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

Yep, I misunderstood how the last two lines work together, but now that
I do, 2*par*nN-R is wrong by a factor of -1.



> 
> 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,

Which is wrong, because the original ray is supposed to be *incoming*
(see the original post).

And it doesn't matter if the direction is reversed on the N vector;
multiplying N by <-1,-1,-1> causes par to be negated, which undoes the
negative scaling of N when par and nN are multiplied together.

Regards,
John

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