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134 Chapter 17: Rendering
Compositing Rendered Elements
In general, you can composite elements using
additive composition, which is independent of the
compositing order.
Themainexceptionsarethebackgroundelement,
atmospheres, and shadows.
• Background: The background is not trimmed
against geometry, the background should be
composited under the other elements.
• Atmosphere: The atmosphere element should
be composited over all other elements.
• Black-and-white shadows: Black-and-white
shadowsshouldbecompositedovertherestof
the image (aside from the atmosphere), to dim
color in the shadowed areas. This technique
does not take colored lighting into account.
In other words, the layers when you composite
using black-and-white shadows appear like this:
Top: At m o s p h e re
Second from top: Shadow element
Middle: Diffuse + Specular + ... (other elements)
Bottom: Background
"Screen" Compositing for Specular and
Reflection Elements
The other exception to additive composition is
when specular or reflection elements have been
generated by certain material shaders. These
shaders generate specular and reflection elements
you must composite differently:
•Anisotropic
•Multi-Layer
• Oren-Nayar-Blinn
Shaders are assigned on a per-material basis,
in the Material Editor. If you render specular
or reflect ion elements in a scene that uses these
shaders, t hen composite them with the diffuse and
other foreground components (aside from colored
shadows, as described above), by overlaying
them using an operation ca lled "Screen" in some
compositing programs.
Screen compositing uses this formula to combine
elements:
Background * (1 - Foreground) + Foreground
Thebackgroundismultipliedbytheinverseofthe
foreground color, and then t he foreground color is
added to the result.
For more information, see the documentation for
the compositing program you use.
Procedures
To have the R ender E lements dialog assign names to
the rendered element files automatically:
1.
In the Render Scene dialog’s Common
Parameters rollout, use the Files button to
assign an output file name and file type for the
(entire) rendered scene.
2. In the Render Elements rollout, use the Add
button to add elements for rendering.
You can render elem ents to fi les without rendering
theentirescenetoafile,butinthiscase,youhave
to assign the files for the elements individually or
"by hand."
To add an element for rendering:
1.
Click Add.
2. In the Render Elements dialog, do one of the
following:
• Click to highlight the name of an element,
and then click OK.
• Double-click the name of an element.
If you have assigned a file name for the entire
rendering, the new element is assigned a file
name automatically. Otherwise, use the Files
button in the Selected Element Parameters