This an adaptation from
http://www.gimpguru.org/Tutorials/ContrastMask/
The original methode (for photo)
1. duplicate layer
2. duplicate layer -> B&W
3. B&W -> negative
4. negative -> Mode = Overlay
5. Overlay -> Gaussian Blur 10-30
An AviSynth adaptation
a = AviSource()
b = a. GrayScale().Levels(0,1,255,255,0)Blur(0.2,0.2)
Layer(a,b,"add",10)
Happy testing