Return the arccosine of the parameter
float acos(float x)
vec2 acos(vec2 x)
vec3 acos(vec3 x)
vec4 acos(vec4 x)x specify the value whose arccosine to return.
acos() returns the angle whose trigonometric cosine is x.
cos(), sin(), asin(), tan(), atan(), Chapter 05: Shaping Functions