NAME
    acos - returns arccosine of scalars and vectors.

SYNPOSIS
      float  acos(float a);
      float1 acos(float2 a);
      float2 acos(float2 a);
      float3 acos(float3 a);
      float4 acos(float4 a);
 
      half   acos(half a);
      half1  acos(half2 a);
      half2  acos(half2 a);
      half3  acos(half3 a);
      half4  acos(half4 a);
 
      fixed  acos(fixed a);
      fixed1 acos(fixed2 a);
      fixed2 acos(fixed2 a);
      fixed3 acos(fixed3 a);
      fixed4 acos(fixed4 a);

PARAMETERS
    a       Vector or scalar of which to determine the arccosine.

DESCRIPTION
    Returns the arccosine of *a* in the range [0,pi], expecting *a* to be in
    the range [-1,+1].

    For vectors, the returned vector contains the arccosine of each element
    of the input vector.

REFERENCE IMPLEMENTATION
    acos for a float scalar could be implemented like this.

      // Handbook of Mathematical Functions
      // M. Abramowitz and I.A. Stegun, Ed.

      // Absolute error <= 6.7e-5
      float acos(float x) {
        float negate = float(x < 0);
        x = abs(x);
        float ret = -0.0187293;
        ret = ret * x;
        ret = ret + 0.0742610;
        ret = ret * x;
        ret = ret - 0.2121144;
        ret = ret * x;
        ret = ret + 1.5707288;
        ret = ret * sqrt(1.0-x);
        ret = ret - 2 * negate * ret;
        return negate * 3.14159265358979 + ret;
      }

PROFILE SUPPORT
    acos is supported in all profiles.

    Support in the fp20 is limited.

SEE ALSO
    the abs manpage, the asin manpage, the cos manpage, the sqrt manpage

