float3 ImportanceSampleGGX( float2 Xi, float Roughness , float3 N )
{
    float a = Roughness * Roughness;
    float Phi = 2 * PI * Xi.x;
    float CosTheta = sqrt( (1 - Xi.y) / ( 1 + (a*a - 1) * Xi.y ) );
    float SinTheta = sqrt( 1 - CosTheta * CosTheta );
    float3 H;
    H.x = SinTheta * cos( Phi );
    H.y = SinTheta * sin( Phi );
    H.z = CosTheta;
    float3 UpVector = abs(N.z) < 0.999 ? float3(0,0,1) : float3(1,0,0);
    float3 TangentX = normalize( cross( UpVector , N ) );
    float3 TangentY = cross( N, TangentX );
    // Tangent to world space
    return TangentX * H.x + TangentY * H.y + N * H.z;
}

float3 SpecularIBL( float3 SpecularColor , float Roughness , float3 N, float3 V )
{
    float3 SpecularLighting = 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 NoV = saturate( dot( N, V ) );
        float NoL = saturate( dot( N, L ) );
        float NoH = saturate( dot( N, H ) );
        float VoH = saturate( dot( V, H ) );
        if( NoL > 0 )
        {
            float3 SampleColor = EnvMap.SampleLevel( EnvMapSampler , L, 0 ).rgb;
            float G = G_Smith( Roughness , NoV, NoL );
            float Fc = pow( 1 - VoH, 5 );
            float3 F = (1 - Fc) * SpecularColor + Fc;
            // Incident light = SampleColor * NoL
            // Microfacet specular = D*G*F / (4*NoL*NoV)
            // pdf = D * NoH / (4 * VoH)
            SpecularLighting += SampleColor * F * G * VoH / (NoH * NoV);
        }
    }
    return SpecularLighting / NumSamples;
}