Calculate direction angle from two vectors?

Question

Say I have two 2D vectors, one for an objects current position and one for that objects previous position. How can I work out the angular direction of travel?

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Answer 1

If you're in C (or other language that uses the same function set) then you're probably looking for the atan2() function. From your diagram:

double theta = atan2(x1-x, y1-y);

That angle will be from the vertical axis, as you marked, and will be measured in radians (God's own angle unit).