Alpha Channel
Fourth channel that stores transparency or a mask
Also known as: Alpha channel · Alpha map · Opacity map · Transparency channel · RGBA
Definition
An alpha channel is a fourth channel stored with a texture's red, green and blue channels, usually describing transparency, where white is opaque and black is transparent. In game art it is used for cutouts such as leaves, fences and hair, for opacity masks and also to hold extra data, such as smoothness in Unity's metallic map or a mask in a packed texture. Not all file formats store alpha, but PNG, TGA and EXR do.
In production
Check what the alpha means in your engine before you use it: in Unreal, a texture's alpha can be read as a separate mask, and in Unity it can be smoothness or transparency depending on the shader. Extend the color past the alpha edge to avoid dark outlines, and turn off sRGB when alpha holds data, not color. Keep alpha-heavy textures small, since they cost memory and often overdraw.
