Center of a polyline

Rhino Developer
1

Sorry but yet another thing that’s bugging me and that I can’t figure out.
The developers kit lists this method for polylines:
CenterPoint Compute the center point of the polyline as the weighted average of all segments.

Here is a polyline set at the origin (the center is a point placed at the center Osnap):
Poly1

Here is the excel calculation for the 8 line segments:
Poly2

So why does Rhino show (2, 2.25) for the centre and excel has (2.11, 2.39)?

SteveOh

2

Hello - can you please post the curve? (I believe Cen osnap averages the vertices on the polyline - see what AreaCentroid gets you.

Yeah… confirmed, at least for Cen Osnap, not sure of the function you are using.

image
image745×475 12.5 KB

Polyline.CenterPoint() is more accurate than Cen osnap but still doing some averaging and not calculating the area centroid.

image
image799×464 7.26 KB 

    mp=Rhino.Geometry.AreaMassProperties.Compute(pLineCrv, .001)
    cenPt = mp.Centroid

-Pascal

3

Pascal,

Thanks for clearing that up. I thought the picture of the curve was in the post but here it is again.

image001.png

Since the method indicated a “weighted average of all segments”, I was multiplying the segment length by the x and y positions of the segment centres. Summing
them up and dividing by the polyline length. I’ll have to think about how/why/when I might use the average of the segment endpoints. But at least I now know what the calculation is doing!!

SteveP

4

Sorry, bigger brains, I did not notice this was on ‘Rhino Developer’, I thought it was on Rhino. I’ll butt out…

-Pascal