> ## Name

> log2 - returns the base-2 logarithm of scalars and vectors

> ## Synopsis

>     float  log2(float a);
>     float1 log2(float1 a);
>     float2 log2(float2 a);
>     float3 log2(float3 a);
>     float4 log2(float4 a);

>     half   log2(half a);
>     half1  log2(half1 a);
>     half2  log2(half2 a);
>     half3  log2(half3 a);
>     half4  log2(half4 a);

>     fixed  log2(fixed a);
>     fixed1 log2(fixed1 a);
>     fixed2 log2(fixed2 a);
>     fixed3 log2(fixed3 a);
>     fixed4 log2(fixed4 a);

> ## Parameters

> a
>   Vector or scalar of which to determine the base-2 logarithm.

> ## Description

> Returns the base-2 logarithm a.

> For vectors, the returned vector contains the base-2 logarithm of each element of the input vector.

> ## Reference Implementation

>     float3 log2(float3 a)
>     {
>       float3 rv;
>       int i;

>       for (i=0; i<3; i++) {
>         rv[i] = log2(a[i]);  // this is the ANSI C standard library log2()
>       }
>       return rv;
>     }

> log2 is typically implemented with a native base-2 logarithm instruction.

> ## Profile Support

> log2 is fully supported in all profiles unless otherwise specified.

> Support in the fp20 is limited to constant compile-time evaluation.

> ## See Also

> exp, log, log10, pow