lambert.h

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#pragma once

#include <cugar/linalg/vector.h>
#include <cugar/basic/numbers.h>
#include <cugar/bsdf/differential_geometry.h>
#include <cugar/spherical/mappings.h>


namespace cugar {


struct LambertBsdf
{
    CUGAR_FORCEINLINE CUGAR_HOST_DEVICE
    LambertBsdf(const Vector3f _color) :
        color(_color) {}

    CUGAR_FORCEINLINE CUGAR_HOST_DEVICE
    Vector3f f(const DifferentialGeometry& geometry, const Vector3f V, const Vector3f L) const
    {
        const Vector3f N = geometry.normal_s;

        const float NoL = dot(N,L);
        const float NoV = dot(N,V);

        return NoL * NoV > 0.0f ? color : Vector3f(0.0f);
    }

    CUGAR_FORCEINLINE CUGAR_HOST_DEVICE
    Vector3f f_over_p(const DifferentialGeometry& geometry, const Vector3f V, const Vector3f L) const
    {
        const Vector3f N = geometry.normal_s;

        const float NoL = dot(N,L);
        const float NoV = dot(N,V);

        return NoL * NoV >= 0.0f ? color * M_PIf : Vector3f(0.0f);
    }

    CUGAR_FORCEINLINE CUGAR_HOST_DEVICE
    void f_and_p(const DifferentialGeometry& geometry, const Vector3f V, const Vector3f L, Vector3f& f, float& p, const SphericalMeasure measure = kProjectedSolidAngle) const
    {
        const Vector3f N = geometry.normal_s;

        const float NoL = dot(N, L);
        const float NoV = dot(N, V);

        f = NoL * NoV > 0.0f ? color : Vector3f(0.0f);
        p = NoL * NoV > 0.0f ? 1.0f / M_PIf : 0.0f;

        if (measure == kSolidAngle)
            p *= fabsf(NoL);
    }

    CUGAR_FORCEINLINE CUGAR_HOST_DEVICE
    float p(const DifferentialGeometry& geometry, const Vector3f V, const Vector3f L, const SphericalMeasure measure = kProjectedSolidAngle) const
    {
        const Vector3f N = geometry.normal_s;

        const float NoL = dot(N, L);
        const float NoV = dot(N, V);

        const float p = NoL * NoV > 0.0f ? 1.0f / M_PIf : 0.0f;

        return (measure == kSolidAngle) ? p * fabsf(NoL) : p;
    }

    CUGAR_FORCEINLINE CUGAR_HOST_DEVICE
    void sample(
        const Vector3f              u,
        const DifferentialGeometry& geometry, 
        const Vector3f              V,
              Vector3f&             L,
              Vector3f&             g,
              float&                p,
              float&                p_proj) const
    {
        const Vector3f N = geometry.normal_s;

        Vector3f local_L = square_to_cosine_hemisphere(u.xy());
        if (dot(V, N) < 0.0f)
            local_L[2] = -local_L[2];

        L = local_L[0] * geometry.tangent +
            local_L[1] * geometry.binormal +
            local_L[2] * geometry.normal_s;

        g      = color * M_PIf;
        p      = fabsf( local_L[2] ) / M_PIf;
        p_proj = 1.0f / M_PIf;
    }

    template <typename RandomGeneratorT>
    CUGAR_FORCEINLINE CUGAR_HOST_DEVICE
    bool invert(
        const DifferentialGeometry& geometry,
        const Vector3f              V,
        const Vector3f              L,
        RandomGeneratorT&           random,
        Vector3f&                   z,
        float&                      p,
        float&                      p_proj) const
    {
        const Vector3f N = geometry.normal_s;

        if (dot(V, N) * dot(L, N) < 0.0f)
            return false;

        const Vector3f local_L(
            dot(L, geometry.tangent ),
            dot(L, geometry.binormal ),
            fabsf(dot(L, geometry.normal_s )));

        p      = M_PIf / cugar::max( local_L[2], 1.0e-8f );
        p_proj = M_PIf;

        const Vector2f u = cosine_hemisphere_to_square( local_L );
        z = Vector3f(u.x, u.y, random.next());
        return true;
    }

    CUGAR_FORCEINLINE CUGAR_HOST_DEVICE
    void inverse_pdf(
        const DifferentialGeometry& geometry,
        const Vector3f              V,
        const Vector3f              L,
        const Vector3f              u,
        float&                      p,
        float&                      p_proj) const
    {
        const Vector3f N = geometry.normal_s;

        if (dot(V, N) * dot(L, N) < 0.0f)
            p = p_proj = 0.0f;
        else
        {
            p      = M_PIf / cugar::max(fabsf(dot(L, N)), 1.0e-8f);
            p_proj = M_PIf;
        }
    }

public:
    Vector3f    color;
};

} // namespace cugar