Maerchenprinz wrote in Wed Aug 03, 2022 2:15 pm:AC3D (and FG, too?) uses, as far as I get it, the centre of a bounding box made out of the model's expansion over x/y/z
Nope - FG (or rather OSG, which is what mainly handles the 3D stuff) uses the origin of the file as reference - you can try it out for yourself, just put a cube, offset it in AC3D or Blender ten meters to the right, then put it into an FG model without offsets - you'll find that it is indeed ten meters to the right, and that you need an offset of -10 along the Y axis to get it back to center. For rotations and scaling, the reference point (around which the vertices are rotated / scaled) is by default also the file's origin, but it can be changed by using a <center><x-m>…</x-m>…</center tag.