ellipses on image

hi; I have a set of ellipses where each ellipse is defined by its center position (x,y) , an orientaion angle teta, major axe a and a minor axe b. i want to find a way in order to draw those ellipses on a given image* Img* . any idea please? thank you

[Image Analyst]

"Burn" it into the image pixels, or just display above the image in the overlay (without changing image pixel values at all)?

Answer 1

[Walter Roberson]

One way is to calculate a circle such as is described in the FAQ. Make it a unit circle as you first calculate it. Multiply the x coordinates by (a/2) and the y coordinates by (b/2). Then use hgtransform() to rotate it around the angle. Finally, add the center coordinates to the transformed results.

[Richard Brown]

almost easier to just do

theta = linspace(0, 2*pi, 100);
plot(a * cos(theta) + xc, b * sin(theta) + yc)

isn't it?

[Walter Roberson]

That sequence does not allow the ellipse to be rotated. Also, it would use "a" and "b" as the size of the semi-major and semi-minor axis rather than as the size of the major and minor axis (that's why I divided by 2)

[Richard Brown]

Sorry, didn't read the question properly:

phi = linspace(0, 2*pi, 100);
Q = [cos(theta), -sin(theta); sin(theta), cos(theta)];
X = bsxfun(@plus, [xc; yc], Q * [a/2 * cos(phi); b/2 * sin(phi)]);
plot(X(1, :), X(2, :))

... yes, your way may be cleaner :)

[Walter Roberson]

I have no objection to using a rotation matrix instead of hgtransform() :-)

Answer 2

[Ryan]

Use pdeellip (if you have the PDE toolbox):

 pdeellip(xc,yc,a,b,phi)

Where:

 (xc,yc) = centroid
 (a,b) = semiaxes
 phi = rotation of ellipse