[Radiance-general] Re: replmarks and mirroring

[Carsten Bauer]

Hi,

I think there might be some misunderstanding going on here..

-1) my joking comment about looking into the mirror is (sad but true ..) 
a bit oversimplified. Of course you always need to specify additional 
information, i.e., mirroring with respect to what, and although 
intuitively one always thinks of a plane, sometimes the 'inversion' 
(x,y,z) -> (-x,-y,-z) is also called a mirror operation,('mirroring with 
respect to the origin'). On inversion, the normal also gets turned around.

-2) I never used replmarks before, but if I get it right from the 
previous discussion, you set up a scene with triangles in CAD, convert 
this to Radiance format and then replace the triangles with, say, instances.

The information about orientation is completely contained in the 
triangles (as long as they are drawn according to the replmarks specs, 
i.e. three different side lengths, of which the second and third 
smallest stand for the local x and y axis. The mentioned cross product X 
(x) Y = Z then is in fact the surface normal of the triangle.

You can now make a simple sketch on paper, draw a triangle with sides 
parallel to the coordinate axes, and 'mirror' it with respect to the, 
say, yz plane. If you simply replace the vertices by their mirrored 
counterparts, you soon end up with an inverted normal, which is in fact 
not what you want.

Now I don't know how formZ stores surface normal information, maybe it 
is stored separately, allowing to override the above effect. And this 
information then gets lost in the conversion, so replmarks gets feeded 
the wrong triangle and thus transforms the instance equally wrong.

Maybe ... just a simple guess..

Anyway, I assume that the error already lies within the mirroring 
operation. You can check this by drawing a simple scene in CAD with two 
mirrored triangles and the look at the converted rad input file. (look 
at the ordering of the two sets of vertices in the polygon data)

-cb