Roland Mas <mas### [at] acratrcojp> wrote in article
<m3s### [at] rpc66acratrcojp>...

>   I confirm my position.  The handedness of a coordinate system is given
by
> the way it calculates cross-products, not by the way it looks because it
> has no canonical way to look...
> 
>   Other suggestions?

The basic coordinate system used by POV-Ray (and many other renderers,
contrary to an earlier assertion that POV-Ray was somehow unique) is
definitely left-handed.  Also, inverting one of the coordinate axes of the
camera does transform the world from right to left handed.  The only
coordinate system of interest is that of the observer, which is defined by
the camera.

The rational for a left-handed coordinate system in computer graphics is
pretty easy to understand.  Given a camera sitting at <0,0,0> and looking
along the positive Z axis, larger Z values correspond to larger depth
values.  So, when removing hidden surfaces, I simply keep the one with the
smallest Z value.  For non-raytracers, the camera is almost always
transformed into this canonical orientation before rendering.

Why would a left-handed cross product look like a right-handed one?  Easy -
right thumb along one positive axis, fingers along a second positive one. 
Fingers curl in the direction of the third (in the positive direction). 
Look into a mirror as you do this.  It still works, but now it's a left
hand doing the pointing and curling.  It's an excellent example of parity,
since someone in a mirror universe would describe their mathematics in
exactly the same terms as you.

The biggest problems with left vs. right have to do with importing models. 
If a different coordinate system was used, the model could be mirrored,
rotated, or both. 

Xander

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