vol_subsurface.cpp

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/*
 * Adapted from Open Shading Language with this license:
 *
 * Copyright (c) 2009-2010 Sony Pictures Imageworks Inc., et al.
 * All Rights Reserved.
 *
 * Modifications Copyright 2011, Blender Foundation.
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 * * Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * * Redistributions in binary form must reproduce the above copyright
 *   notice, this list of conditions and the following disclaimer in the
 *   documentation and/or other materials provided with the distribution.
 * * Neither the name of Sony Pictures Imageworks nor the names of its
 *   contributors may be used to endorse or promote products derived from
 *   this software without specific prior written permission.
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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 */

#include <OpenImageIO/fmath.h>

#include <OSL/genclosure.h>

#include "osl_closures.h"

CCL_NAMESPACE_BEGIN

using namespace OSL;

// Computes scattering properties based on Jensen's reparameterization
// described in:
//    http://graphics.ucsd.edu/~henrik/papers/fast_bssrdf/

class SubsurfaceClosure : public VolumeClosure {
public:
    float  m_g;
    float  m_eta;
    Color3 m_mfp, m_albedo;
    static float root_find_Rd(const float Rd0, const float A) {
        // quick exit for trivial cases
        if (Rd0 <= 0) return 0;
        const float A43 = A * 4.0f / 3.0f;
        // Find alpha such that f(alpha) = Rd (see eq.15). A simple bisection
        // method can be used because this function is monotonicaly increasing.
        float lo = 0, hi = 1;
        for (int i = 0; i < 20; i++) { // 2^20 divisions should be sufficient
            // eval function at midpoint
            float alpha = 0.5f * (lo + hi);
            float a1 = sqrtf(3 * (1 - alpha));
            float e1 = expf(-a1);
            float e2 = expf(-A43 * a1);
            float Rd = 0.5f * alpha * (1 + e2) * e1 - Rd0;
            if (fabsf(Rd) < 1e-6f)
                return alpha; // close enough
            else if (Rd > 0)
                hi = alpha; // root is on left side
            else
                lo = alpha; // root is on right side
        }
        // didn't quite converge, pick result in the middle of remaining interval
        return 0.5f * (lo + hi);
    }
    SubsurfaceClosure() { }

    void setup()
    {
        ior(m_eta);

        if (m_g >=  0.99f) m_g =  0.99f;
        if (m_g <= -0.99f) m_g = -0.99f;

        // eq.10
        float inv_eta = 1 / m_eta;
        float Fdr = -1.440f * inv_eta * inv_eta + 0.710 * inv_eta + 0.668f + 0.0636 * m_eta;
        float A = (1 + Fdr) / (1 - Fdr);
        // compute sigma_s, sigma_a (eq.16)
        Color3 alpha_prime = Color3 (root_find_Rd(m_albedo[0], A),
                                     root_find_Rd(m_albedo[1], A),
                                     root_find_Rd(m_albedo[2], A));
        Color3 sigma_t_prime = Color3 (m_mfp.x > 0 ? 1.0f / (m_mfp[0] * sqrtf(3 * (1 - alpha_prime[0]))) : 0.0f,
                                       m_mfp.y > 0 ? 1.0f / (m_mfp[1] * sqrtf(3 * (1 - alpha_prime[1]))) : 0.0f,
                                       m_mfp.z > 0 ? 1.0f / (m_mfp[2] * sqrtf(3 * (1 - alpha_prime[2]))) : 0.0f);
        Color3 sigma_s_prime = alpha_prime * sigma_t_prime;

        sigma_s((1.0f / (1 - m_g)) * sigma_s_prime);
        sigma_a(sigma_t_prime - sigma_s_prime);
    }

    bool mergeable (const ClosurePrimitive *other) const {
        const SubsurfaceClosure *comp = (const SubsurfaceClosure *)other;
        return m_g == comp->m_g && VolumeClosure::mergeable(other);
    }

    size_t memsize () const { return sizeof(*this); }

    const char *name () const { return "subsurface"; }

    void print_on (std::ostream &out) const {
        out << name() << " ()";
    }

    virtual Color3 eval_phase(const Vec3 &omega_in, const Vec3 &omega_out) const {
        float costheta = omega_in.dot(omega_out);
        float ph = 0.25f * float(M_1_PI) * ((1 - m_g * m_g) / powf(1 + m_g * m_g - 2.0f * m_g * costheta, 1.5f));
        return Color3 (ph, ph, ph);
    }
};



ClosureParam closure_subsurface_params[] = {
    CLOSURE_FLOAT_PARAM (SubsurfaceClosure, m_eta),
    CLOSURE_FLOAT_PARAM (SubsurfaceClosure, m_g),
    CLOSURE_COLOR_PARAM (SubsurfaceClosure, m_mfp),
    CLOSURE_COLOR_PARAM (SubsurfaceClosure, m_albedo),
    CLOSURE_STRING_KEYPARAM("label"),
    CLOSURE_FINISH_PARAM(SubsurfaceClosure) };

CLOSURE_PREPARE(closure_subsurface_prepare, SubsurfaceClosure)

CCL_NAMESPACE_END