Keying - Nuke 101: Professional Compositing and Visual Effects - page 320

7. You can look at the alpha channel now by hovering the mouse pointer over the
viewer and pressing the A key. The matte won't be perfect, but it should look
something like FIGURE 7.28 .
Figure 7.28. The starting matte for Keylight1.
You can now start to tweak the matte. Keylight has a lot of different properties,
butthemainoneisScreenGain.Thisisamultiplier,whichhardensthematte.By
harden I mean that it pushes the contrast up—the dark grays towards black and
the light grays towards white.
8. Change the Screen Gain property to 2 ( FIGURE 7.29 ) .
Figure 7.29. Fine hair detail is lost when Screen Gain is pushed too hard.
As you can see, the background part of the matte is completely black now.
However, that's been achieved at the expense of the fine detail in the hair. This
value should be used with a lot of caution and should rarely ever reach these high
values.
There are controls similar to Screen Gain that are a little finer. Find them under
the Tuning submenu below Screen Gain.
9. Bring back the Screen Gain parameter to 1.1 or so.
10. Click to twirl down the triangular arrow in order to display the Tuning submenu
( FIGURE 7.30 ) .
Figure 7.30. The Tuning submenu.