I want to calculate the density of a volumetric Emission material using the distance inward from the mesh surface. I've been able to do this with a cube and several coordinate systems: generated, object, I can display the u,v-coordinates as colors on the surface of a uv-mapped mesh. I can use the Mapping Node to rotate these coordinates around the X- or Y-axis and see the w-coordinate in shades of blue.
Within the Blender manual pages for Input Nodes, the description of the Texture Coordinate Node makes no mention of unsupported coordinate systems for volume materials. But the description of the Geometry Node states "For volume shaders, only the position and incoming vector are available."
Is the w-coordinate of the UV coordinate system supported for volume materials? I fear that the w-coordinate cannot be defined within a mesh of arbitrary shape because each interior point can be projected back to the mesh along many normal vectors. But I'm not sure I understand the w-coordinate well.