Shaders and Texturing - An Introduction to Computer Graphics for Artists - page 204

Fig. 11.27 This wrought iron fence is made of four transparency mapped planes

11.5.4.6 Transparency

Transparency maps allow artists to make transparent or semi-transparent surfaces
like glass or plastic, but also allow masking of diffuse maps to create the appearance
of very detailed geometry, such as wrought iron fences or complicated lead patterns
in a stained glass window. A transparency map is a grayscale image where black
pixels are rendered as completely transparent and white pixels are completely
opaque (Fig. 11.27 ). All values in between represent steadily decreasing transpar-
ency as they advance from black towards white.

11.5.4.7 Specular

A specular map is used to defi ne different levels of refl ectivity on a surface. If a
wooden surface is partly wet in some places but dry in others, a specularity map
might be appropriate as a way to specify which parts are wet and which are not.
Another example is a refl ective surface that is dusty in some places but not in others.
The dusty areas would have a lower specular value than the clean sections. This can
be described with a specularity map (Fig. 11.28 ).

11.5.5 Weathering

Many surfaces will look unnatural if they are too clean. For an architectural render-
ing or an advertisement, you will probably want your objects to be pristine, but for
almost everything else, you will want dirt maps . Dirt maps don't have to actually
represent dirt, though they often do, but any form of weathering at all. This can be
rust, oxidization, paint, graffi ti, bird droppings, or anything else that could conceiv-
ably be added to a clean surface (Fig. 11.29 ).