Re: [Algorithms] Pack My Spheres Up Your Frustum

[Daniel R. <dan...@ho...>]

hi list, rehi pierre

For sure AABB collision tests are the fastest of all,
but spheres tend to be faster in building / maintaining
the arround dynamic objects (objects which rotate, or have
changing sub-parts).

And secondly VF and occlusion culling normaly CUTS a
ray or a plane through your BV, and handling spheres
and ray / plane intersections is much easyier than
handling boxes and ray / plane intersections.

Daniel "SirLeto" Renkel [D.R...@Fu...]


>From: "Pierre Terdiman" <p.t...@wa...>
>Reply-To: gda...@li...
>To: <gda...@li...>
>Subject: Re: [Algorithms] Pack My Spheres Up Your Frustum
>Date: Fri, 13 Oct 2000 14:05:37 +0200
>
> > My question would be why use spheres at all? It is more or less a myth
>that they are faster.
>
>
>100% agreed, BTW. Spheres are certainly not the fastest BV for basic
>intersection tests. For example, here's a sphere-sphere intersection test,
>from Magic:
>
>bool MgcTestIntersection (const MgcSphere& rkS0, const MgcSphere& rkS1)
>{
>     MgcVector3 kDiff = rkS1.Center() - rkS0.Center();
>     MgcReal fSqrLen = kDiff.SquaredLength();
>     MgcReal fRSum = rkS0.Radius() + rkS1.Radius();
>     MgcReal fRSumSqr = fRSum*fRSum;
>
>     return fSqrLen <= fRSumSqr;
>}
>
>And here's my AABB-AABB intersection code:
>
>   __forceinline bool    Intersect(const AABB& a)
>       {
>        float tx = mCenter.x - a.mCenter.x; float ex = a.mExtents.x +
>mExtents.x; if((IR(tx)&0x7fffffff) > IR(ex)) return false;
>        float ty = mCenter.y - a.mCenter.y; float ey = a.mExtents.y +
>mExtents.y; if((IR(ty)&0x7fffffff) > IR(ey)) return false;
>        float tz = mCenter.z - a.mCenter.z; float ez = a.mExtents.z +
>mExtents.z; if((IR(tz)&0x7fffffff) > IR(ez)) return false;
>        return true;
>       }
>
>When no intersection occurs, the AABB code may exit early, and in that case
>it's a clear winner (that's why the way Gomez wrote his Gamasutra code is
>*not* good IHMO). In the worst case, when two AABBs collide, there's 6
>floating-point operations involved, which is approximatively the same as 
>for
>the two first lines of the Sphere-Sphere test.
>
>The difference is probably not very important (I didn't use rdtsc on them 
>to
>check), but in many cases (if not all) the AABB code is just as fast, or
>even faster than the Sphere one. At the very least, you can't really say 
>the
>Sphere code is "faster". The assertion "spheres are faster", is IMHO wrong.
>
>So use the Carmack way, and never trust anything without checking it
>yourself (that's the demomaker way of life, BTW. One day I'll tell you the
>story of Niclas Thisell...)
>
>Needless to say, my preference goes to AABBs, especially since good 
>bounding
>spheres are so much difficult to compute (Dave, you know what I mean!).
>
>Pierre
>
>
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