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I am very new with the use of 3D graphics in Mathematica, so my question might be very naive. I need to Import an STL file, apply transformations to it (rotation and translation), and export it back to an STL file.

I have seen the STL-related questions already posted, but they don’t seem to help me. It’s not just the visualisation of the 3D graphics that I need to rotate, but the entire set of facets, with respect to the origin.

Please let me know if I need to clarify things, or provide more details.

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    $\begingroup$ Very briefly: after importing your *. stl file as Import[filename, "GraphicsComplex"], apply RotationTransform[]/TranslationTransform[] to the point list in the first argument of GraphicsComplex[], and then export Graphics3D[GraphicsComplex[newpoints, otherstuff]]. $\endgroup$ – J. M.'s ennui♦ Jul 28 '15 at 2:34
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In version 10.2 (perhaps going back to 10.0, but I cannot verify that), by default STL's are imported as MeshRegion objects. Assuming that casefn is a string representing the path to an STL file, you can see all the elements available for import from the file

Import[casefn, "Elements"]
(*{"BinaryFormat", "BoundaryMeshRegion", "Comments", 
"CoordinateTransform", "FacetNormals", "Graphics3D", 
"GraphicsComplex", "MeshRegion", "PolygonCount", "PolygonData", 
"PolygonObjects", "Summary", "VertexData", "VerticalAxis"} )*

If you use the default MeshRegion, then use TransformedRegion with the *Transform family of geometric transform functions to transform the region

case = Import[casefn, "STL"]; 
center = Mean /@ RegionBounds[case]; 
centered = TransformedRegion[case, TranslationTransform[-center]]

of course, you would use RotationTransform for your problem, but shearing, scaling, reflection, and other geometric transforms are also available. Personally, I do not like the default appearance of MeshRegion objects with edges displayed, so I use a different style

MeshRegion[Import[casefn, "STL"], PlotTheme -> "Polygons"]
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Here is a quick answer (based on @J.M.'s comment) for a basic rotation:

 STLdata = Import["MyFile.stl", "GraphicsComplex"];

 RotatedSTL = 
   Graphics3D[
     GeometricTransformation[STLdata, RotationTransform[30 Degree, {1, 1, 1}]], Axes -> True]

 Export["MyFileRotated.stl", RotatedSTL, {"STL","BinaryFormat" -> False}]
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