Rotation Matrix calculates by column not by row
- by pinnacler
I have a class called forest and a property called fixedPositions that stores 100 points (x,y) and they are stored 250x2 (rows x columns) in MatLab. When I select 'fixedPositions', I can click scatter and it will plot the points.
Now, I want to rotate the plotted points and I have a rotation matrix that will allow me to do that.
The below code should work:
theta = obj.heading * pi/180;
apparent = [cos(theta) -sin(theta) ; sin(theta) cos(theta)] * obj.fixedPositions;
But it wont. I get this error.
??? Error using == mtimes
Inner matrix dimensions must agree.
Error in == landmarkslandmarks.get.apparentPositions at 22
apparent = [cos(theta) -sin(theta) ; sin(theta) cos(theta)] * obj.fixedPositions;
When I alter forest.fixedPositions to store the variables 2x250 instead of 250x2, the above code will work, but it wont plot. I'm going to be plotting fixedPositions constantly in a simulation, so I'd prefer to leave it as it, and make the rotation work instead.
Any ideas?
Also, fixed positions, is the position of the xy points as if you were looking straight ahead. i.e. heading = 0. heading is set to 45, meaning I want to rotate points clockwise 45 degrees.
Here is my code:
classdef landmarks
properties
fixedPositions %# positions in a fixed coordinate system. [x, y]
heading = 45; %# direction in which the robot is facing
end
properties (Dependent)
apparentPositions
end
methods
function obj = landmarks(numberOfTrees)
%# randomly generates numberOfTrees amount of x,y coordinates and set
%the array or matrix (not sure which) to fixedPositions
obj.fixedPositions = 100 * rand([numberOfTrees,2]) .* sign(rand([numberOfTrees,2]) - 0.5);
end
function obj = set.apparentPositions(obj,~)
theta = obj.heading * pi/180;
[cos(theta) -sin(theta) ; sin(theta) cos(theta)] * obj.fixedPositions;
end
function apparent = get.apparentPositions(obj)
%# rotate obj.positions using obj.facing to generate the output
theta = obj.heading * pi/180;
apparent = [cos(theta) -sin(theta) ; sin(theta) cos(theta)] * obj.fixedPositions;
end
end
end
P.S. If you change one line to this: obj.fixedPositions = 100 * rand([2,numberOfTrees]) .* sign(rand([2,numberOfTrees]) - 0.5);
Everything will work fine... it just wont plot.
