Touring the Interface with a Basic Composite - Nuke 101: Professional Compositing and Visual Effects - page 42

• Images that don't have incorporated alpha channels are straight.
• If black areas in the alpha channel are not black in the RGB channels, the image is
straight.
•Ifyouarecreatingthealphachannel(usingakey,ashapecreationnode,oranother
method), you should check whether your operation created a premultiplied image
or not. As you learn the alpha-channel creation tools, I will cover premultiplied
again.
Another reason you need to know the premultiplication state of your image is color cor-
rection. Why? Because you can't color correct premultiplied images. You are probably
not aware of this fact, as other software packages hide it from you. In Nuke, however, it
is something that needs to be taken care of by the user—and that means you. Unprem-
ultiplying is performed by dividing the premultiplied RGB channels with the alpha, thus
reversing the multiplication. This produces a straight image that you can then color cor-
rect, then reapply the multiplication. You will learn how to do this later in the chapter.
Here are some dos and don'ts:
• The Merge node expects a premultiplied image as the foreground image for its
Over (most basic and default) operation.
• Never color correct premultiplied images. Chapter 4 is all about color correction.
• Always transform (move around) and filter (blur and so forth) premultiplied im-
ages.
YoucontrolpremultiplicationwithtwonodesintheMergetoolbox:PremultandUnprem-
ult.
The rule says that if an image's RGB channels aren't black where the alpha channel is
black, then it isn't a premultiplied image. When you looked at Read2 you noticed exactly
this. The black areas in the alpha channel were a light purple color in the RGB channels
( FIGURE 2.10 ) . This means this image is a ... what? A straight image!
Figure 2.10. This image shows both the alpha and RGB channels.