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Vector rotation to Euler angles

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I'm trying to convert vector rotation angles from a game into euler angles. There is no roll in the game so I don't need to calculate that. I'm just extracting these figures from the game memory.

I wrote this little function here..
void ForwardVectorToEuler2d(real_vector3d *forward, real_euler_angles2d *euler)
euler->yaw = atanf(forward->j/forward->i);
euler->pitch = atanf(forward->k/sqrt((forward->j*forward->j)+(forward->i*forward->i)));
if(euler->yaw < 0.0f)
euler->yaw += CAST(real, 2.0f * M_PI);

Pitch is converted correctly. The problem is the converted yaw goes from 0-360 twice in a full turn. So when I turn the player 180 degrees, it goes from 0-360 degrees, then when I continue rotating the final 180 degrees, it does 0-360 again. I need the converted yaw to be 0-360 degrees in a full turn. Hopefully that makes sense.

Update: For the heck of it, I tried using atan2f instead for yaw and it worked perfectly. Can anyone explain this?

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It's simple. The atan2f function considers the signs of its two arguments, thus yielding the correct result. By using the atanf function, you loose the sign information.

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