6.1.1. General Concept

DIRSIG has a mapping system which allows a scene builder to assign attributes at a resolution smaller than individual facets in the scene. (Figure 6-1)

Maps use an external data source, like a PGM image, together with an entry in the DIRSIG config (.cfg) file. As an example, material mapping translates image digital counts (DCs) to material IDs. See Figure 6-2.

Using individual facets, it is possible to reproduce the effects of a map. However, this may require thousands of tiny facets to accurately reproduce high spatial frequency information. Thus, memory efficiency is one key benefit of using maps.

Maps can also take advantage of existing data or data generated by external models. A temperature map (which maps pre-calculated temperatures onto spatial locations) of a nuclear power plant's cooling pond, is a good example of this.

Detailed information about which attributes can be varied using maps is given in Property Mapping How-To.

6.1.2. Associating Maps with Facets

Maps are assigned to material IDs. The process begins when a ray intersects a facet with a mapped material ID. The mapping algorithm translates from worldspace coordinates to coordinates inside the map's image. Earlier releases of DIRSIG only supported coordinate mapping using a tiling algorithm, while DIRSIG 4 also supports a UV algorithm.

Both methods use a ground sampling distance (GSD) which relates the size of one image pixel to worldspace. This value is set inside the maps section of the config file.

Once the coordinates have been translated, the maps are applied as shown in Figure 6-3