How to make Ellipse rotates around focus

How to make Ellipse rotates around focus

[James Tursa]

You rotate all of the points on the ellipse. I think we need more details from you. Do you have an equation, a set of points, or ...?

Answer 1

[Peter O]

Haoang,

Are you looking to have the ellipse rotate in a plot?

You might be interested in applying a rotation matrix. See:

https://en.wikipedia.org/wiki/Rotation_matrix

If you have the (x,y) points of your ellipse, you can multiply them by the rotation matrix for each frame or angle of interest.

For instance, (this is pseudocode -- it won't run without having the xy points, and it's possible to be a little more clever to remove the inner loop with arrayfun and broadcasting.)

my_ellipse_xy % an Nx2 set of x,y points
angles = linspace(0,2*pi)
origin_x = 1.0
origin_y = 2.1
x = my_ellipse_xy(:,1) - origin_x
y = my_ellipse_xy(:,2) - origin_y
for ix=1:numel(angles)
    q = angles(ix)
    R = [cos(q), -sin(q);
        sin(q), cos(q)]
    ellipse_rot = zeros(numel(x),2)
    for jx=1:numel(x)
        % Apply the rotation at this angle.
        % The transpose operation gets it into the Nx2 rows again.
        % (2x2)*(2x1) is a column vector otherwise.
        % If you use a 2xN approach, the transpose is not needed.
        ellipse_rot(jx,1:2) = transpose(R*[x(jx);y(jx)])
    end
    %do something with the rotated vector here
end

Does this help?