topic Re: Geometry Question - CircularArc2d in .NET Forum

Geometry Question - CircularArc2d

Anonymous — Wed, 05 Mar 2008 13:09:26 GMT

Given I have a valid CircularArc2d object:
CircularArc2d ca2d = new CircularArc2d(bulgevertex.Vertex, mPolygonLoop[j + 1].Vertex, bulgevertex.Bulge, false);

what is the formula for finding the midpoint on the arc?

... I am challenged geometrically.

dennis

Re: Geometry Question - CircularArc2d

Anonymous — Wed, 05 Mar 2008 15:38:16 GMT

This is in vba but it is the math you want.
[code]
Function MidPtofPolyArc(iSeg As Integer) As Variant

Dim P1 As Variant, P2 As Variant
Dim MidPt(2) As Double
Dim dBulge As Double
Dim Height As Double, Dist As Double
Dim deltaX As Double, deltaY As Double
Dim Slope As Double
Dim dLength As Double
Dim MidArc(2) As Double

P1 = oPline.Coordinate(iSeg)
P2 = oPline.Coordinate(iSeg + 1)

deltaX = P1(0) - P2(0)
deltaY = P1(1) - P2(1)

Dist = Length(P1, P2) / 2

If deltaY >= 0 Then
Dist = -Dist
End If

MidPt(0) = (P1(0) + P2(0)) / 2
MidPt(1) = (P1(1) + P2(1)) / 2
MidPt(2) = oPline.Elevation

dBulge = oPline.GetBulge(iSeg)
Height = dBulge * Dist

If deltaY = 0 Then
MidArc(0) = MidPt(0)
MidArc(1) = MidPt(1) + Height
ElseIf deltaX = 0 Then
MidArc(0) = MidPt(0) + Height
MidArc(1) = MidPt(1)
Else
Slope = deltaY / deltaX
Slope = -1 / Slope 'Invert slope for perpendicular bisector
'X = X1 + distance / dLength * DX
'proving for DeltaX=1 in slope direction
'slope=DeltaY / 1 => DeltaY=slope
dLength = Sqr(Slope ^ 2 + 1)
MidArc(0) = MidPt(0) + (Height / dLength) * 1
MidArc(1) = MidPt(1) + (Height / dLength) * Slope
End If

MidPtofPolyArc = MidArc

End Function

[/code]

Re: Geometry Question - CircularArc2d

jbooth — Wed, 05 Mar 2008 15:47:07 GMT

Try using the GetPointAtDist function (I might be wrong though).

I'm pretty sure there's a function in the curve class (CircularArc2d derives from it) that can help you.

Re: Geometry Question - CircularArc2d

Anonymous — Wed, 05 Mar 2008 16:22:33 GMT

Point2d midpoint = ca2d.EvaluatePoint( 0.5 );

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Given I have a valid CircularArc2d object:
CircularArc2d ca2d = new CircularArc2d(bulgevertex.Vertex, mPolygonLoop[j + 1].Vertex, bulgevertex.Bulge, false);

what is the formula for finding the midpoint on the arc?

... I am challenged geometrically.

dennis

Re: Geometry Question - CircularArc2d

Anonymous — Wed, 05 Mar 2008 19:51:17 GMT

thanks to all for responding ...

EvalutePoint() is exactly what was needed.

... but I am still challenged geometrically 😉

dennis