purposes.