An STL file is made of floating-point triangles. You get X,Y,Z for each of the 3 corners. It should be integer tetrahedrons. You'd get X,Y,Z for each of the 4

An STL file is made of floating-point triangles. You get X,Y,Z for each of the 3 corners. It should be integer tetrahedrons. You'd get X,Y,Z for each of the 4 corners. Tetrahedrons let you fill the middle. Each tetrahedron should have fill data, for the whole thing or corner-by-corner for gradients. This would allow variable density, elasticity, conductivity, opacity, hardness, etc. Integer math solves the problem of variable rounding that often screws up STL files. Being unit-independent is much better than having one particular unit. You may think millimeters are great, but to others this might as well be angstroms or chains or twips or rods or parsecs. Unit conversion introduces rounding error. I could go for an optional free-form text field that suggests a unit to use, with the software defaulting to the suggested unit if recognized. Software should let you define the unit.