How can I find the angle between two vectors, including directional information?
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Sebastian Echeverri on 24 Feb 2015
Hello, I am a graduate student, and I am working on a script that tracks the position of animals during a courtship. I have position data in the form of XY coordinates from two points on each animal's body taken from top down filming. I use these two points to create a vector that defines the animal's orientation. My script needs to calculate the angle between these two vectors, but also include directional information - IE, go from -180 through 0 to 180 degrees, depending on where the vectors are placed (see image).
This is the code that I currently have. It gives me the desired angle (I believe), but is NOT directional. 60 degrees to either side spits out as 60 degrees no matter which it is.
angle_maleToFemale_radians = acos(dot(maleFemaleVector,femaleVector)/(norm(maleFemaleVector)*norm(femaleVector))); angle_maleToFemale_degrees(index) = radtodeg(angle_maleToFemale_radians); angle_maleToFemale_degrees(index) = 180 - angle_maleToFemale_degrees(index);
Roger Stafford on 24 Feb 2015
Edited: Roger Stafford on 24 Feb 2015
If v1 = [x1,y1] and v2 = [x2,y2] are the components of two vectors, then
a = atan2d(x1*y2-y1*x2,x1*x2+y1*y2);
gives the angle in degrees between the vectors as measured in a counterclockwise direction from v1 to v2. If that angle would exceed 180 degrees, then the angle is measured in the clockwise direction but given a negative value. In other words, the output of 'atan2d' always ranges from -180 to +180 degrees.
One further observation: Besides the greater range of 'atan2d' as compared with 'acosd', the former does not suffer the inaccuracies that occur with 'acosd' for angles near zero and 180 degrees.