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How to make Ellipse rotates around focus

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Haoang Sun
Haoang Sun 2020-3-11
回答： Peter O 2020-3-11
How to make Ellipse rotates around focus
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James Tursa
James Tursa 2020-3-11
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 ...?

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回答（1 个）

Peter O
Peter O 2020-3-11
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?

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