Graphics Reference
In-Depth Information
To do this, it is often necessary to be able to work with different components of an image separately. For ex-
ample, you can create an appealing glow effect by isolating the bright specular highlights of an image, blurring
them, and overlaying the blurred highlights upon the original image. Although there are several ways to isolate
bright points, being able to work with specular highlights on their own is a convenient way of doing this. Like-
wise, you might want to composite a CG character into a video scene, which requires that the CG character's
shadowappearstofallrealisticallyontothebackgroundvideoimage.Inthissituation,workingwiththeshadow
pass only enables you to tweak the exact influence of the shadow on the background, so that you can get that
aspect of the image exactly right without influencing other parts of the image that may need to be tweaked in
differentways.Youmightwanttopre-rendertime-andresource-intensivepropertiesofavideoinadvance,such
as ray-traced reflections and transparency, ambient occlusion, and indirect lighting, while keeping the color of
an object so that it can be quickly changed on the fly in the compositor. This can save enormous amounts of
rendering time if you need to experiment with different colors for an object in a fully rendered scene.
In Chapter 4 you saw how to separate elements of the render into render passes. This chapter deals in part
with how to put these elements back together. The way this is done is to use render layers as input nodes in
Blender's node-based compositor.
What Are Nodes?
Very generally, nodes are a way of organizing and combining data. Blender currently has node-based systems
forworkingwithmaterialdata,texturedata,andcompositedata.Allofthesesystemsshareacommonstructure.
Every node represents some data or some operation to be performed on data. Every node in a node graph has
eitheroneormoreinputsockets,oneormoreoutputsockets,oroneormoreofboth.Thelinks,orlinesbetween
node sockets, define the input-output relationships between the nodes. All input sockets are on the left side of
nodes, and all output sockets are on the right side of nodes, so the node graphs always read from left to right
When we say node graphs read from left to right, we mean logically. The actual placement of the nodes is
completely free. It's a good idea try to maintain left-to-right placement of your nodes to keep the graphs
readable.
If you're having trouble getting your head around the idea of composite nodes, you can start by thinking of
them as something similar to 2D layers in Photoshop or GIMP but without the constraint of having one directly
on top of the next. Unlike 2D layers, composite nodes are nonlinear and structurally flexible. If 2D layers are a
train track, nodes are a system of streets.
A Closer Look at Render Layers and Nodes
The best way to understand composite nodes is to work with them in rendering a scene. In the following ex-
ample, you'll look at composite nodes and render passes within the context of a 3D scene that incorporates a
variety of light effects including diffuse and specular lighting with lamps, ambient occlusion and environment
lighting,andindirectlightingbyemittingobjects.Thescenealsoincludesray-tracedreflections.Itisinstructive
to break the render down into individual render passes and then try to reconstruct the original render using the
compositor.
You can create your own 3D scene with these effects to follow along here, or you can use the scene I've
prepared for you in the downloadable files for this chapter, called scene.blend . However, be aware that if
 
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