# How to rotate imellipse

11 views (last 30 days)
erik on 23 May 2013
Answered: Tim Jackman on 25 Sep 2018
Hello fellow Matlab users,
I'm currently using the imellipse function to draw an ellipse on an image. I would like to rotate the ellipse interactively. However I cannot get this to work. Does anyone have experience with rotating ellipses? Please share, it would help me out a lot!
With kind regards,
Erik Groot Jebbink

Image Analyst on 23 May 2013
See my demo. It doesn't use imellipse() so you can't have handles to click and drag out new a size or angle. So you'd need to have a GUI with some sliders to allow the user to set new parameters for the major axis length, minor axis length, center location, and angle or orientation.
clc; % Clear the command window.
workspace; % Make sure the workspace panel is showing.
format longg;
format compact;
fontSize = 20;
% Parameterize the equation.
t = linspace(0, 360,1000);
phaseShift = 20;
xAmplitude = 2;
yAmplitude = 1;
x = xAmplitude * sind(t + phaseShift);
y = yAmplitude * cosd(t);
% Now plot the rotated ellipse.
plot(x, y, 'b-', 'LineWidth', 2);
axis equal
grid on;
xlabel('X', 'FontSize', fontSize);
ylabel('Y', 'FontSize', fontSize);
title('Rotated Ellipses', 'FontSize', fontSize);
text(-1.75, 1.4, 'Parametric --', 'Color', 'b', 'FontSize', fontSize);
% Now plot another ellipse and multiply it by a rotation matrix.
% http://www.maa.org/joma/Volume8/Kalman/General.html
rotationAngle = 30;
transformMatrix = [cosd(rotationAngle), sind(rotationAngle);...
-sind(rotationAngle), cosd(rotationAngle)]
xAligned = xAmplitude * sind(t);
yAligned = yAmplitude * cosd(t);
xyAligned = [xAligned; yAligned]';
xyRotated = xyAligned * transformMatrix;
xRotated = xyRotated(:, 1);
yRotated = xyRotated(:, 2);
hold on;
plot(xRotated, yRotated, 'g-', 'LineWidth', 2);
% Plot a line at 30 degrees
slope = tand(30);
x1 = min(x(:));
y1 = slope * x1;
x2 = max(x(:));
y2 = slope * x2;
line([x1 x2], [y1 y2], 'Color', 'r');
text(-1.75, 1.25, 'Rotation Matrix --', 'Color', 'g', 'FontSize', fontSize);
text(-1.75, 1.1, '30 Degree Line --', 'Color', 'r', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]); Tim Jackman on 25 Sep 2018
Beginning in R2018b, you can create an interactive ellipse ROI using the drawellipse function. This new ROI supports interactive rotation.

Matt J on 23 May 2013
I believe imellipse objects are 4 DOF only :-(

erik on 27 May 2013
Image Analyst, thank you for your contribution. I created a small gui with some sliders to move the ellipse around. YUKAi LIU on 10 May 2018