rotating a line using rotation matrix

I have this function thats supposed to rotate a 2x2 matrix (arm) by theta.

I think i have to multiply the arm matrix by a rotation matrix to be given the new arm but i can't figure out how to do it. I keep getting error messages saying that arm and the rotation matrix is of different sizes.

Answer 1

[KSSV]

L = rand(2,2) ;  % line 
% Rotation matrix 
R = @(theta) [cos(theta) -sin(theta) ; sin(theta) cos(theta)] ;
% Get mean 
m = mean(L) ;
L1 = m+(L-m)*R(pi/2) ; % rotate line by 45 degrees

plot(L(:,1),L(:,2),'r',L1(:,1),L1(:,2),'b')

[Paul Jackson]

I tried applying it like this

R = [cos(theta) -sin(theta);sin(theta) cos(theta)];

m = mean(arm);

[arm_rot]=m+(arm-m)*R(theta);

I got the error message

Error using *

Incorrect dimensions for matrix multiplication.

[KSSV]

What is size of arm? It should be compatible to multiply with 2*2 rotation matrix.

[Paul Jackson]

I cant actually see it but i just checked the dimensions using height and width command and it turned out to be height=2 and width=10

how would you apply the rotation to this?

[KSSV]

Transpose it before finding mean and transpose after calculations are done.

[Walter Roberson]

arm.' * R

would be 10 x 2 * 2 x 2, giving a 10 x 2 result. You could then transpose that to 2 x 10.

(arm.' * R).'

You might also be able to just use

R * arm

[Paul Jackson]

That worked, thanks guys!