Color3f sample(BSDFQueryRecord &bRec, const Point2f &sample_) const {
Point2f sample(sample_);
if (Frame::cosTheta(bRec.wi) <= 0)
return Color3f(0.0f);
bRec.measure = ESolidAngle;
// 1. Select diffuse or specular
bool useSpecular = true;
if (sample.x() <= m_specSamplingWeight) {
sample.x() /= m_specSamplingWeight;
} else {
sample.x() = (sample.x() - m_specSamplingWeight) / m_diffSamplingWeight;
useSpecular = false;
}
if (useSpecular) {
// this is a tricky one
// See http://mathinfo.univ-reims.fr/IMG/pdf/Using_the_modified_Phong_reflectance_model_for_Physically_based_rendering_-_Lafortune.pdf
float sinTheta = std::sqrt(1.0f - std::pow(sample.y(), 2.0f/(m_exp + 1.0f)));
float cosTheta = std::pow(sample.y(), 1.0f/(m_exp + 1.0f));
float phi = 2.0f * M_PI * sample.x();
float cosPhi, sinPhi;
sincosf(phi, &sinPhi, &cosPhi);
// direction from the lobe axis (i.e. reflection of wi) in lobe coordinate
Vector3f lobeAxis = reflect(bRec.wi);
Vector3f lobeWo(
sinTheta * std::cos(phi),
sinTheta * std::sin(phi),
cosTheta
);
bRec.wo = Frame(lobeAxis).toWorld(lobeWo);
// check that we are on the hemisphere
if(Frame::cosTheta(bRec.wo) <= 0.0f)
return Color3f(0.0f);
} else {
/* Warp a uniformly distributed sample on [0,1]^2
to a direction on a cosine-weighted hemisphere */
bRec.wo = squareToCosineHemisphere(sample);
}
/* Relative index of refraction: no change */
bRec.eta = 1.0f;
// the importance-weighted sample is given by
// f / pdf * cos(wo)
Color3f f_eval = eval(bRec);
float pdf_eval = pdf(bRec);
if(pdf_eval <= 0.0f) return Color3f(0.0f); // let's not explode here
return f_eval / pdf_eval * Frame::cosTheta(bRec.wo);
}
/// Draw a a sample from the BRDF model
Color3f sample(BSDFQueryRecord &bRec, const Point2f &sample) const {
if (Frame::cosTheta(bRec.wi) <= 0)
return Color3f(0.0f);
bRec.measure = ESolidAngle;
/* Warp a uniformly distributed sample on [0,1]^2
to a direction on a cosine-weighted hemisphere */
bRec.wo = squareToCosineHemisphere(sample);
/* eval() / pdf() * cos(theta) = albedo. There
is no need to call these functions. */
return m_albedo;
}
