Polyarea for noncontinuous shapes

2011 12 8
2 个回答
8 次查看（30 天）

0 个投票

Using a series of points, I would like to calculate the area the shape creates. For example, the following: x=(-.24 -.29 -.19 -.31 -.14 -.15 -.34 -.03 -.31 -.01 -.23 -.19 -.13 -.09) y=(-.01 -.07 -.06 -.05 -.03 0 .05 .07 .11 .15 .19 .21 .26 .27) doesn't graph as a continuous ellipsoid and as such the polyarea function doesn't work appropriately. I tried converting the points from cart2pol and back using pol2cart but that only creates a continual ellipsoid for shapes where the point (0,0) lies inside. For shapes where (0,0) does not lie inside (such as the example with the points given) this does not work. Can anyone suggest a way to correctly calculate the area of these? Thank you!

3 个评论
显示 1更早的评论 隐藏 1更早的评论

Sean de Wolski
Sean de Wolski 2011-12-8
It's not clear what you're after. Perhaps you could post a picture of the expectations (points/shapes etc.)?
In the mean time, I might recommend reading Steven Lord's reply in this old newsreader thread:
http://www.mathworks.com/matlabcentral/newsreader/view_thread/256591
Laura Kloepper
Laura Kloepper 2011-12-8
Ah, thank you for that Sean. I'm now able to reconstruct the curve but my problem of finding the area enclosed by the curve still exists. The final output for the curve according to the tutorial (xb and yb) are both 2xn and (as far as I understand it) polyarea only works with 1xn inputs. Is there a way around this?
Sean de Wolski
Sean de Wolski 2011-12-8
I don't follow on xb/yb being 2xn? They're lists of x/y coordinates right? So why shouldn't they be 1xn or nx1? I think this is a place where picture=='1000words';

请先登录，再进行评论。

请先登录，再回答此问题。

回答（2 个）

Laura Kloepper
Laura Kloepper 2011-12-8

0 个投票

When the original points listed are plotted, it looks like this: http://i40.tinypic.com/swz22p.jpg which has x and y values of 1x53 each.
When the curve is calculated with Delaunay triangulation it looks like this: http://i40.tinypic.com/2e5pikw.jpg This is ultimately what I'd like to calculate the area of.
But the xb and yb values are each 2x53, hence why the polyarea function won't work. Ideas?

2 个评论
显示 无 隐藏 无

Sean de Wolski
Sean de Wolski 2011-12-8
Are the rows of xb and yb identical? How were they generated from your above points?
Laura Kloepper
Laura Kloepper 2011-12-8
Yes. They are identical and were generated from the delaunay curve fitting tutorial you suggested. Also, when I run the exact script from the tutorial (#7) it generates 2x192 for both xb and yb.

请先登录，再进行评论。

Sean de Wolski
Sean de Wolski 2011-12-8

0 个投票

So you do want the convex hull! That simplifies it a lot.
x=[-.24 -.29 -.19 -.31 -.14 -.15 -.34 -.03 -.31 -.01 -.23 -.19 -.13 -.09];
y=[-.01 -.07 -.06 -.05 -.03 0 .05 .07 .11 .15 .19 .21 .26 .27];
tri = DelaunayTri(x',y');
[junk, the_area] = convexHull(tri);
Or you can skip the Delaunay triangulation all together and use the second output of convhull:
[junk, the_area] = convhull(x',y');

1 个评论
显示 -1更早的评论 隐藏 -1更早的评论

Laura Kloepper
Laura Kloepper 2011-12-8
Awesome! Thanks so much Sean. You have no idea how much this helps!

请先登录，再进行评论。

类别

在 帮助中心 和 File Exchange 中查找有关 Delaunay Triangulation 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by