PBREffect directional lights still impact metalness when they are supposed to be off

[walbourn]

The current implementation of directional lights in the PBR effect mean that even when they are 'off' by default, they still add to the specular metalness factors.

The helper LightSurface in PBRCommon.fxh needs to properly ignore a light that has it's color set to 0,0,0.

[walbourn]

Looking at the shader in more detail, it should end up contributing zero if the light is set to 0,0,0. If you don't call EnableDefaultLighting, it ends up looking a lot more 'metal' than it should.

[walbourn]

After reviewing the shader some more, I'm beginning to think it's not a bug after all. The 'specular radiance' is taken from the IBL and not based on the directional lights at all.

[walbourn]

To be clear, if I change the LightSurface function to this:

    // Accumulate light values
    for (int i = 0; i < numLights; i++)
    {
        if (step(dot(lightColor[i], lightColor[i]), EPSILON) > 0.f)
        {
            // light vector (to light)
            const float3 L = normalize(-lightDirection[i]);

            // Half vector
            const float3 H = normalize(L + V);

            // products
            const float NdotL = saturate(dot(N, L));
            const float LdotH = saturate(dot(L, H));
            const float NdotH = saturate(dot(N, H));

            // Diffuse & specular factors
            float diffuse_factor = Diffuse_Burley(NdotL, NdotV, LdotH, roughness);
            float3 specular = Specular_BRDF(alpha, c_spec, NdotV, NdotL, LdotH, NdotH);

            // Directional light
            acc_color += NdotL * lightColor[i] * (((c_diff * diffuse_factor) + specular));
        }
    }

It made no difference.