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Topic: create polygon coordinates
Replies: 5   Last Post: Jan 20, 2013 10:40 AM

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Roger Stafford

Posts: 5,892
Registered: 12/7/04
Re: create polygon coordinates
Posted: Jan 19, 2013 2:14 PM
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"Jessica" wrote in message <kdenag$nth$1@newscl01ah.mathworks.com>...
> I have coordinates such as this:
>
> 384 360 60 69
>
> that represent the left, top, width, and height of an ellipse. How can I turn these values into a list that specifies a polygon?
>
> I would like to use this polygon list to find out whether particular X and Y coordinates fall inside this ellipse:
>
> k_inside1= inpolygon(X_Coordinate, Y_Coordinate, X_Polygon3, Y_Polygon3);
>
> Thanks!

- - - - - - - - -
It seems a shame to approximate an ellipse with a polygon to see if points lie inside it, when a direct test with the simple equation of the ellipse would accomplish the same thing with much better accuracy and simplicity.

However, here are n vertices of a polygon approximating an ellipse. Let the ellipse be defined by the equation

(x-x0)^2/a^2 + (y-y0)^2/b^2 = 1

Then the vertices are given by:

t = linspace(0,2*pi,n); % <-- You choose n
x = a*cos(t)+x0;
y = b*sin(t)+y0;

Note: The spacing along the polygon is not uniform here. To do that would require the use of elliptic integrals.

Roger Stafford



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