Originally Posted by Angy86 View Post
I also tried adjusting the levels slightly to get a more pronounced look, is this bad for the quality or does it have no impact?
Code:
Levels(0, 1, 235, -15, 255, coring=false)
There is no impact on the intrinsic quality (provided that you do not crush below Y=0 and above Y=255), but on the overall look. Do it accordingly to your preference (and add dither=true to the command).
You can also play with Tweak() in addition to Levels().
As you have seen, any "automatic" levels adjustement does not work very well, so it is a difficult work. Some valid tutorials with basic introduction here:
https://www.cambridgeincolour.com/tutorials/histograms1.htm
https://www.cambridgeincolour.com/tutorials/histograms2.htm

For serious color correction, you need something like DaVinci Resolve prior to lossy encoding and after the AviSynth processing (which does not accept HuffYUV input, so bridge codec should be used).