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You have an entity A that you want to turn towards another entity B. One way to determine the direction to turn is to compute the cross-product of the direction vector of A with the vector from AB. If the value of z is positive, then you turn right. If the value of z is negative, you turn left. Is this the only way of determining which way to turn?

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This is pretty much the standard way to determine direction. Speed and the like are of course also factors, and if you can translate the problem to 2D there are more methods. Also, direction to turn might depend on current state (like you don't want jitter around the 0 degree line). Any specific reason you'd need another method? There are other ways (like simple trig - sin = opposing side length/sloped side length), but in the end they all boil down to calculating the angle's direction. – Kaj Aug 11 '10 at 17:35
up vote 5 down vote accepted

If you're storing your rotations as quaternions (which you probably should), you could use a quaternion lerp function to lerp from your initial rotation to the rotation you want to be at (i.e the rotation given by the vector AB with whatever your up vector is).

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It sounds like this is essentially a 2D problem; if it's truly a 3D problem (one spaceship face another spaceship in free space) then you should definitely use quaternions. Traditional vectors will have odd turn patterns. But according to your question, you're just turning left or right. If you can simplify the problem down to two (x,y) positions and the angle that entity A is facing, then you can use simple 2D math:

tan((yB-yA)/(xB-xA)) is the desired angle (of A facing B). Turn towards that.

You need to account for crossing 0 degrees; for example, if the target angle is 359 degrees and you're currently looking at a 1 degree angle, you want to just turn 2 degrees clockwise rather than turning 358 degrees counterclockwise.

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