lta.h

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#pragma once
 
#include "Math/TVector3.h"
#include "Math/math.h"
 
#include <Common/config.h>
#include <Common/assertion.h>
#include <Common/primitive_type.h>
 
#include <cmath>
 
namespace ph::lta
{
 
inline real pdfA_to_pdfW(
    const real pdfA,
    const math::Vector3R& dAPosToTargetPos,
    const math::Vector3R& dANormal)
{
    const real distSquared = dAPosToTargetPos.lengthSquared();
    const real signedPdfW = pdfA / dAPosToTargetPos.normalize().dot(dANormal) * distSquared;
    return std::isfinite(signedPdfW) ? std::abs(signedPdfW) : 0.0_r;
}
 
inline real importance_scatter_Ns_corrector(
    const math::Vector3R& Ns,
    const math::Vector3R& Ng,
    const math::Vector3R& L,
    const math::Vector3R& V)
{
    PH_ASSERT_NE(Ng.dot(V) * Ns.dot(L), 0.0_r);
 
    return std::abs((Ns.dot(V) * Ng.dot(L)) / (Ng.dot(V) * Ns.dot(L)));
}
 
inline real importance_BSDF_Ns_corrector(
    const math::Vector3R& Ns,
    const math::Vector3R& Ng,
    const math::Vector3R& V)
{
    PH_ASSERT_NE(Ng.dot(V), 0.0_r);
 
    return std::abs(Ns.dot(V) / Ng.dot(V));
}
 
inline real tamed_importance_scatter_Ns_corrector(
    const math::Vector3R& Ns,
    const math::Vector3R& Ng,
    const math::Vector3R& L,
    const math::Vector3R& V)
{
    const auto factor = importance_scatter_Ns_corrector(Ns, Ng, L, V);
#if PH_STRICT_ASYMMETRIC_IMPORTANCE_TRANSPORT
    return factor;
#else
    return math::safe_clamp(factor, 0.0_r, 10.0_r);
#endif
}
 
inline real tamed_importance_BSDF_Ns_corrector(
    const math::Vector3R& Ns,
    const math::Vector3R& Ng,
    const math::Vector3R& V)
{
    const auto factor = importance_BSDF_Ns_corrector(Ns, Ng, V);
#if PH_STRICT_ASYMMETRIC_IMPORTANCE_TRANSPORT
    return factor;
#else
    // To my testing, the corrector for BSDF dominates the unbearable variance. Clamping to 10 seems to
    // work well. Mitsuba 0.6 uses another approach: discards the photon if `Ng.dot(V) < 1e-2f`.
    return math::safe_clamp(factor, 0.0_r, 10.0_r);
#endif
}
 
}// end namespace ph::lta