bsdf.h

/*
    Lightmetrica : A research-oriented renderer

    Copyright (c) 2014 Hisanari Otsu

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#pragma once
#ifndef LIB_LIGHTMETRICA_BSDF_H
#define LIB_LIGHTMETRICA_BSDF_H

#include "generalizedbsdf.h"
#include "surfacegeometry.h"

LM_NAMESPACE_BEGIN

class BSDF : public GeneralizedBSDF
{
public:

    LM_ASSET_INTERFACE_DEF("bsdf", "bsdfs");
    LM_ASSET_DEPENDENCIES("texture");

public:

    BSDF() {}
    virtual ~BSDF() {}

protected:

    LM_FORCE_INLINE static Math::Float ShadingNormalCorrectionFactor(
        const TransportDirection& transportDir, const SurfaceGeometry& geom,
        const Math::Vec3& localWi, const Math::Vec3& localWo, const Math::Vec3& worldWi, const Math::Vec3& worldWo)
    {
        // Prevent light leak
        // In some cases wi and wo are same side according to the shading normal
        // but opposite side according to the geometry normal.
        auto wiDotNg = Math::Dot(worldWi, geom.gn);
        auto woDotNg = Math::Dot(worldWo, geom.gn);
        auto wiDotNs = Math::CosThetaZUp(localWi);
        auto woDotNs = Math::CosThetaZUp(localWo);
        if (wiDotNg * wiDotNs <= 0 || woDotNg * woDotNs <= 0)
        {
            return Math::Float(0);
        }

        // Special handling for adjoint case
        // Be careful of the difference of the notation between Veach's thesis;
        // in the framework, wo is always the propagating direction.
        if (transportDir == TransportDirection::LE)
        {
            // |w_i, N_s| * |w_o, N_g| / |w_i, N_g| / |w_o, N_s| 
            return wiDotNs * woDotNg / (woDotNs * wiDotNg);
        }

        return Math::Float(1);
    }

};

LM_NAMESPACE_END

#endif // LIB_LIGHTMETRICA_BSDF_H