Number of regions in alpha shape
N = numRegions( 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.
Find Number of Regions in 2-D Alpha Shape
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
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.