Rendering: Faster, Processor! - Blender Foundations: The Essential Guide to Learning Blender 2.6 - page 349

Figure 12.28 The Render Layers selector in the Image Editor.
Further to the right on the header is a series of buttons that determine which image channels are displayed.
We've worked with Alpha before when making objects transparent in Chapter 7, but didn't mention that
each image you render has a full Alpha channel included with it. In addition to having an R, G, and B
color, every pixel in your rendered image has an Alpha value, ranging from 0.0 to 1.0. If you remember
to think of “Alpha” as “opacity” (instead of “transparency”), it will be obvious that pixels with an Alpha
of 1.0 will be completely opaque—the normal state of things. Pixels with an Alpha of 0.0 will be com-
pletely transparent.
The buttons, which are highlighted in Figure 12.28 , tell the Image Editor to show only RGB, RGB with
Alpha, or just Alpha. In the combined image (RGB + Alpha), transparency is shown with a gray-and-white
checkerboard pattern.
One of the reasons you may not have noticed this yet is because we have been rendering with a Sky for
the background. The Alpha selector on the Shading panel of the Render properties controls what the
renderer puts in the background when there are no objects. In the case of these space monkeys, a lot of
the image is comprised of this kind of space. Set to Sky , empty space is filled with whatever colors or
textures are designated in the World properties. This is what we have been doing so far. For simple renders
with no compositing, it is an easy way to put something in the background.
Now we're all grown up, though, and it's time to leave our diapers behind.
The other settings of the Alpha selector are Straight Alpha and Premultiplied . Straight Alpha renders
the full pixel color of an object's edge and assumes that you'll use the Alpha channel later to make it blend
nicely with the background. Premultiplied shows up already nicely blended. If you're only going to be
working within Blender, there's no reason to use Straight Alpha for now. The monkey_orrery scene in the
Web Bucket has been set to use Premultiplied, which is often shortened to “Premul” in the interface.
All this talk about Alpha ought to be leading up to something, right? Let's dive back into the compositor
to find out.
Do the standard dance of choosing Compositing Nodes from the Node Editor header, and enable both
Use Nodes and Backdrop . Disconnect the initial Render Layers node from the final Composite node,