> >
> > (As an unrelated side note, it is highly recommended to /not/ use
> > all-lowercase variable names; such names might be used as new keywords
> > in future versions without special notice.)
> >
>
> Thanks!
>
> Now I am afraid because I use lower case for variable names almost
> exclusively.
>
> :(

I too am afraid. Many of my local variables are all lower case, though my #macro
names do start with e.g. AA_......

Something that doesn't seem to be well known (or maybe clipka has warned against
it in the past) is if you #declare a variable before you pass it into a #macro
and change it using #local in the #macro, when the #macro finishes it retains
the changed value. Obviously (since clipka says so) you can pass back the 3rd
component of a 3 vector, but then you would have to unpack the <x,y>
intersection point from the <x,y,z> vector. I would just add a 3rd argument to
line_intersection() and use that as the switch.

#version 3.7;
#include "colors.inc"
global_settings { assumed_gamma 1.0 }
//
#macro line(p1, p2)
 #local A = (p1.y - p2.y);
 #local B = (p2.x - p1.x);
 #local C = (p1.x * p2.y - p2.x*p1.y);
 <A,B,-C>
#end

#macro line_intersection(l1, l2, unique_int_point)
 #local D  = l1.x * l2.y - l1.y * l2.x;
 #local Dx = l1.z * l2.y - l1.y * l2.z;
 #local Dy = l1.x * l2.z - l1.z * l2.x;
 #if (D != 0)
  #local out_x = Dx / D;
  #local out_y = Dy / D;
  #local out_value = <out_x,out_y>;
     #local unique_int_point = 1;
 #else
  #local out_value = <0,0>;
  #local unique_int_point = 0;
 #end
 out_value
#end

#declare p1 = <0,1>;
#declare p2 = <2,3>;
#declare p3 = <2,6>;
//#declare p3 = <2,3>;
#declare p4 = <0,4>;
#declare L1 = line(p1, p2);
#declare L2 = line(p3, p4);
#declare unique_int_point = 1;
//
#declare R  = line_intersection(L1, L2, unique_int_point);
//
#if(unique_int_point = 1)
  #debug concat("\n\n\n Intersection detected: <", vstr(2,R,",",0,-1),">\n\n\n")
  sphere{R 0.1 pigment{Blue}}
#else
  #debug concat("\n\n\n No single intersection point detected.\n\n\n")
#end
//
cylinder{p1 p2 0.05 pigment{Red}}
cylinder{p3 p4 0.05 pigment{Green}}
//
camera{orthographic location <0,0,-100> look_at <0,-0.00001,0> angle 12}
light_source{<0,0,-100> color rgb <1,1,1>  shadowless}
background{White}

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