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POTheoretically-plausible collision sorting algorithm
 

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  1. POTheoretically-plausible collision sorting algorithm
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    <p>Say you have a bunch of objects to check for collision. Say you're an enterprising hobbyist programmer learning collision physics. Say you've heard of this marvelous thing where you sort by X, Y or Z position and the X, Y, or Z "worst-case" bounds which allows you to discard whole swathes of objects that "can't possibly" collide. (I haven't implemented swept/continuous collision yet, hence the quote marks).</p> <p>Once that was implemented on the X-axis, I added further Z and Y filters , sorting the X, Z and Y axis lists in turn and breaking when I reached an object that could not be collided with.</p> <p>That all worked rather well, although any speed increase from the last filter was negligible.</p> <p>Then I thought "If I sort them on the X-axis, on the Z-axis if that check returns equivalency, then on the Y-axis if that check returns equivalency, I should be able to toss out sorting the secondary lists." This would drop about 40% from the time taken.</p> <p>So I made it so. And it didn't work - Objects fell through the ground-plane. Other objects hit the ground-plane. It looked like the first set of objects fell through and all others bounced.</p> <p>So, two questions - Could this work and please check my code for errors:</p> <pre><code> bodies.Sort( (a, b) =&gt; { // Should sort the list in X, then Z, then Y order. int signX, signY, signZ; return (signX = Math.Sign((a.LocalPosition.X - a.Bounds.X) - (b.LocalPosition.X - b.Bounds.X))) == 0 ? (signZ = Math.Sign((a.LocalPosition.Z - a.Bounds.Z) - (b.LocalPosition.Z - b.Bounds.Z))) == 0 ? (signY = Math.Sign((a.LocalPosition.Y - a.Bounds.Y) - (b.LocalPosition.Y - b.Bounds.Y))) == 0 ? 0 : signY : signZ : signX; } ); Contact info; List&lt;BodyEnt&gt; checkBodiesZ = new List&lt;BodyEnt&gt;(); List&lt;BodyEnt&gt; checkBodiesY = new List&lt;BodyEnt&gt;(); for (int t = 0; t &lt; bodies.Count; ++t) { checkBodiesZ.Clear(); for (int t2 = t + 1; t2 &lt; bodies.Count; ++t2) { // If we've reached the last object we could collide with, exit. if (bodies[t].LocalPosition.X + bodies[t].Bounds.X &lt;= bodies[t2].LocalPosition.X - bodies[t2].Bounds.X) break; checkBodiesZ.Add(bodies[t2]); } // checkBodiesZ.Sort((a, b) =&gt; { return Math.Sign((a.LocalPosition.Z - a.Bounds.Z) - (b.LocalPosition.Z - b.Bounds.Z)); }); checkBodiesY.Clear(); for (int t2 = 0; t2 &lt; checkBodiesZ.Count; ++t2) { if (bodies[t].LocalPosition.Z + bodies[t].Bounds.Z &lt;= checkBodiesZ[t2].LocalPosition.Z - checkBodiesZ[t2].Bounds.Z) break; checkBodiesY.Add(checkBodiesZ[t2]); } // One of the two sorts I removed in this test. // checkBodiesY.Sort((a, b) =&gt; { return Math.Sign((a.LocalPosition.Y - a.Bounds.Y) - (b.LocalPosition.Y - b.Bounds.Y)); }); for (int t2 = 0; t2 &lt; checkBodiesY.Count; ++t2) { if (bodies[t].LocalPosition.Y + bodies[t].Bounds.Y &lt;= checkBodiesY[t2].LocalPosition.Y - checkBodiesY[t2].Bounds.Y) break; info = bodies[t].CheckCollision(checkBodiesY[t2]); // .CheckCollision() returns null if no collision detected. if (info != null) contacts.Add(info); } } </code></pre> <p>Thanks. :)</p>
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    1. USNarf the Mouse
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    1. COHave you tried stepping through your code to understand *why* it didn't work?
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    2. CO*Considers applying own head to wall* ...I think I've been coding too long today. I'll be back with results.
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    3. CO...Definitely too long. Can't think my way through a proper sorting algorithm for this. Any ideas?
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