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to reflect this. For example, a pixel that is 30% covered by the object will have
an alpha of 0.30.
To antialias the edges, the alpha must be premultiplied to darken these edge
pixels. (This is equivalent to compositing the image over a black image). So
it is natural, in a sense, for rendered images to have premultiplied alpha. If
you do not premultiply the alpha of a rendered image, then just looking at
the RGB you will see jaggies on the edges of objects. You would need to
composite it against black using the alpha channel whenever you wanted to
display it.
NOTE To control whether or not the renderer uses the environment map's alpha
channel in creating the alpha for the rendered image, choose Customize >
Preferences > Rendering, and then turn on Use Environment Alpha in the
Background Antialiasing group.
If Use Environment Alpha is turned off (the default), the background receives
an alpha value of 0 (completely transparent). If Use Environment Alpha is
turned on, the alpha of the resulting image is a combination of the scene and
background image's alpha channel. Also, when you render to
TGA files on
page 7162 with Pre-Multiplied Alpha set to off, turning on Use Environment
Alpha prevents incorrect results.
TIP If you plan to composite objects in another program such as Combustion or
Photoshop, render the objects against a black background. Otherwise, a fringe of
environment or background color can appear around the objects.
Procedural Maps
Unlike a bitmap, which is an image produced by a fixed matrix of colored
pixels like a mosaic, a procedural map is generated by a mathematical
algorithm. Consequently, the types of controls you might find for a procedural
map will vary depending on the capabilities of the procedure.
Glossary | 7901