In article <7k739u4hvrtp1um8nhc8n77d554ohj92ch@4ax.com>,
 Peter Popov <pet### [at] vipbg> wrote:

> Well, what I do is inverse-transform the point of intersection
> (accounting for translation) so that it is in the object's own
> coordinate sphere, then use a simple formula to determine the normal
> of a unit object,

This is the usual way of doing this part.


> then transform that normal back to global space (ignoring 
> translation, of course). This works. And of course it might just turn 
> out that I am doing what you're doing, only slower <g>. I need to 
> explore this better.

It doesn't work. Think about what it's doing...
Assume the translation is scale < 1, 0.5, 1>. When you transform the 
normal back into the global space as a direction, you end up halving the 
y component, making it more perpendicular to the y axis. This is the 
opposite of what you want to do...the normal should point *more* along 
the y axis.

And again with the sheared box...say you shear the +y axis in the +x 
direction. You inverse-transform the intersection point and compute the 
normal of an intersection on the top of the box as < 0, 1, 0>. Your 
method will shear this vector, when it should remain < 0, 1, 0>.

I don't fully understand why using the transpose here works, but it does 
in my own experiments, and POV seems to use it.

-- 
Christopher James Huff <chr### [at] maccom>
POV-Ray TAG e-mail: chr### [at] tagpovrayorg
TAG web site: http://tag.povray.org/

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