Documentation for ATAN2 in GLSL because the GLSL docs suck.
Above is the implementation output for atan2 or GLSL's atan(y,x)
function.
The range here in degrees is 0 to pi (0-180 degrees)
and -pi to 0 (181 to 359 degrees
.
This can be converted to proper radians 0 to 2pi
using the following function:
float atan2(float y, float x) {
const float PI2 = 6.2831853071795864769252867665590;
return mod(atan(y,x) + (PI2*0.5), PI2);
}