include/nori/bsdf.h

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/*
    This file is part of Nori, a simple educational ray tracer

    Copyright (c) 2012 by Wenzel Jakob and Steve Marschner.

    Nori is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License Version 3
    as published by the Free Software Foundation.

    Nori 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/>.
*/

#if !defined(__BSDF_H)
#define __BSDF_H

#include <nori/object.h>

NORI_NAMESPACE_BEGIN


/**
 * \brief Convenience data structure used to pass multiple
 * parameters to the evaluation and sampling routines in \ref BSDF
 */
struct BSDFQueryRecord {
        /// Incident direction (in the local frame)
        Vector3f wi;

        /// Outgoing direction (in the local frame)
        Vector3f wo;

        /// Measure associated with the sample
        EMeasure measure;

        /// Create a new record for sampling the BSDF
        inline BSDFQueryRecord(const Vector3f &wi)
                : wi(wi), measure(EUnknownMeasure) { }

        /// Create a new record for querying the BSDF
        inline BSDFQueryRecord(const Vector3f &wi,
                        const Vector3f &wo, EMeasure measure) 
                : wi(wi), wo(wo), measure(measure) { }
};

/**
 * \brief Superclass of all bidirectional scattering distribution functions
 */
class BSDF : public NoriObject {
public:
        /**
         * \brief Sample the BSDF and return the importance weight (i.e. the
         * value of the BSDF * cos(theta_o) divided by the probability density 
         * of the sample with respect to solid angles). 
         *
         * \param bRec    A BSDF query record
         * \param sample  A uniformly distributed sample on \f$[0,1]^2\f$
         *
         * \return The BSDF value divided by the probability density of the sample
         *         sample. The returned value also includes the cosine
         *         foreshortening factor associated with the outgoing direction,
         *         when this is appropriate. A zero value means that sampling
         *         failed.
         */
        virtual Color3f sample(BSDFQueryRecord &bRec, const Point2f &sample) const = 0;

        /**
         * \brief Evaluate the BSDF for a pair of directions and measure
         * specified in \code bRec
         * 
         * \param bRec
         *     A record with detailed information on the BSDF query
         * \return
         *     The BSDF value, evaluated for each color channel
         */
        virtual Color3f eval(const BSDFQueryRecord &bRec) const = 0;

        /**
         * \brief Compute the probability of sampling \c bRec.wo
         * (conditioned on \c bRec.wi).
         *
         * This method provides access to the probability density that
         * is realized by the \ref sample() method.
         *
         * \param bRec
         *     A record with detailed information on the BSDF query
         *
         * \return
         *     A probability/density value expressed with respect
         *     to the specified measure
         */

        virtual float pdf(const BSDFQueryRecord &bRec) const = 0;
        
        /**
         * \brief Return the type of object (i.e. Mesh/BSDF/etc.) 
         * provided by this instance
         * */
        EClassType getClassType() const { return EBSDF; }
};

NORI_NAMESPACE_END

#endif /* __BSDF_H */