I have two macros that calculate the intersection of two lines and 
return a 2D vector <x,y>.


################################################


#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)
	#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>;
	#else
		#local out_value = false;
	#end
	out_value
#end

#declare L1 = line(<0,1>, <2,3>);
#declare L2 = line(<2,3>, <0,4>);
#declare R = line_intersection(L1, L2);
//#if (R != false)
     #debug concat("\n\n\nIntersection detected: <", vstr(2,R,",",0,-1), 
">\n\n\n")
//#else
//    #debug concat"No single intersection point detected")
//#end


################################################


Normally this works okay, but it is possible for the macro to fail if 
the lines are coinciding. But, how can I signal a failure? I tried 
passing the result to a conditional statement, but POVRay then complains 
that it needs a float value but got a color value. There also is no 
"null" or "undefined" value in POVRay SDL. I would appreciate some tips, 
thanks.


Mike

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