Alpha is a type of data, found in 32-bit bitmap files, that assigns transparency to the pixels in the image.
Alpha channel shown in black, on the right
A 24-bit truecolor file contains three channels of color information: red, green, and blue, or RGB. 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 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 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 and maps. You can see this in the rendered frame window by clicking the Display Alpha Channel button on the toolbar: In the resulting display, black pixels are fully transparent, white pixels are opaque, and gray pixels show degrees of transparency. To return to the regular display, click Display Alpha Channel again.
To output a rendered image with alpha, save in an alpha-compatible format such as TIFF or Targa With Targa, the default settings include saving alpha; with TIFF, be sure to turn on the Store Alpha Channel checkbox.