9

Aliasing/Antialiasing 907
Aliasing/Antialiasing
Pyramid is aliased on left, antialiased on right.
Aliasingisthestaircaseeffectattheedgeofaline
or area of color when it’s displayed by an array of
discrete pixels.
Antialiasing smoothes the staircase effect that
occurs when diagonal or curved lines or borders
are drawn on raster displays consisting of squar e
or rectangular pixels. Antialiasing can b e either on
or off. Turn this off only when you’re rendering
test images and want g reater speed. Leave it on at
all other times.
You can a lso turn antialiasing off for the Material
Editor sample slots to speed up redra w of the
sample objects. Click the Options button in the
Material Editor and turn on the Antialias togg le on
the Material Editor Options dialog. Default=off.
Note: To control whether or not a background
image is affected by the renderer’s antialiasing
filter, choose Customize > Preferences >
Rendering and then turn on Filter Background in
the Background group. Default=off.
Alpha Cha nnel
Alphachannelshowninblack,ontheright
Alpha is a type of data, found in 32-bit bitmap
files, that assig ns transparency to the pixels in t he
image.
A 24-bit truecolor file contains three channels of
color information: red, green, and blue, or RGB
(page 3–1001). Each channel has a particular
intensity or value at each pixel. The intensity of
each channel determines the color of the pixel in
the image.
Byaddingafourth,alphachannel,thefilecan
specify the transparency, or opacity, of each of the
pixels. An alpha va lue of 0 is tr ansparent, an alpha
value of 255 is opaque, and values in between are
semi-transparent. Transparency is important for
compositing (page 3–922) operations, such as those
in Video Post, where several images are blended
together in layers.
An alpha channel is particularly useful for the
partly transparent pixels around the aliased (page
3–907) edge of an object in a rendered image.
These pixels are used for compositing. An image
suchastheoneshownabovecanbecomposited
smoothly onto a different background if an alpha
channel is produced and saved with the image.
Each channel of a truecolor bitmap file is defined
by 8 bits, providing 256 levels of intensity. Th us, an
RGB file is 24-bit with 256 levels each of red, green,
and blue. An RGBA file (red, green, blue, alpha)