How to detect number of rotations in a trajectory?

Hello!

I have the x and y coordinates of a fish trajectory (nose tracking) within a tank, obtained from a video of 2 min. I am searching for an algorithm to detect the number of rotations of the fish within that period of time (being a rotation a change in direction of 360 degrees).

Do you have any suggestions about the best way to approach this?

[Les Beckham]

If you provide a sample of the data (save it to a .mat file and attach it here using the paperclip icon in the INSERT section of the question/comment editor) you will be more likely to get an answer.

Answer 1

[Supraja]

I understand that you want to count the number of rotations based on the x and y coordinates.

You can use functions “cross” and “circshift” to calculate the number of rotations.

Here are the documentation links for the same:

https://www.mathworks.com/help/matlab/ref/cross.html

https://www.mathworks.com/help/matlab/ref/circshift.html?s_tid=doc_ta

Sample code is attached below:

% Example x and y coordinates 
x = [0, 1, 2, 3, 4, 3, 2, 1, 9, -1, -2, -1, 0]; 
y = [0, 1, 2, 18, 4, 3, 2, 1, 0, -1, -2, -1, 0]; 

% Create vectors from consecutive points 
v = [x(2:end) - x(1:end-1); y(2:end) - y(1:end-1)]; 

% Calculate cross product between consecutive vectors 
cross_product = v(1, :) .* circshift(v(2, :), -1) - v(2, :) .* circshift(v(1, :), -1); 

% Count the number of sign changes 
num_rotations = sum(cross_product(1:end-1) .* cross_product(2:end) < 0); 

disp(['Number of rotations: ', num2str(num_rotations)]);