Error in glossy_f()

[tigrazone]

when we read https://www.pbr-book.org/3ed-2018/Reflection_Models/Fresnel_Incidence_Effects

Spectrum FresnelBlend::f(const Vector3f &wo, const Vector3f &wi) const {
    auto pow5 = [](Float v) { return (v * v) * (v * v) * v; };
    Spectrum diffuse = (28.f/(23.f*Pi)) * Rd *
        (Spectrum(1.f) - Rs) * 
        (1 - pow5(1 - .5f * AbsCosTheta(wi))) *
        (1 - pow5(1 - .5f * AbsCosTheta(wo)));
    Vector3f wh = wi + wo;
    if (wh.x == 0 && wh.y == 0 && wh.z == 0) return Spectrum(0);
    wh = Normalize(wh);
    Spectrum specular = distribution->D(wh) /
        (4 * AbsDot(wi, wh) *
         std::max(AbsCosTheta(wi), AbsCosTheta(wo))) *
         SchlickFresnel(Dot(wi, wh));
    return diffuse + specular;
}

and if we compare with glossy_f():

vec3 glossy_f(const Material mat, const vec3 wo, const vec3 wi, const vec3 n_s,
              float hl, float nl, float nv, float beckmann_term) {
    const vec3 h = normalize(wo + wi);
    const vec3 f_diffuse =
        (28. / (23 * PI)) * vec3(mat.albedo) * (1 - mat.metalness) *
        (1 - pow5(1 - 0.5 * dot(wi, n_s))) * (1 - pow5(1 - 0.5 * dot(wo, n_s)));
    const vec3 f_specular = beckmann_term *
                            fresnel_schlick(vec3(mat.metalness), hl) /
                            (4 * hl * max(nl, nv));
    return f_specular + f_diffuse;
}

(1 - mat.metalness) is Rd mat.metalness is Rs and then right version of calc f_diffuse is

    const vec3 f_diffuse =
        (28. / (23 * PI)) * vec3(mat.albedo) * (1 - mat.metalness) * mat.metalness *
        (1 - pow5(1 - 0.5 * dot(wi, n_s))) * (1 - pow5(1 - 0.5 * dot(wo, n_s)));

Real working #define pow5(x) ((x)(x)(x)(x)(x)) can show you this error and my code is fixes it

[yuphin]

Rd stands for diffuse reflectance and Rs is the specular reflectance. Consequently, the BRDF for diffuse surfaces is Rd / PI (I.e albedo is Rd). So I think the current version should be correct. Regarding the pow5 fix, I'll add it when I start rework/reorganize the BSDF backend.

[tigrazone]

maybe this issue connected with https://github.com/yuphin/Lumen/issues/25