Christopher James Huff wrote:
>In article <cja### [at] netplexaussieorg>,
> Christopher James Huff <cja### [at] earthlinknet> wrote:
>
>> The point will be in a plane perpendicular to a line between the two
>> known points and passing through the midpoint. You can easily calculate
>> this plane and find the intersection of the ray with it (with trace() or
>> by using the plane equation), but as I mentioned, with a line it can go
>> off to infinity, and with a ray there is sometimes no solution.
>
>#macro CompPoint(pA, pB, rayStart, rayDir)
>// The midpoint and plane normal.
>    #local midPt = (pA + pB)/2;
>    #local Norm = vnormalize(pB - pA);
>
>// Compensate for the plane not passing through the origin.
>    #local lRayStart = rayStart - midPt;
>
>// Solve for intersection distance with line.
>    #local T = (vdot(Norm, lRayStart)/vdot(Norm, rayDir))
>
>// If T is > 0, there is a ray intersection at rayStart + rayDir*T.
>// Otherwise, no intersection. You'll need to fill in the blank.
>#end
>
>Untested, I may have made an error, or trace() might be a better idea,
>it would certainly be simpler.


I think this line;
#local lRayStart = rayStart - midPt;

- should have been:
#local lRayStart = midPt - rayStart;


And there should be no need to normalize
the normal vector.

Here's how I would have written that macro:


#macro CompPoint(pA, pB, pRayStart, vRayDir)

    #local pMid = (pA + pB)/2;
    #local vNorm = pB - pA;

    #local T =
      vdot(pMid - pRayStart, vNorm)/vdot(vRayDir, vNorm);

    (pRayStart + T*vRayDir)

#end // CompPoint


And here's how I would test the ray:

#declare pHit = CompPoint(p0, p1, pRay, vRay);

#if (vdot(pHit - pRay, vRay) < 0)
  #debug "There is no point on the ray that has"
  #debug "equal distances to both points."
#end


But I have not tested my code either,
so I too could be wrong.

Tor Olav

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