Compositing CGI with Bigger Node Trees - Nuke 101: Professional Compositing and Visual Effects - page 132

9. Go to frame 70 and view Merge6.
The hair element has been added. However, there is a little problem. The hair and
the head are perfectly cut so they fit each other. This means the hair has been cut
in such a way that the part that should be behind the head is not there anymore.
When doing a normal Over operation in the Merge, this actually results in a little
error due to the math involved. In these cases, an Over operation isn't enough.
Let's see what this means.
10. Switch to the alpha channel by hovering the mouse pointer in the Viewer and
pressing the A key.
Notice the thin black line where the hair and the head come together. You can fix
this with a different Over operation designed especially for these kinds of situ-
ations ( FIGURE 3.48 ).
Figure 3.48. This thin gray line in the matte shouldn't be there.
11. In Merge6's Properties panel, change the Operation from Over to Disjoint-over.
Lookattheplacewheretheheadandhaircometogether.Youcanseethelinehas
disappeared and the matte looks correct now ( FIGURE 3.49 ) .
Figure 3.49. The Disjoint-over operation saves the day.