Alpha mapping is a process by which a texture is applied to an object in three-dimensional (3D) Computer-Generated Imagery (CGI) that creates transparency. The term “opacity” is often used in computer graphics, and full opacity means that an object is solid, while no opacity indicates a transparent object. Alpha mapping is the creation of a texture map for an object that does not provide color or texture, but instead indicates varying levels of opacity for it. The map for this is typically created as a grayscale object: where white indicates full opacity, black is transparent, and varying shades of gray represent gradients between these two extremes.
While alpha mapping may seem complex, the process and methodology behind it is actually quite simple. Objects in 3D CGI are created as a series of shapes or polygons that are connected together to create a larger shape. A square box, for example, can be made using only six polygons, with a single four-sided shape on each side to create a box. More complex objects are made in much the same way, though they can consist of many more polygons as necessary to create curves and other details.
Onto this shape, which is often called a “mesh,” are applied a number of different “maps” to add colors, images, and textures. That simple box can have a texture map applied to it with the appearance of wooden boards, so that each side appears to have been assembled from wood. This is often a fairly complicated process that involves a number of different maps that are combined to make the mesh as realistic as possible.
Alpha mapping is simply one part of this larger process that adds additional realism and can be used to add detail in a number of ways. Different types of images can be used during alpha mapping, though the image used is typically a simple grayscale graphic. This means that it does not contain any color, but instead consists only of black, white, and shades of gray. The image created in this way is referred to as an “alpha map.”
Once the map is applied to an object, then parts of it that are white in the alpha map appear as normal, while parts that are black become completely transparent. Different shades of gray can be used in alpha mapping to indicate varying degrees of opacity. Tones closer to white are more solid, while those toward black are more transparent. This does not actually change the mesh itself, only its appearance.
Alpha mapping is important for the creation of realistic computer graphics that are not overly intensive for rendering or display. If an artist wants to create a hole in the box in the previous example, then doing so would require a change in the number of polygons to create the shape of the hole in one of the sides. The use of alpha mapping, however, allows an artist to simply create a map with a black area for the hole, which becomes transparent in the final image.