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# Collision Detection Needs Fixing

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2 replies to this topic

### #1nes8bit  Members

Posted 21 February 2000 - 09:23 AM

I need help with my function In3DTriangle. It does not correctly check if the point you specify is in the triangle. I have a feeling that it has something to do with my dotproducts. This function checks if the point is in the triangle by creating 3 planes arround the triangle and checking if the point is on all the planes front sides. The other function below works. It is just there for readibility. Sorry for no HTML formatting. Function In3DTriangle(PointToCheck As D3DVECTOR, CircleRadius As Single, p1 As D3DVECTOR, p2 As D3DVECTOR, p3 As D3DVECTOR) As Boolean On Error Resume Next Dim PlaneNormals(2) As D3DVECTOR Dim OpposingVector As D3DVECTOR ''Check Bounding Circle First If Not Abs(GetPlaneDistance(PointToCheck, p1, p2, p3)) < CircleRadius Then Exit Function ''Do this for Side One OpposingVector.X = (p1.X + p3.X) / 2 OpposingVector.Y = (p1.Y + p3.Y) / 2 OpposingVector.Z = (p1.Z + p3.Z) / 2 PlaneNormals(0).X = p2.X - OpposingVector.X PlaneNormals(0).Y = p2.Y - OpposingVector.Y PlaneNormals(0).Z = p2.Z - OpposingVector.Z ''Do this for Side Two OpposingVector.X = (p1.X + p2.X) / 2 OpposingVector.Y = (p1.Y + p2.Y) / 2 OpposingVector.Z = (p1.Z + p2.Z) / 2 PlaneNormals(1).X = p3.X - OpposingVector.X PlaneNormals(1).Y = p3.Y - OpposingVector.Y PlaneNormals(1).Z = p3.Z - OpposingVector.Z ''Do this for Side Three OpposingVector.X = (p2.X + p3.X) / 2 OpposingVector.Y = (p2.Y + p3.Y) / 2 OpposingVector.Z = (p2.Z + p3.Z) / 2 PlaneNormals(2).X = p1.X - OpposingVector.X PlaneNormals(2).Y = p1.Y - OpposingVector.Y PlaneNormals(2).Z = p1.Z - OpposingVector.Z ''This is just to fix my vectors dx.VectorNormalize PlaneNormals(0) dx.VectorNormalize PlaneNormals(1) dx.VectorNormalize PlaneNormals(2) ''now check if it is in the triangle If dx.VectorDotProduct(PointToCheck, PlaneNormals(0)) > 0 And _ dx.VectorDotProduct(PointToCheck, PlaneNormals(1)) > 0 And _ dx.VectorDotProduct(PointToCheck, PlaneNormals(2)) > 0 Then In3DTriangle = True End If End Function Function GetPlaneDistance(PointToCheck As D3DVECTOR, p1 As D3DVECTOR, p2 As D3DVECTOR, p3 As D3DVECTOR) As Single On Error Resume Next Dim V1 As D3DVECTOR, V2 As D3DVECTOR, CP As D3DVECTOR Dim TempSingle As Single V1.X = p1.X - p2.X V1.Y = p1.Y - p2.Y V1.Z = p1.Z - p2.Z V2.X = p1.X - p3.X V2.Y = p1.Y - p3.Y V2.Z = p1.Z - p3.Z dx.VectorCrossProduct CP, V1, V2 TempSingle = (V1.X ^ 2 + V1.Y ^ 2 + V1.Z ^ 2) CP.X = CP.X / TempSingle CP.Y = CP.Y / TempSingle CP.Z = CP.Z / TempSingle GetPlaneDistance = dx.VectorDotProduct(PointToCheck, CP) End Function It is using DX7 with Visual Basic. Object DX is valid also.

### #2Spellbound  Members

Posted 21 February 2000 - 11:49 AM

Actually you're computing your PlaneNormals wrong, the way you do it only works if all sides are of equal length.

What you wan't to do is to compute the triangles normal first (like you do in GetPlaneDistance) then for each PlaneNormal do a crossproduct with the triangle normal and the edge. There is no need to normalize these as you only want to see if the point is inside or outside.

(I don't know any VB so I write it in pseudo code instead)

Compute the normals like this:

VECTOR E0, E1, E2, TriangleNormal;VECTOR PlaneNormals[3];E0 = P1 - P0;E1 = P2 - P1;E2 = P0 - P2;TriangleNormal = Normalize(CrossProduct(E0, E1));PlaneNormals[0] = CrossProduct(E0, TriangleNormal);PlaneNormals[1] = CrossProduct(E1, TriangleNormal);PlaneNormals[2] = CrossProduct(E2, TriangleNormal);

You must also do the testing like this:

VectorDotProduct(PointToCheck - P0, PlaneNormals(0)) > 0VectorDotProduct(PointToCheck - P1, PlaneNormals(1)) > 0VectorDotProduct(PointToCheck - P2, PlaneNormals(2)) > 0

Your GetPlaneDistance is also wrong in the last test:

GetPlaneDistance = DotProduct(PointToCheck - P0, CP)

Edited by - Spellbound on 2/21/00 5:52:35 PM

Edited by - Spellbound on 2/22/00 4:53:26 PM

### #3nes8bit  Members

Posted 22 February 2000 - 10:35 AM

I figured some calculation was kinda weird. Also I thought I replied to this post? Oh well. I was wondering if GetPlaneDistance was wrong. Is that what you are talking about? I tried and it works. Trust me. Anyhoo, I''ll continue trying to understand the new math concepts you posted for the next hour.

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