MATLAB Answers

0

Rotate rectangle x number of degrees when using imrect or alternative function

Asked by Right Grievous on 5 Apr 2014
Latest activity Edited by Image Analyst
on 5 Apr 2014
Hi everybody,
I would like to allow a user to draw a rectangle over a plot to highlight some data. This seems entirely possible using imrect, however I sometimes need the rectangle to be rotated 60 degrees to the x axis. I can't find any mention of rotation in the help or by using get(h).
I've looked for some help on this function but strangely it doesn't seem to be widely used. Does anybody know how to rotate a rectangle when using imrect, or are there alternatives which do allow it?
Thanks,
Rod.

  0 Comments

Sign in to comment.

1 Answer

Answer by Image Analyst
on 5 Apr 2014
 Accepted Answer

You can get the vertices of the corners. Then use the rotation matrix (see Wikipedia) to rotate it about its center by any angle. Then redraw the rectangle with plot().

  2 Comments

I think I'm going to use impoly instead and manually calculate the coordinates for the rotated rectangle. I only have to calculate this once and then I can have the same polygon created on the figure each time - the user can move it into position.
I'm still at a loss as to why imrect does not have an input for orientation. Did Mathworks not foresee someone wanting a rectangle that was maybe not parallel to the axes?
My next challenge is rotating the contents of the polygon, so I will probably use your suggestion for that purpose. It seems awfully complicated, is there a Matlab function for a rotation matrix or does it have to be coded manually?
Thanks,
Rod.
Rod, it's trivial. Just a 2 by 2 matrix [cos, -sin, sin, cos] multiplied by the "old" coordinates. See the attached demo to rotate an ellipse, and a star.
rotationArray = [cos(rotationAngle), -sin(rotationAngle); sin(rotationAngle), cos(rotationAngle)];
% Rotate the diagram
rotatedArray = vertices * rotationArray;

Sign in to comment.