float3 PrefilterEnvMap( float Roughness , float3 R )
{
    float3 N = R;
    float3 V = R;
    float3 PrefilteredColor = 0;
    const uint NumSamples = 1024;
    for( uint i = 0; i < NumSamples; i++ )
    {
        float2 Xi = Hammersley( i, NumSamples );
        float3 H = ImportanceSampleGGX( Xi, Roughness , N );
        float3 L = 2 * dot( V, H ) * H - V;
        float NoL = saturate( dot( N, L ) );
        if( NoL > 0 )
        {
            PrefilteredColor += EnvMap.SampleLevel( EnvMapSampler , L, 0 ).rgb * NoL;
            TotalWeight += NoL;
        }
    }
    return PrefilteredColor / TotalWeight;
}