MATLAB: Set of points form a polygon

polygon detectionsub region finding

Hello,

Is there a way to tell if a set of vertices can form a closed polygon or not in genear or in matlab ?

Example:

[1,1], [3,1],[2,2] form a triangle when connected.

How do you conclude that these points can form a polygon.

Is there any algorithm to follow ?.

Thank you.

Best Answer

If your list of points are ordered of the boundary of the polygonal, you can detect if the list of points self intersect thus NOT represent a valid polygonal by using polyshape, catch the warning or get the number of regions, if it's larger than 1 it's not valid:

EDIT: you can also use ISSIMPLIFIED function

>> x=rand(1,5)
x =
    0.1419    0.4218    0.9157    0.7922    0.9595
>> y=rand(1,5)
y =
    0.6557    0.0357    0.8491    0.9340    0.6787
>> issimplified(polyshape(x,y,'Simplify',false))
ans =
  logical
   0 % <= NOT VALID

Related Solutions

MATLAB: How to use inpolygon function

A polygon is defined by its vertices(corner points), being pairs of (x,y) coordinates. In your case, these coordinates are stored in a single N-by-2 matrix.

polygon1= [0.1624   -0.6477;
         0.2247   -0.6477;
         0.2247    0.3874;    
         0.1624    0.5219]
P = rand(1,2)/2 % we will look at 1 point, but in poly can accept multiple points!
tf = inpolygon(P(:,1), P(:,2),polygon1(:,1), polygon1(:,2))
plot(polygon1([1:end 1],1), polygon1([1:end 1],2),'b.-',P(:,1), P(:,2),'r*') % a plot is often informative
if any(tf==1),
   title('Some point(s) is (are) in or on the edge of polygon 1') ;
else
   title('All points are outside polygon 1') ;
end

I hope this helps.

MATLAB: Does the “polyshape” function deletes the vertices in the input when there are no duplicated points or intersections

Please confirm if the "polyshape" function is used in the default way or not.

If it is used in a default way, it tries to detect and fix intersections, remove collinear points, remove overlapping points, etc.

After the release of MATLAB R2018b, there is a new name/value pair for polyshape(), "KeepCollinearPoints"=true/false, for the keeping the collinear points purpose. By default 'KeepCollinearPoints' is set to false.

If you want to keep the collinear points then you can set 'KeepCollinearPoints' to true.

As of MATLAB R2018a, the reason for the "polyshape" function to delete half the total number of vertices in the input when there are no duplicate points or intersections is most likely as the input vertices contain collinear points.

Regarding the other query, if there is a way to set an absolute threshold distance so that the function can treat the close points as duplicates, currently, there is no such threshold value.

However, as a workaround, you can call the "polyshape" function with a name/value pair, as follows:

>> p2 = polyshape(points(path,1), points(path,2), 'simplify', false);

This will keep all the input points without simplifying it, as follows:

p2 =

polyshape with properties:

Vertices: [142×2 double]

NumRegions: 1

NumHoles: 0

You can use the "simplify" function, "p3 = simplify(p2)", later if you need to simplify the input data. Please note that "p3" will be the same as polygon in the script.

Best Answer

Related Solutions

MATLAB: How to use inpolygon function

MATLAB: Does the “polyshape” function deletes the vertices in the input when there are no duplicated points or intersections

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