Alpha

Alpha is an integer variable that is often included as the "fourth" number in a RGB color format. Alpha is an opacity rating from 0 - 255, where 0 is completely invisible and 255 is completely visible. In a VMT file, the "$alpha" parameter is normalized from 0 to 1.0 instead.

To store an individual alpha value in each pixel, an alpha channel is used as the fourth channel of the TGA file. This allows variable opacity, for example, in a single texture, such as the lettered Petrol signs in Half-Life 2. The texture's alpha channel can also be used to define self illumination instead of transparency.