area of curve?

Hi
had trouble understanding how to get the area of curve - found an example for a brep volume in the primer but it seems to work differently than for curves
I did the following:

For Each crv As oncurve In x
Dim centroid As New on3dpoint
Dim normal As New On3dVector
Dim cMass As New OnMassProperties
Dim area As New Double
crv.AreaMassProperties(centroid, normal, cMass)
area = cMass.Area()
print(area)
Next

but the areas are coming out as 0...

I feel like it's because I need to be giving the centre and normal vectors but I don't know these offhand - I assumed that these sorts of things are generated by the method. Any ideas?

Replies to This Discussion

Permalink Reply by Dirk Anderson on June 10, 2009 at 9:54pm

I've got no idea why the area calculation is slightly different to one generated by a component (or Rhino caclulation) - it's also the same for a surface VB area calc.
I've set up a basepoint at the start of the curve and used a static z vector as a plane input.

Sub RunScript(ByVal inCrv As OnCurve)
Dim tol As Double = RhUtil.RhinoApp().ActiveDoc().AbsoluteTolerance()

Dim bBox As New OnBoundingBox
inCrv.GetBoundingBox(bBox)

Dim bPt As New On3dPoint
bPt = inCrv.PointAt(0)

Dim normal As New On3dVector(0, 0, 1)
Dim cMass As New OnMassProperties
Dim area As New Double
inCrv.AreaMassProperties(bPt, normal, cMass, Nothing, Nothing, tol)
area = cMass.Area()

A = area
End Sub

Permalink Reply by Rajaa Issa on June 11, 2009 at 4:40pm

Dirk solution is good. the one thing I would change is to use bBox.Center() as "Center" point for the AreaMassProperty method.
Also, if input curves are on world xy_plane, then using zaxis as "Normal" is appropriate, otherwise I woud find curve plane and use the plane normal instead.

Permalink Reply by Dirk Anderson on June 11, 2009 at 6:28pm

Sorry... yep, the BBox was a test to check if the area calcs were different. Rajaa, would you know why the area calcs differ between (Rhino or GH Area Component) and (VB area calculation)?

Permalink Reply by Rajaa Issa on June 12, 2009 at 6:10am

Hi Dirk,
Can you send me a sample file with curves and sample code you used to compare?

Permalink Reply by Dirk Anderson on June 14, 2009 at 6:30pm

Rajaa, thanks very much for looking at this :D

Attachments:

crvAreaVB.3dm, 45 KB
crvAreaVB.ghx, 22 KB

Permalink Reply by Rajaa Issa on June 25, 2009 at 1:53pm

Hi Dirk,
Sorry it took sometime to get back to you.

We found a bug in the low level curve area calculation in some special cases. This will be fixed. For now, please use the attached work around to get accurate result.

Thanks for your report.

Attachments: