mitsuba/include/mitsuba/render/sahkdtree2.h

/*
    This file is part of Mitsuba, a physically based rendering system.

    Copyright (c) 2007-2014 by Wenzel Jakob and others.

    Mitsuba 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.

    Mitsuba 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
#if !defined(__MITSUBA_RENDER_SAHKDTREE2_H_)
#define __MITSUBA_RENDER_SAHKDTREE2_H_

#include <mitsuba/core/aabb.h>
#include <mitsuba/render/gkdtree.h>
#include <mitsuba/core/warp.h>

MTS_NAMESPACE_BEGIN

/**
 * \brief Implements the 2D surface area heuristic for use
 * by the \ref GenericKDTree construction algorithm.
 * \ingroup librender
 */
class SurfaceAreaHeuristic2 {
public:
    /**
     * \brief Initialize the surface area heuristic with the bounds of
     * a parent node
     *
     * Precomputes some information so that traversal probabilities
     * of potential split planes can be evaluated efficiently
     */
    inline SurfaceAreaHeuristic2(const AABB2 &aabb) {
        m_extents = aabb.getExtents();
        m_normalization = 1.0f / (m_extents.x + m_extents.y);
    }

    /**
     * Given a split on axis \a axis that produces children having extents
     * \a leftWidth and \a rightWidth along \a axis, compute the probability
     * of traversing the left and right child during a typical query
     * operation.
     */
    inline std::pair<Float, Float> operator()(int axis, Float leftWidth, Float rightWidth) const {
        return std::pair<Float, Float>(
            (m_extents[1-axis] + leftWidth) * m_normalization,
            (m_extents[1-axis] + rightWidth) * m_normalization);
    }

    /**
     * Compute the underlying quantity used by the tree construction
     * heuristic. This is used to compute the final cost of a kd-tree.
     */
    inline static Float getQuantity(const AABB2 &aabb) {
        return aabb.getSurfaceArea();
    }
private:
    Float m_normalization;
    Vector2 m_extents;
};

/**
 * \brief Specializes \ref GenericKDTree to a two-dimensional
 * tree to be used for flatland ray tracing.
 *
 * One additional function call must be implemented by subclasses:
 * \code
 * /// Check whether a primitive is intersected by the given ray.
 * /// Some temporary space is supplied, which can be used to cache
 * /// information about the intersection
 * bool intersect(const Ray2 &ray, IndexType idx,
 *     Float mint, Float maxt, Float &t, void *tmp);
 * \endcode
 *
 * This class implements an epsilon-free version of the optimized ray
 * traversal algorithm (TA^B_{rec}), which is explained in Vlastimil
 * Havran's PhD thesis "Heuristic Ray Shooting Algorithms".
 *
 * \author Wenzel Jakob
 * \ingroup librender
 */
template <typename Derived>
    class SAHKDTree2D : public GenericKDTree<AABB2, SurfaceAreaHeuristic2, Derived> {
public:
    typedef GenericKDTree<AABB2, SurfaceAreaHeuristic2, Derived> Parent;
    typedef typename KDTreeBase<AABB2>::SizeType                 SizeType;
    typedef typename KDTreeBase<AABB2>::IndexType                IndexType;
    typedef typename KDTreeBase<AABB2>::KDNode                   KDNode;

    using Parent::m_nodes;
    using Parent::m_aabb;
    using Parent::m_indices;

protected:
    void buildInternal() {
        SizeType primCount = cast()->getPrimitiveCount();
        KDLog(EInfo, "Constructing a SAH kd-tree (%i primitives) ..", primCount);
        GenericKDTree<AABB2, SurfaceAreaHeuristic2, Derived>::buildInternal();
    }

    /// Cast to the derived class
    inline Derived *cast() {
        return static_cast<Derived *>(this);
    }

    /// Cast to the derived class (const version)
    inline const Derived *cast() const {
        return static_cast<const Derived *>(this);
    }

    /// Ray traversal stack entry for Wald-style incoherent ray tracing
    struct KDStackEntry {
        const KDNode * __restrict node;
        Float mint, maxt;
    };

