As we said before, every element that participates in the DIRSIG rendering process is assigned a material. This includes physical objects that the user has constructed within the scene. For example, the grass on the terrain, the leaves on a tree, the bricks on a building, the windows in a vehicle, etc. Materials are also assigned to some not so obvious but user defined elements in the scene including the gases in a factory stack plume. In addition, the DIRSIG model also automatically creates some materials that are assigned to elements the user does not have geometric access to including the atmosphere, Sun, Moon, etc.
The primary function of the DIRSIG model is to perform radiative transfer between a large number of scene elements. The material is assigned to each element that participates in this process must include both surface and bulk optical properties. Since, the model also supports thermal region simulations, each material also contains thermodynamic properties so that the thermal prediction model can simulate the surface temperature for a given set of environmental conditions.
The thermodynamic aspects of the materials are discussed in detail in the chapter entitled "Thermal Modeling". However, in summary the user should be aware that the temperature of a given optical element is a function of the material's specific thermodynamic properties (thermal conductivity, heat capacity, etc.) and it's optical properties. For example, from practical experience we know that a surface painted with a dark paint will get warmer in the Sun than the same material painted a lighter color.
The optical properties of a material are the driving force in the DIRSIG radiative transfer process. At this time, the model supports three primary types of materials:
Opaque, or non-transmissive materials. This material type is commonly used for most non-transmission objects including soils, building materials, etc.
Plate-style transmission materials. This material type is used for materials that have constant thickness and hence constant transmission. It can be used to model panes of glass, a single tree leaf, etc.
Volume-style transmissive materials. This material type is used for irregularly shaped volumes which will have different transmissions depending on the path through the volume. It can be used to model a homogeneous plume or cloud.
These three material types require slightly different inputs when they are configured in the material database (a topic that will be discussed later in this section).