Ganbar R. - NUKE 101. Professional Compositing and Visual Effects - 2011

ToURING THE INTERFACE WITH A BASIC CoMPoSITE 25
FIGURE 2.7 Both inputs are
now connected, creating a
composite.
FIGURE 2.8 All this
light purple discoloration
wasn’t here
before.
Maybe you don’t have an alpha channel? Let’s have a look.
4. Click Read2 and press 1 on the keyboard to view it in the Viewer.
5. While hovering your mouse pointer over the Viewer, press the A key to view the
alpha channel (FigUrE 2.9).
You can clearly see an alpha image here. It represents the area where the doll is.
The black area represents parts of the doll image that should be discarded when
compositing. So why aren’t they being discarded?
6. Press the A key again to switch back to viewing the RGB channels.
Notice that the black areas in the alpha channel are a light purple color, same as
the discoloration in your composite. Maybe this is the source of your problems?
Normally, the Merge node assumes the foreground input is a premultiplied image.
What’s that you say? You’ve seen that term premultiplied before but never quite figured
out what it means? Read on.
FIGURE 2.9 The doll’s
alpha channel.