Keying - Nuke 101: Professional Compositing and Visual Effects - page 324

level of detail starts to deteriorate. A good way of getting a better key is to use the tree
itself to build up a series of keys together that produce the best result.
Thisisreallywherenode-basedcompositingsystems,andNukeinparticular,shine.Being
able to easily combine mattes using the tree can result in a very good key and can utilize
several keyer nodes together, taking the best from each.
Combining Keyer Nodes Using the Tree
In my opinion, the IBK produced a far superior result compared to the other two keyer
nodesdiscussedintheprevioussection.Thehairdetailwasreallyfine,andtheedgeswere
great. The main problem, however, was that the area that's supposed to be background
wasn't black in the matte. After all, edges are really what keying is all about. It is very
easy to draw a shape with the RotoPaint node and fill in things that are supposed to be
white in the center (or core) of the matte, and it is very easy to do the same for areas that
are supposed to be black (also called garbage ). But the actual edge, that's what you are
looking for when keying.
TheresultoftheKeylightnodewasOK,butnotgreat.Ontheotherhand,playingwiththe
Screen Gain property for the Keylight node produces a very hard matte with well-defined
white and black areas. Maybe you can use the Keylight node together with the IBK nodes
to create a perfect key.
First let's use Keylight1 to create a garbage matte—a matte that will define unwanted
areas.
1. You might want to save this Nuke script and then save it again with a different
name. You are going to degrade the key your Keylight node is producing, so you
might want to keep it for posterity looking the way it does now. I saved mine as
chapter07_v01 and chapter07_v02.
2. Increase Keylight1's Screen Gain property to something like 1.5. This should
make the matte a perfect black on the outside and white on the inside. The hair
detail will still be gray, which is fine.
3. If increasing Screen Gain isn't enough, you can change the Clip Black and Clip
White properties in the Screen Matte area as you did before. I brought down the
Clip White property to 0.1 to get rid of some gray patches on the side of the girl's
arm and really whiten up the hair ( FIGURE 7.34 ) .
Figure 7.34. A crunchy matte made up of mainly black and white.