Material ID

Figure mapped using a multi/sub-object material: material IDs identify the component sub-materials.

A surface's material ID is the value that determines which sub-material the surface will use when you apply a Multi/Sub-Object to the object to which the surface belongs.

Geometric primitives have default material identification number assignments, just as they have default smoothing groups. The default material ID assignment depends on the type of geometry. Most curved objects such as spheres have a single material ID. Boxes have six IDs, one for each side. Cylinders have three: ID number 1 and 2 for the two caps, and ID number 3 for the sides. Hedra have three: one for each of their P, Q, and R axes.

When you apply a Multi/Sub-Object material, the materials will match the Multi/Sub-Object material ID numbers to the material ID numbers on the faces of the object. Faces keep a record of the ID number, and not of the material name. If the material is anything but Multi/Sub-Object, the material is assigned to the object's entire surface.

Assigning some defining material ID number to each object before they become compound object operands can be a useful technique for being able to select the separate pieces after they're combined.

You can use material IDs for continuous surfaces that require separate paints or finishes. For example, a car constructed from different types of materials, such as a colored metal body, chrome parts, glass windows, and so on.

You can use the Material modifier to assign material ID numbers. Also you can reassign material IDs using the Editable Mesh > Surface Properties rollout, or Edit Mesh modifier > Edit Surface rollout.