How to plot the exterior edges only of an alphaShape

I am trying to plot an alphaShape using the code below

x = rand(10,1)*5;
y = rand(10,1)*5;
shape = alphaShape(x,y);
alphas = alphaSpectrum(shape);
shape.Alpha = alphas(1);
ax = app.mapPlt;
plot(shape,'Parent',ax,'EdgeColor','none','LineStyle','--','LineWidth',0.75);

When I plot this shape, the exterior edges are blank. Changing the last line to

plot(shape,'Parent',ax,'EdgeColor','black','LineStyle','--','LineWidth',0.75);

will plot the exterior esdges but it will also plot the the interior edges. Is there a way to plot the exterior edges only?

Answer 1

[John D'Errico]

When you have a problem like this, learn how to investigate what you can do with the class.

x = rand(10,1)*5;
y = rand(10,1)*5;
shape = alphaShape(x,y);
>> methods(shape)

Methods for class alphaShape:

alphaShape          alphaTriangulation  boundaryFacets      inShape             numRegions          plot                volume              
alphaSpectrum       area                criticalAlpha       nearestNeighbor     perimeter           surfaceArea         

methods tells you what methods have been provided. Here, do any of them sound interesting? I hope so.

edges = boundaryFacets(shape)
edges =
     1     4
     4     8
     8     6
     6    10
    10     3
     3     9
     9     2
     2     5
     5     1
     
plot(x(edges),y(edges),'-ro')
hold on
plot(x,y,'.')

So a fairly boring alpha shape, since all but one of the points were on the perimeter.

[Mustafa Al Homsi]

Thanks John for the reply and for pointing out the "method'" trick. I usually have to go to the documentation to access this information. This way is more convenient.

Now regarding plotting the edges of an aplhaShape, I am familiare with the approch you mentioned above but this approch end up plotting an array of points. What I really want, which maybe I should have stressed in my question, is to still plot the alphaShape but only show the perimiter edges and not the interior edges. This is becuase later in the code, I take adavantage of alphaShape patch plotting properties such as "FaceColor" and "FaceAlpha". Do you know of a way to do this?

[John D'Errico]

Is that not exactly what I did? I plotted only the perimeter edges of the shape, and they were indeed ploted as edges. So what is the problem? Are you confused that I then plotted the original point set on top of the perimater too? What do you think the red lines are in the figure as plotted?

[Mustafa Al Homsi]

I need to fill in the inside of the shape and not leave it blank. I usually do that by setting the "FaceColor" patch property when I plot it as an alphaShape but with this approch I can't do that unless you know of another way to do it.

[Mustafa Al Homsi]

To be more specific, here is the code I am using

x = rand(10,1)*5;
y = rand(10,1)*5;
shape = alphaShape(x,y);
alphas = alphaSpectrum(shape);
shape.Alpha = alphas(1);
plot(shape,'EdgeColor','none','FaceAlpha',0.4,'FaceColor','black','LineStyle','--','LineWidth',0.75);

and this code generates the plot below with no edges

If I remove the 'EdgeColor' specifcation, the code will plot all edges on the perimiter and inside the shape as shown below

I obviously can plot the alphaShape with no edges and then just plot the perimeter edges as an array of points like you have shown. I just thought there might be a more straightforward way to manipulate the alphaShape plot command.

[John D'Errico]

You know how to plot the shape, filled, with NO edges.

Do you know how to then add a second set of edges on top?

help hold

So just then add the perimeter edges on top. Make them any color you wish.

[Román Comelli]

Is there any easy way to do this with 3D polytopes?