I need to call up a vertex from polyshape command

Him im looking for a simple command or code to call up a vertex from a polyshape array.

Specifically I want to draw a line inside a polyshape that comes from one vertex to another vertex. I want to call these vertexs randomly then have Matlabe draw a line between them. my starting code is the following

pgon=polyshape([0,5*cosd(18),5*cosd(-54),-5*cosd(-54),-5*cosd(18)],[5,5*sind(18),5*sind(-54),5*sind(-54),5*sind(18)]);

plot(pgon)

Answer 1

[William Rose]

@Bruce Griffin

If you have a polygon pgon, of unknown size, get the number of vertices:

pgon=polyshape(cosd(0:40:320),sind(0:40:320));
plot(pgon); hold on; axis equal; axis tight
N=length(pgon.Vertices);
fprintf('Number of vertices=%d.\n',N)

Number of vertices=9.

Use N to select vertices randomly:

vpair=randi(N,[1,2]);  %return 2 random numers between 1 and N inclusive

Draw a line beteween the 2 vertices

plot(pgon.Vertices(vpair,1),pgon.Vertices(vpair,2),'-r')

The randomly chosen pair could be the same vertex twice, or adjacent vertices. You could add code to prevent this.

Good luck.

[William Rose]

@Bruce Griffin,

[Edit: Replace vpair(2)-vpair(1) with abs(vpair(2)-vpair(1)) in code below.]

Here is code that will assure that the "random" vertices are always at least one vertex apart:

delta=1; 
while delta<2, 
    vpair=randi(N,[1,2]); 
    delta=mod(abs(vpair(2)-vpair(1)),N); 
end

Use the code above in place of vpair=randi(N,[1,2]);

[William Rose]

@Bruce Griffin,

Putting it all together:

pgon=polyshape(cosd(0:20:340),sind(0:20:340));
plot(pgon); hold on; axis equal; axis tight
N=length(pgon.Vertices);
M=8;  % number of lines to draw between randomly chosen vertices
for i=1:M
    delta=1; 
    while delta<2, 
        vpair=randi(N,[1,2]); 
        delta=mod(abs(vpair(2)-vpair(1)),N); 
    end
    plot(pgon.Vertices(vpair,1),pgon.Vertices(vpair,2),'-r')
end

OK.

[Bruce Griffin]

Thank you very much. I appreciate your responce.