The definition of material properties in RADIANCE is generally controlled by the application of modifiers. There are several classes of modifiers, most of which can be freely combined to get more interesting results.
The most basic modifier class are the materials. Every geometric element in a scene needs some material assigned, or it would be invisible. In some special cases, several materials can be mixed, which is the purpose of the mixture class of modifiers.
Materials define the fundamental properties of a surface. There are material types with just plain opaque properties, some with a metallic look, others with transparent and refractive behaviour and a few unusual ones for modelling fog and smoke, true mirrors, or other special features.
Last but not least, RADIANCE differs from most other rendering engines by the fact that light sources normally are not "virtual" (and invisible) objects, but light is defined as a material type that turns geometry elements into luminous surfaces. This very closely reflects the situation in the real world, where light is generally emitted by physical objects, and doesn't just appear "out of nowhere".
While most material types above basically have constant color and reflectance values, materials in the real world often don't. The modifier class of patterns can be used to modify any material, in order to define changes in color and reflectance depending on any number of parameters.
Patterns are procedural modifiers, that use functions and variables as input. Modifications can depend on the location in space, the orientation of the surface, the distance from the viewpoint, a random function and many other quantities.
An unlimited number of patterns can be used to modify any material, so that their effects will be superimposed on each other.
When just manipulating the color and reflectivity of a surface, we may notice that some objects still have a rather dull look, while the real thing would show a much more interesting surface structure. This is the result of the "micro structure" of the surface, that interacts with direct light to create a play of light and shadow.
With RADIANCE, we can use texture modifiers to simulate this kind of effect. A texture will manipulate the normal vector used by the program when evaluating a specific point on a surface. As a consequence, even a otherwise flat surface will reflect more light in places where the normal vector points closer towards a light source than in others. A very simple example would be a corrugated metal sheet, where the normal vector swings back and forth according to a sine wave along one axis.
An unlimited number of textures can be used to modify any material, so that their effects will be superimposed on each other, and on the effects of any patterns which happen to modify the same material.
In some cases, it may be convenient to assign two different modifiers to a surface, with a function to control which one takes precedence at each point. The modifier class of mixtures takes care of this problem. A mixture allows you to blend two materials into each other. Of course, you can also mix patterns or textures that modify a material.
Mixtures can also be applied recursively to each of the foreground and background of another mixture, and they may be used to control transparency by not assigning a material to one of the branches.
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