Keying - Nuke 101: Professional Compositing and Visual Effects - page 315

21. Check Luminance Match.
You can see that only a few hairs were added to the matte.
22. Move the Screen Range property a little so that you reduce the amount of noise
on the background a little—without changing the foreground. I left mine at 0.9.
It doesn't seem like this property did any good to the overall key, so you should
turn it off.
23. Uncheck Luminance Match.
Thisisasfarasyoucangetthematte.Itishardlyperfect,butforthisgreenscreen
elementitisthebestyoucando.Youwillgettousethismattelater,usinganother
technique. For now, you can still adjust the spill a bit more.
The IBKGizmo has some controls remaining at the very bottom for edge correc-
tion. These properties are Screen Subtraction, Use Bkg Luminance, and Use Bkg
Chroma. Let's see what these do ( FIGURE 7.22 ) .
Figure 7.22. The bottom three properties for the IBKGizmo node.
24. Switch to viewing the RGB channels of Merge1.
25. Screen Subtraction is checked by default in IBKGizmov3_1. Uncheck and check
it again to see what it does. Leave it on.
This property subtracts the original greenscreen element from the result. This re-
duces spill on the edges of the matte, and it does a very good job at that.
26. Use Bkg Luminance is unchecked by default. Check it to see its effect, but leave
it unchecked when you're finished.
This property uses the background's luminance to color correct the foreground
around the edges of the matte. In this case, its effect isn't helpful.
27. Use Bkg Chroma is unchecked by default. Check it to see its effect, and leave it
on when you're finished.
This property uses the background's chroma to color correct the foreground around the
edges of the matte. In this case it creates a better composite, and so you should leave it on
( FIGURE 7.23 ) .
Figure 7.23. The final composite as it stands at this stage.