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/*******************************************************************************
* subsurface.cpp
*
* This module implements subsurface light transport.
*
* Author: Christoph Lipka
*
* ---------------------------------------------------------------------------
* Persistence of Vision Ray Tracer ('POV-Ray') version 3.7.
* Copyright 1991-2013 Persistence of Vision Raytracer Pty. Ltd.
*
* POV-Ray is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License as
* published by the Free Software Foundation, either version 3 of the
* License, or (at your option) any later version.
*
* POV-Ray 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 Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
* ---------------------------------------------------------------------------
* POV-Ray is based on the popular DKB raytracer version 2.12.
* DKBTrace was originally written by David K. Buck.
* DKBTrace Ver 2.0-2.12 were written by David K. Buck & Aaron A. Collins.
* ---------------------------------------------------------------------------
* $File: //depot/public/povray/3.x/source/backend/lighting/subsurface.cpp $
* $Revision: #1 $
* $Change: 6069 $
* $DateTime: 2013/11/06 11:59:40 $
* $Author: chrisc $
*******************************************************************************/
// frame.h must always be the first POV file included (pulls in platform config)
#include "backend/frame.h"
#include "backend/math/vector.h"
#include "backend/lighting/subsurface.h"
// this must be the last file included
#include "base/povdebug.h"
namespace pov
{
/// Computes the BRDF approximation of the diffuse reflectance term.
inline double DiffuseReflectance(double A, double alphaPrime)
{
double root = sqrt(3*(1-alphaPrime));
return (alphaPrime/2) * ( 1 + exp(-(4.0/3.0)*A*root) ) * exp(-root);
}
SubsurfaceInterior::SubsurfaceInterior(double ior) :
precomputedReducedAlbedo(ior)
{}
SubsurfaceInterior::PrecomputedReducedAlbedo::PrecomputedReducedAlbedo(double ior)
{
reducedAlbedo[ReducedAlbedoSamples] = 1.0;
double Fdr = FresnelDiffuseReflectance(ior);
double A = (1+Fdr)/(1-Fdr);
double alphaPrime = 1.0;
double Rd = 1.0;
int it = 0;
for (int i = ReducedAlbedoSamples-1; i > 0; i --)
{
double Rd0 = 0.0;
double Rd1 = Rd;
double diffuseReflectance = double(i)/ReducedAlbedoSamples;
double alphaPrime0 = 0.0;
double alphaPrime1 = alphaPrime;
while(fabs(Rd-diffuseReflectance) >= EPSILON)
{
double p = (diffuseReflectance-Rd0)/(Rd1-Rd0);
alphaPrime = alphaPrime0 + p*(alphaPrime1-alphaPrime0);
Rd = DiffuseReflectance(A, alphaPrime);
if (Rd < diffuseReflectance)
{
alphaPrime0 = alphaPrime;
Rd0 = Rd;
}
else
{
alphaPrime1 = alphaPrime;
Rd1 = Rd;
}
it ++;
}
reducedAlbedo[i] = alphaPrime;
}
reducedAlbedo[0] = 0.0;
}
double SubsurfaceInterior::PrecomputedReducedAlbedo::operator()(double diffuseReflectance) const
{
double Rd = clip(diffuseReflectance, 0.0, 1.0);
double i = diffuseReflectance * ReducedAlbedoSamples;
int i0 = floor(i);
int i1 = ceil(i);
double p = (i-i0);
return (1-p)*reducedAlbedo[i0] + p*reducedAlbedo[i1];
}
DblRGBColour SubsurfaceInterior::GetReducedAlbedo(const RGBColour& diffuseReflectance) const
{
return DblRGBColour(((const PrecomputedReducedAlbedo&)precomputedReducedAlbedo)(diffuseReflectance.red()),
((const PrecomputedReducedAlbedo&)precomputedReducedAlbedo)(diffuseReflectance.green()),
((const PrecomputedReducedAlbedo&)precomputedReducedAlbedo)(diffuseReflectance.blue()));
}
} // end of namespace
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