> # NAME

> acos - returns arccosine of scalars and vectors.

> # SYNOPSIS

>       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

> abs, asin, cos, sqrt