MATLAB: How to draw a ellipses with known foci

ellipses

Hi all,

I have a fixed foci in an image in Matlab, and I am wondering how to draw a ellipses with this known foci and get the pixels within the ellipses, and finally group these pixels into several sets according to their different distances to one foci point?

Thanks.

Best Answer

Joy, you can use the FAQ http://matlab.wikia.com/wiki/FAQ#How_do_I_create_an_ellipse.3F to get x and y coordinates, or use Joseph's code - either way. Then, to write into an image, you can just run through your x,y list:

for k = 1 : length(y);
  row = round(y);
  column = round(x);
  binaryImage(row, column) = true;
end

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1) if (x-x0)^2 + (y-y0)^2 < r^2, the point (x,y) is inside the circle,
2) if (x-x0)^2 + (y-y0)^2 == r^2, the point (x,y) is on the circle, and
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MATLAB: Define an ellipse with a given angle and find if a point lies inside its boundaries

The most easy approach is defining the ellipse by the two foci F1 and F2. Then an ellipse is the set of points, which sum of distances to F1 and F2 is 2*a, where a is the semi-major axis length. Convert this to the polar form to draw the ellipse, but use this to determine points inside:

P      = [x, y];  % A point in 2D
dist   = norm(P - F1) + norm(P - F2);
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Best Answer

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