> If the writer is amenable to suggestions, then try recommending the use
> of mesh2 structures rather than individual polygons. POV doesn't have
> this rounding error with meshes. Meshes render faster, use much less
> memory and have smaller file sizes. Meshes also provide the ability to
> handle extra features, like smoothness and uv_mapping.
>
I think/hope that moving to triangles rather than polygons will avoid the
matrix inversion issue - haven't had enough leisure time to do this
experiment yet...
However, I think that moving to meshes would be rather more complex.  The
original data is imported into the CAD program (CYCAS, from www.cycas.de)
from a DXF file, and I guess the DXF represents the solid as individual
triangular or polygonal elements.  Going from this description to an
efficient mesh representation may be more effort than it's worth.  I'm only
trying to draw a bathroom, after all!

I'm now planning to write a simple program to analyse the POV input text
file and replace polygons by triangles where I can, in the hope that this
will fix the problem.

Overall I'm still surprised that POV can be flummoxed by such a simple
issue; it almost makes the polygon primitive unworkable.  A less
numerically-sensitive approach would be to define a polygon by defining the
(3D) origin of the plane, then two orthogonal vectors that lie in the
plane, then the points as linear combinations of those vectors.  That way
the points would be coplanar by definition.  I guess a further alternative
would be to define the plane through the first three points, then to
project all subsequent ones onto the plane.  But I'm a newcomer: perhaps
the polygon primitive just isn't designed to be used in this way at all.

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