Alpha compositing. This image's alpha channel falls off to zero at its base.
In computer graphics, alpha compositing is the process of combining an image with a background to create the appearance of partial or full transparency. It is often useful to render image elements in separate passes, and then combine the resulting multiple 2D images into a single, final image in a process called the composite. For example, compositing is used extensively when combining computer-rendered image elements with live footage. Description[edit] What is alpha channel. Main » TERM » A » In graphics, a portion of each pixel's data that is reserved for transparency information. 32-bit graphics systems contain four channels -- three 8-bit channels for red, green, and blue (RGB) and one 8-bit alpha channel.
The alpha channel is really a mask -- it specifies how the pixel's colors should be merged with another pixel when the two are overlaid, one on top of the other. Typically, you wouldn't define the alpha channel on a pixel-by-pixel basis, but rather per object. Different parts of the object would have different levels of transparency depending on how much you wanted the background to show through. This allows you to create rectangular objects that appear as if they are irregular in shape -- you define the rectangular edges as transparent so that the background shows through. "After Effects does not support the current OS language." AE TUTS.