Quote:Original post by markwwRemember that (in C++ at least), the return value of atan2() is in radians.
Hmm if it's from the positive x-axis, then shouldn't:atan2(4.0, -0.3);
be greater than 90 degrees since it is already in quadrant II? I'm probably mis-using it -
angle in a circle
Quote:Original post by OrenGL
There is a 2d version of the cross product that gives a scalar... Oh my rusty brain...
It's called "perpdot".
For two vectors a and b,
a dot b = |a| * |b| * cos(angle)
And you know the lengths of the vectors (=radius).
Therefore,
cos(angle) = (a dot b) / r2
So you can find the angle using the acos() function.
Edit: Sorry didn't notice that one of the vectors is always along the X axis. In that case the atan2() methods are perfect. You can use the above method using cosines if you need to find the angle between any two points on the circle.
[Edited by - Verminox on October 15, 2007 9:00:55 AM]
a dot b = |a| * |b| * cos(angle)
And you know the lengths of the vectors (=radius).
Therefore,
cos(angle) = (a dot b) / r2
So you can find the angle using the acos() function.
Edit: Sorry didn't notice that one of the vectors is always along the X axis. In that case the atan2() methods are perfect. You can use the above method using cosines if you need to find the angle between any two points on the circle.
[Edited by - Verminox on October 15, 2007 9:00:55 AM]
The "acos of the dot product" solution has the advantage that it works for vectors in more dimensions. It has the disadvantage that it doesn't give you a sign.
If you are familiar with complex numbers, you can represent both points by complex numbers (representing point (x,y) as x+iy) and then the ratio of both numbers is a complex number whose argument is the angle you are looking for. Again, use atan2() to extract the argument.
Oh, and you should get in the habit of using radians for everything, at least internally in your programs. If you ever need to display the angle for a user to see, then you can multiply it by 180/PI before you show it.
If you are familiar with complex numbers, you can represent both points by complex numbers (representing point (x,y) as x+iy) and then the ratio of both numbers is a complex number whose argument is the angle you are looking for. Again, use atan2() to extract the argument.
Oh, and you should get in the habit of using radians for everything, at least internally in your programs. If you ever need to display the angle for a user to see, then you can multiply it by 180/PI before you show it.
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