90° clockwise rotation of 2D coordinates

Hi. I cannot understand why the 90° clockwise rotation of the attached 2D coordinates (matrix.txt) is not done.

I have tried two different ways but I do not get the desired result.

1st way (using "rot90"):

matrix = importdata('matrix.txt');
Rmatrix = rot90(matrix,3);       % I have to rotate 3 times 90° counterclockwise
figure
plot(Rmatrix(1,:), Rmatrix(2,:), 'k.')
axis equal
xlim([0 512]);
ylim([0 512]);

2nd way (using rotation matrix, more convenient):

matrix = importdata('matrix.txt');
theta = 90;
R = [cos(theta) -sin(theta); sin(theta) cos(theta)];
Rmatrix = R*matrix';
figure
plot(Rmatrix(1,:), Rmatrix(2,:), 'k.')
axis equal
xlim([0 512]);
ylim([0 512]);

[Voss]

rot90 is not for this. Use the rotation matrix approach.

Remove the xlim and ylim lines of code to see the result, and go from there.

[Adam Danz]

As @Voss mentioned, rot90 rotates the array of data. Look how this 4x3 matrix turns into a 3x4 matrix and the top row moves to the first column and it gets fliped as if you moved your monitor 90deg counter clockwise.

x = randi(9,4,3)

x = 4×3

3 5 8 2 2 5 1 4 3 5 6 2

rot90(x)

ans = 3×4

8 5 3 2 5 2 4 6 3 2 1 5

What is the problem with your second method using the rotation matrix?

Answer 1

[Matt J]

2nd way (using rotation matrix, more convenient):

Your implementation of this has several problems:

1. You try to rotate by 90 degrees once, instead of 3 times
2. Your rotation angle theta is expressed in degrees instead of radians
3. You are rotating about the origin, which means the rotated coordinates cannot lie in the range [0,512].

Here's what you might have intended:

matrix = importdata('matrix.txt');
theta = 90*3;
R = [cosd(theta) -sind(theta); sind(theta) cosd(theta)];

c=mean(matrix,1);

Rmatrix = (matrix-c)*R'+c;

plot(matrix(:,1), matrix(:,2), 'k.'); hold on
plot(Rmatrix(:,1), Rmatrix(:,2), 'r.'); hold off
axis equal

Answer 2

[Matt J]

matrix = importdata('matrix.txt');

p=polyshape(matrix(:,1), matrix(:,2),'Simplify',1);

Warning: Polyshape has duplicate vertices, intersections, or other inconsistencies that may produce inaccurate or unexpected results. Input data has been modified to create a well-defined polyshape.

[cx,cy]=centroid(p);

prot=rotate(p,90*3,[cx,cy]);

plot([p,prot]); axis equal