    /// Ray traversal stack entry for Havran-style incoherent ray tracing
    struct KDStackEntryHavran {
        /* Pointer to the far child */
        const KDNode * __restrict node;
        /* Distance traveled along the ray (entry or exit) */
        Float t;
        /* Previous stack item */
        uint32_t prev;
        /* Associated point */
        Point2 p;
    };

    /**
     * \brief Ray tracing kd-tree traversal loop (Havran variant)
     *
     * This is generally the most robust and fastest traversal routine
     * of the methods implemented in this class.
     */
    FINLINE bool rayIntersectHavran(const Ray2 &ray, Float mint, Float maxt,
            Float &t, void *temp) const {
        KDStackEntryHavran stack[MTS_KD_MAXDEPTH];

        /* Set up the entry point */
        uint32_t enPt = 0;
        stack[enPt].t = mint;
        stack[enPt].p = ray(mint);

        /* Set up the exit point */
        uint32_t exPt = 1;
        stack[exPt].t = maxt;
        stack[exPt].p = ray(maxt);
        stack[exPt].node = NULL;

        bool foundIntersection = false;
        const KDNode * __restrict currNode = m_nodes;
        while (currNode != NULL) {
            while (EXPECT_TAKEN(!currNode->isLeaf())) {
                const Float splitVal = (Float) currNode->getSplit();
                const int axis = currNode->getAxis();
                const KDNode * __restrict farChild;

                if (stack[enPt].p[axis] <= splitVal) {
                    if (stack[exPt].p[axis] <= splitVal) {
                        /* Cases N1, N2, N3, P5, Z2 and Z3 (see thesis) */
                        currNode = currNode->getLeft();
                        continue;
                    }

                    /* Typo in Havran's thesis:
                       (it specifies "stack[exPt].p == splitVal", which
                        is clearly incorrect) */
                    if (stack[enPt].p[axis] == splitVal) {
                        /* Case Z1 */
                        currNode = currNode->getRight();
                        continue;
                    }

                    /* Case N4 */
                    currNode = currNode->getLeft();
                    farChild = currNode + 1; // getRight()
                } else { /* stack[enPt].p[axis] > splitVal */
                    if (splitVal < stack[exPt].p[axis]) {
                        /* Cases P1, P2, P3 and N5 */
                        currNode = currNode->getRight();
                        continue;
                    }
                    /* Case P4 */
                    farChild = currNode->getLeft();
                    currNode = farChild + 1; // getRight()
                }

                /* Cases P4 and N4 -- calculate the distance to the split plane */
                Float distToSplit = (splitVal - ray.o[axis]) * ray.dRcp[axis];

                /* Set up a new exit point */
                const uint32_t tmp = exPt++;
                if (exPt == enPt) /* Do not overwrite the entry point */
                    ++exPt;

                KDAssert(exPt < MTS_KD_MAXDEPTH);
                stack[exPt].prev = tmp;
                stack[exPt].t = distToSplit;
                stack[exPt].node = farChild;

                /* Intrestingly, this appears to be faster than the
                   original code with the prevAxis & nextAxis table */
                stack[exPt].p = ray(distToSplit);
                stack[exPt].p[axis] = splitVal;
            }

            /* Reached a leaf node */
            for (IndexType entry=currNode->getPrimStart(),
                    last = currNode->getPrimEnd(); entry != last; entry++) {
                const IndexType primIdx = m_indices[entry];

                bool result = cast()->intersect(ray, primIdx, mint, maxt, t, temp);

                if (result) {
                    maxt = t;
                    foundIntersection = true;
                }
            }

            if (stack[exPt].t > maxt)
                break;

            /* Pop from the stack and advance to the next node on the interval */
            enPt = exPt;
            currNode = stack[exPt].node;
            exPt = stack[enPt].prev;
        }

        return foundIntersection;
    }
};

MTS_NAMESPACE_END

#endif /* __MITSUBA_RENDER_SAHKDTREE2_H_ */