How do I convert imported STL files to 3D images?

After importing a 3D image in STL format, my possibilities in regard to "3D Volumetric Image Processing" seem rather limited. All examples are given with TIFF files. Is there a way to convert from STL to TIFF or work with an STL image in some other way?

Here is something quick and dirty to convert from a Graphics3D to Image3D (the second argument is sort of a quality knob, the higher the number the more slices are taken):

img3Dify[gr3d_Graphics3D, qu_] := Module[{pr, br, slices},
pr = PlotRange /. AbsoluteOptions[gr3d, PlotRange];
br = Map[Norm@Differences[#] &, pr];
slices = ParallelMap[
Image[Show[gr3d, ViewPoint -> Top,
Lighting -> {{"Ambient", White}}, Background -> None,
PlotRange -> {pr[], pr[], #}]] &,
Partition[FindDivisions[pr[], qu*2], 2, 1]];
Image3D[slices, BoxRatios -> br, ColorFunction -> Hue]]

on an example image

spikey = Import["ExampleData/spikey.stl", "Graphics3D"] img3Dify[spikey, 40] • Thank you! That was actually just what I needed…! Jan 21 '14 at 17:15

I found that with Mathematica 10 one can use the region functions/distance+watershed to achieve something similar but much faster. Something along these lines can generate ~1 million voxels in a couple of seconds in my machine:

voxelify[region_?RegionQ, res_?NumericQ] :=
Module[{f, bounds, boxsize, voxsize, data, d2},
f = RegionDistance[region];
bounds = RegionBounds[region];
boxsize = -Subtract @@ # & /@ bounds;
voxsize = Max[boxsize]/res;
data = ParallelArray[f[{##}] &, Round[boxsize/voxsize], bounds];
d2 = HeavisideTheta[data - voxsize];
2 - ReplaceAll[WatershedComponents[Image3D[1 - d2]], {0 -> 2}]
]

And then

spikey = Import["ExampleData/spikey.stl", "MeshRegion"];
Image3D[voxelify[spikey, 100]] • I get this error message (MMA 11.3): ParallelArray::nopar1: Array[f\$11729[{##1}]&,{100,95,81},{{-1.54435,1.54435},{-1.46876,1.46876},{-1.2494,1.2494}},List] cannot be parallelized; proceeding with sequential evaluation. Mar 28 '19 at 9:12
• Actually I get the same error message using ParallelArray in any version of MMA, including 10.0, 10.3 and 11.3. Apr 1 '19 at 17:42