When you create a layer, you are assigning a source input to a geometric object that defines the surface on which the source is mapped. You can replace the assigned surface object with a different one, as well as disconnect a surface object in the Schematic and replace it with another. By default, when you create a layer, the surface type is determined by the last geometric setting chosen in the Layer Editor.

There are five surface types available in Reaction: bilinear, box, frustum, sphere, and plane. The bilinear surface is a planar surface with four vertices, one at each corner. The vertices are joined by straight line segments using linear interpolation. The box, frustum, and sphere surfaces are 3D primitives meshes with no control vertices.