# Find Joint Angle for Animation

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Daniel on 11 Jul 2012
I have a stick figure animation and would like to display joint angles on the plot... I am having trouble calculating the joint angles...
The angles appear to flip (ex change from 170 degrees to 10 degrees) and at times all together wrong...
Here is an example of my code... based on code I found online...
u1(:,1)=M1(:,2); u1(:,2)=M2(:,2); u1(:,3)=M3(:,2);
v1(:,1)=M1(:,1); v1(:,2)=M2(:,1); v1(:,3)=M3(:,1);
for i=1:length(u1);
CosTheta1(i,1)= dot(u1(i,:),v1(i,:))/(norm(u1(i,:))*norm(v1(i,:)));
ThetaInDegrees1(i,1)=acos(CosTheta1(i,1))*180/pi;
end
M1 is point #1, M2 is point #2 & M3 is point #3. Column 1 is Y data & Column 2 is X data. u1 & v1 are 3x1908. For my current file 'i' is is 1908.
I want to find the angle <123 with the vertex at M2.
Why am I getting wrong values? How do I prevent flipping?
Thanks ~Dan

Richard Brown on 12 Jul 2012
Edited: Richard Brown on 13 Jul 2012
You are taking the dot products of the wrong vectors! You need to take the vectors emanating from M2. The following should do the trick
u1 = M1 - M2;
v1 = M3 - M2;
% Easiest to normalise first
u1 = bsxfun(@rdivide, u1, hypot(u1(:, 1), u1(:, 2)));
v1 = bsxfun(@rdivide, v1, hypot(v1(:, 1), v1(:, 2)));
% Can compute the angle in one vectorised hit
theta_deg = acos(dot(u1, v1, 2)) * 180/pi;
edit I initially wrote the code for the M's being transposed. Fixed now.
edit2 Missing parenthesis added in last line
Richard Brown on 13 Jul 2012
oops! I wrote the code without actually testing it