void SubsurfaceFromDiffuse(const Spectrum &Kd, float meanPathLength,
float eta, Spectrum *sigma_a, Spectrum *sigma_prime_s) {
float A = (1.f + Fdr(eta)) / (1.f - Fdr(eta));
float rgb[3];
Kd.ToRGB(rgb);
float sigma_prime_s_rgb[3], sigma_a_rgb[3];
for (int i = 0; i < 3; ++i) {
float alphap = RdToAlphap(rgb[i], A);
float sigma_tr = 1.f / meanPathLength;
float sigma_prime_t = sigma_tr / sqrtf(3.f * 1.f - alphap);
sigma_prime_s_rgb[i] = alphap * sigma_prime_t;
sigma_a_rgb[i] = sigma_prime_t - sigma_prime_s_rgb[i];
}
*sigma_a = Spectrum::FromRGB(sigma_a_rgb);
*sigma_prime_s = Spectrum::FromRGB(sigma_prime_s_rgb);
}
void subsurfaceFromDiffuse(const Spectrum &Kd, double meanPathLength,
double eta, Spectrum *sigma_a, Spectrum *sigma_prime_s) {
double A = (1.f + Fdr(eta)) / (1.f - Fdr(eta));
double rgb[3];
Kd.toRGB(rgb);
double sigma_prime_s_rgb[3], sigma_a_rgb[3];
for (int i = 0; i < 3; ++i) {
// Compute $\alpha'$ for RGB component, compute scattering properties
double alphap = RdToAlphap(rgb[i], A);
double sigma_tr = 1.f / meanPathLength;
double sigma_prime_t = sigma_tr / sqrtf(3.f * (1.f - alphap));
sigma_prime_s_rgb[i] = alphap * sigma_prime_t;
sigma_a_rgb[i] = sigma_prime_t - sigma_prime_s_rgb[i];
}
*sigma_a = Spectrum::fromRGB(sigma_a_rgb);
*sigma_prime_s = Spectrum::fromRGB(sigma_prime_s_rgb);
}
