Touring the Interface with a Basic Composite - Nuke 101: Professional Compositing and Visual Effects - page 41

Figure 2.9. The doll's alpha channel.
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,theMergenodeassumestheforegroundinputisa premultiplied image .
What's that you say? You've seen that term premultiplied before but never quite
figured out what it means? Read on.
Merging premultiplied images
The Merge node expects the foreground image to have both RGB channels and an alpha
channel.Thealphaisusedtodeterminewhichpixelsoftheforegroundimagewillbeused
for the composite. Another assumption is that the foreground RGB channels have been
multipliedbythealphachannel.Thisresultsinanimagewhereallblackareasinthealpha
channel are also black in the RGB channels. This multiplication is usually the result of
3D software renders or that of a composite partway through the tree. It is also the user's
choice, in whatever software package, to multiply the RGB channels with the alpha chan-
nel, thus producing a premultiplied image, whether users are aware of this or not.
Nuke's Merge node expects the foreground image to be a premultiplied one, so it is im-
portant to know whether your image has been premultiplied or not. An image that hasn't
been premultiplied is called either a straight image or an unpremultiplied image. How do
youknowwhetheranimageispremultiplied?Generallyspeaking,youshouldasktheper-
son who gave you the image with the incorporated alpha if he or she premultiplied the
image (that person might very well be you). But here are a few rules of thumb:
• Most 3D software renders are premultiplied.