On Fri, 01 Jan 1999 22:30:24 -0500, John VanSickle
<van### [at] erolscom> wrote:

>Ronald L. Parker wrote:
>> 
>> On Thu, 31 Dec 1998 23:07:01 +0200, Margus Ramst <mar### [at] peakeduee>
>> wrote:
>> 
>> >Hi all, I'm once again having math trouble :(
>> >A macro I'm writing needs to calculate reflected vectors (it simulates
>> >reflected caustics in Superpatch, I'll probably post it soon...)
>> >However, the function I use to calculate the reflected vectors has 2
>> >shortcomings:
>> 
>> Why not determine the component of the ray that does not lie along the
>> 
>> normal vector, negate it, and add it back to the componenent that does
>> lie along the normal?  something like this (untested) code:
>> 
>> #macro mirror( R, N )
>>   #local nN = vnormalize( N );
>>   #local par = vdot( R, nN );
>>   #local perp = R-par*nN;
>>   par*nN-perp
>> #end
>
>The par*nN term needs to be doubled.  I posted the correct code earlier.

If you don't mind my asking... um... which one, and why?  The doubling

you seek is there, I think, in that if you put the last two lines
together you get par*nN-(R-par*nN) = 2*par*nN-R .  I was just trying 
to make it look a little less like magic and a little more like simple

math.

Or perhaps the difference can best be illuminated (heh) by some bad
ASCII art:

A
\   |
 \  |
  \ |
   \|
N---*
   /|\
  / | \
 /  |  \
/   |   \
R   P    B

Assuming all the vectors are pointing away from the * at the center,
and the line marked with a P is of course the reflecting surface, I
was trying to provide the equation for the ray marked 'A'.  I assume
you're assuming that R is pointing inward, in which case your answer
is correct for A.  With my assumptions (admittedly chosen to make my
job easier) and your equation, one would get B.  That's why I made
sure to make my assumptions about the parameters known:

>// This assumes that R is relative to the intersection point and
>// points AWAY from the intersection point.  This is probably not
>// the way you have it set up if you're using the SuperPatch.

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