[java3d] AFFINE?????

[The Casteels]

Does anyone have a "plain english" definition of the inner workings of a
Transform3D object. I've been trying to work with one to figure out how
they work and am now getting the following error:

Exception in thread "main" javax.media.j3d.BadTransformException:
TransformGroup
: non-affine transform
        at
javax.media.j3d.TransformGroup.setTransform(TransformGroup.java:79)


I've been trying to look up what this is and how to set it, but am only
getting more confused.

Here's the part of my code causing the problem:

   Matrix4d m1 = new Matrix4d();
   Matrix4d m2 = new Matrix4d();

   Transform3D inv = new Transform3D();
   Transform3D invd = new Transform3D();
   tg.getTransform(inv);
   inv.get(m1);

   m2.setElement(0,0,-m1.getElement(0,0));
   m2.setElement(0,1,-m1.getElement(0,1));
   m2.setElement(0,2,-m1.getElement(0,2));

   m2.setElement(3,0,0);
   m2.setElement(3,1,0);
   m2.setElement(3,2,0);
   m2.setElement(3,3,1);
   invd.set(m2);

   tg.setTransform(invd);


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