2008

Alpha Channel
Alpha channel shown in black, on the right
Alpha is a type of data, found in 32-bit bitmap files, that assigns transparency
to the pixels in the image.
A 24-bit truecolor file contains three channels of color information: red, green,
and blue, or RGB on page 7908 . 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.
By adding a fourth, alpha channel, the file can specify the transparency, or
opacity, of each of the pixels. An alpha value of 0 is transparent, an alpha
value of 255 is opaque, and values in between are semi-transparent.
Transparency is important for
compositing on page 7741 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 on page 7708 edge of an object in a rendered image. These pixels
are used for compositing. An image such as the one shown above can be
composited 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. Thus, an RGB file is 24-bit with 256 levels each of red,
green, and blue. An RGBA file (red, green, blue, alpha) is 32-bit, with the extra
8 bits of alpha providing 256 levels of transparency.
3ds Max creates the alpha channel automatically when you render. Any
background pixels in the rendered image are fully transparent, and the alpha
channel also accounts for any other transparency that you create via materials,
Glossary | 7709