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When I put a value into acos/asin, I always get a value -Pi/2 < angle < Pi/2. But, I want to know how to get an angle 0 < angle < 2*Pi. Someone help please?

if(theta < 0.0f)
theta += 2*PI;

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That really only works for angles in the fourth quadrant. Angles that are Pi/2 < angle < 3*Pi/2 are still erroneous.

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You will have to find another way of doing this, not by asin/acos...
the reason? the sin and cos functions arent inversible in it entire domain (thats it, [0,2*pi) ). The reason to this is that there are always two angles that have the same sin...
for example:

sin(pi/6)=0.5
sin(5*pi/6)=0.5

so, asin(0.5) would be pi/6 or 5*pi/6??
for that reason, the image of asin is defined only in 2 quadrants.

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