numRegions

R2026b

Number of regions in alpha shape

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Syntax

N = numRegions(shp)

Description

N = numRegions(shp) returns the number of distinct regions that make up the alpha shape. For an alpha radius of Inf, the alpha shape is the convex hull, and the number of regions is one. As the value of the alpha radius decreases the shape can break into separate regions, depending on the point set.

example

Examples

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Open Live Script

Create and plot a set of 2-D points.

th = (pi/12:pi/12:2*pi)';
x1 = [reshape(cos(th)*(1:5), ...
      numel(cos(th)*(1:5)),1);
      0];
y1 = [reshape(sin(th)*(1:5), ...
      numel(sin(th)*(1:5)),1);
      0];
x = [x1;x1+15;];
y = [y1;y1];
plot(x,y,".")
axis equal

Scatter plot of two circular point sets arranged in concentric rings, one centered near the origin and one centered near x=15

Create an alpha shape using an alpha radius of 7 and query the number of distinct regions in the shape.

shp = alphaShape(x,y,7);
nregions = numRegions(shp)
nregions = 
1

Use a smaller alpha radius of 2.5 to better capture the boundary and then retrieve the new number of distinct regions.

shp.Alpha = 2.5;
nregions = numRegions(shp)
nregions = 
2

Plot the alpha shape to check the boundary quality.

plot(shp)

Alpha shape with two filled circular regions computed with alpha radius 2.5, one centered near the origin and one centered near x=15

Input Arguments

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Alpha shape, specified as an alphaShape object. For more information, see alphaShape.

Example: shp = alphaShape(x,y) creates a 2-D alphaShape object from the (x,y) point coordinates.

Version History

Introduced in R2014b

See Also