Le 27/12/2012 19:28, Albun nous fit lire :
> 
> Well, i try to change color with  vector's comparison. Subtraction , or equal
> can't give a result to change color.
> And  substraction (Pos_-Pos_compare) can't be recognize. I want to change color
> when the two vectors are the same, in substraction equal to 0 ,=0.
> I use trace() for one vector.
> 

Caveat: 3D vectors are using floating point arithmetic, strict equality
is unlikely to occurs due to noise on floating point operations. (e.g. :
A=<2,4,5>/10; B=<2,4,5>/20; C=B+B+B; D=A+B; ... does D == C ? not sure!)

I would recommend computing the square length of the difference, and
testing it for "small enough" (not =0).

#declare DeltaLenSquared = vdot((Pos_ - Pos_compare),(Pos_ - Pos_compare));
#if (DeltaLenSquared < 0.01)

If you want slower (due to a square root):

#declare DeltaLen = vlength(Pos_ - Pos_compare);
#if (DeltaLen < 0.1)


> ////
> 
> #declare PointA = VEq(Pos_,Pos_compare);
> 
> 
> sphere{Pos_,.25
>  #if (0=PointA)
>  pigment{White}
> #else
>  pigment{Gray30}  #end }
> 
> 
> ////
> 
> Thanks so
> 
> 
> 
>

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