This dialog page allows selection of the two materials associated with a surface. All materials defined in the current FRED document may be assigned to the surface. In addition, new materials can be added to the FRED document from this dialog by using the "Create New" button and existing materials can be edited using the "Edit/View" button. The materials added from catalogs cannot be edited directly at this time. The process for adding a catalog material or new material is described in the help topics Create New Material and Add Catalog Material.
How do material assignments work in the raytrace? The first piece of information that is relevant to the interpretation of materials assignments is the fact that every source begins its life immersed in a material. By default, the source immersion material is air but this can be modified by simply dragging/dropping a specific material node onto the source node in the usual manner. Since the source is immersed in a material, this means that the rays begin their lives immersed in the same material as the source node.
Now, every surface in the system defines a boundary between two materials and these two materials are designated on the surface dialog as "Material 1" and "Material 2". Given that a ray incident on this surface has its own immersion material assignment, what are the possible combinations of materials? 1. Ray's immersion material is the same as Material 1 on the surface If the ray's immersion material matches Material 1, then we know that Material 1 is the incident material and any reflected rays must remain immersed in Material 1. Any transmitted rays will cross the boundary and become immersed in Material 2. There is no ambiguity regarding the materials in reflection and transmission. 2. Ray's immersion material is the same as Material 2 on the surface If the ray's immersion material matches Material 2, then we know that Material 2 is the incident material and any reflected rays must remain immersed in Material 2. Any transmitted rays will cross the boundary and become immersed in Material 1. There is no ambiguity regarding the materials in reflection and transmission. 3. Ray's immersion material is the same as both Material 1 and Material 2 on the surface The surface is bounded by the same materials and there is no ambiguity regarding the which material is applied in reflection or transmission. 4. Ray's immersion material is different from both Material 1 and Material 2 on the surface The surface is bounded by two materials but the ray is incident from a third material. This means that the geometry has been constructed in an unphysical manner. The incident ray will be halted and the error message, "Num rays halted because could not resolve proper material (warn:10)", will be printed to the output window indicating that some number of rays in the raytrace have experienced this error condition. The user should then debug the system to find out where the inconsistent geometry definition (coincident surfaces, improperly closed volumes, etc.) exists in the model.
Given the above logic regarding surface material assignments, it should be evident that the ordering of materials at a surface (putting a material at Material 1 or Material 2) does not matter. The interpretation of Material 1 and Material 2 (which material is used in reflection, which is used in transmission) is determined entirely by the incident ray's immersion material.
This feature can be accessed by selecting the Materials tab in a surface dialog box.
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