# ellipses on image

3 views (last 30 days)
thoraya on 20 Jun 2012
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 on 8 Jul 2012
"Burn" it into the image pixels, or just display above the image in the overlay (without changing image pixel values at all)?

Walter Roberson on 8 Jul 2012
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 on 9 Jul 2012
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 on 9 Jul 2012
I have no objection to using a rotation matrix instead of hgtransform() :-)

Ryan on 9 Jul 2012
Edited: Ryan on 9 Jul 2012
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