sahkdtree4.h

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/*
    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/>.
*/

#if !defined(__SAH_KDTREE4_H)
#define __SAH_KDTREE4_H

#include <mitsuba/render/sahkdtree3.h>

MTS_NAMESPACE_BEGIN

typedef TAABB<Point4> AABB4;

/**
 * \brief Implements the four-dimensional surface area heuristic for use
 * by the \ref GenericKDTree construction algorithm.
 */
class SurfaceAreaHeuristic4 {
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 SurfaceAreaHeuristic4(const AABB4 &aabb) : m_aabb(aabb) {
                const Vector4 extents(aabb.getExtents());
                const Float temp = 1.0f / (extents.x * extents.y
                                + extents.y*extents.z + extents.x*extents.z);

                m_temp0 = Vector4(
                        extents.y * extents.z * temp,
                        extents.x * extents.z * temp,
                        extents.x * extents.y * temp,
                        0.0f);

                m_temp1 = Vector4(
                        (extents.y + extents.z) * temp,
                        (extents.x + extents.z) * temp,
                        (extents.x + extents.y) * temp,
                        1.0f / extents.w);
        }

        /**
         * 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 {
                if (axis == 3 && m_temp1.w == std::numeric_limits<Float>::infinity()) {
                        return std::pair<Float, Float>(
                                std::numeric_limits<Float>::infinity(),
                                std::numeric_limits<Float>::infinity()
                        );
                }

                return std::pair<Float, Float>(
                        m_temp0[axis] + m_temp1[axis] * leftWidth,
                        m_temp0[axis] + m_temp1[axis] * rightWidth);
        }

        /**
         * 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 AABB4 &aabb) {
                const Vector4 extents(aabb.getExtents());
                Float result = 2 * (extents[0] * extents[1] + extents[1] * extents[2]
                                  + extents[2] * extents[0]);
                if (extents[3] != 0)
                        result *= extents[3];
                return result;
        }
private:
        Vector4 m_temp0, m_temp1;
        AABB4 m_aabb;
};

/**
 * This class specializes \ref GenericKDTree to a four-dimensional
 * tree to be used for spacetime ray tracing. One additional function call
 * must be implemented by subclasses:
 *
 * /// 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 Ray &ray, IndexType idx,
 *     Float mint, Float maxt, Float &t, void *tmp);
 *
 * 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
 */
template <typename Derived>
        class SAHKDTree4D : public GenericKDTree<AABB4, SurfaceAreaHeuristic4, Derived> {
public:
        typedef typename KDTreeBase<AABB4>::SizeType  SizeType;
        typedef typename KDTreeBase<AABB4>::IndexType IndexType;
        typedef typename KDTreeBase<AABB4>::KDNode     KDNode;

protected:
        void buildInternal() {
                SizeType primCount = this->cast()->getPrimitiveCount();
                KDLog(EInfo, "Constructing a 4D SAH kd-tree (%i primitives) ..", primCount);
                GenericKDTree<AABB4, SurfaceAreaHeuristic4, Derived>::buildInternal();
        }

        /**
         * \brief Hashed mailbox implementation
         */
        struct HashedMailbox {
                inline HashedMailbox() {
                        memset(entries, 0xFF, sizeof(IndexType)*MTS_KD_MAILBOX_SIZE);
                }

                inline void put(IndexType primIndex) {
                        entries[primIndex & MTS_KD_MAILBOX_MASK] = primIndex;
                }

                inline bool contains(IndexType primIndex) const {
                        return entries[primIndex & MTS_KD_MAILBOX_MASK] == primIndex;
                }

                IndexType entries[MTS_KD_MAILBOX_SIZE];
        };

        /// 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 */
                Point 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.
         */
        template<bool shadowRay> FINLINE
                        bool rayIntersectHavran(const Ray &ray, Float mint, Float maxt,
                        Float &t, void *temp) const {
                KDStackEntryHavran stack[MTS_KD_MAXDEPTH];
                #if 0
                static const int prevAxisTable[] = { 2, 0, 1 };
                static const int nextAxisTable[] = { 1, 2, 0 };
                #endif

                #if defined(MTS_KD_MAILBOX_ENABLED)
                HashedMailbox mailbox;
                #endif

                /* 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 = this->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 (axis == 3) {
                                        if (ray.time <= splitVal)
                                                currNode = currNode->getLeft();
                                        else
                                                currNode = currNode->getRight();
                                        continue;
                                } else 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;

                                #if 1
                                /* 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;
                                #else
                                const int nextAxis = nextAxisTable[axis];
                                const int prevAxis = prevAxisTable[axis];
                                stack[exPt].p[axis] = splitVal;
                                stack[exPt].p[nextAxis] = ray.o[nextAxis]
                                        + distToSplit*ray.d[nextAxis];
                                stack[exPt].p[prevAxis] = ray.o[prevAxis]
                                        + distToSplit*ray.d[prevAxis];
                                #endif

                        }

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

                                #if defined(MTS_KD_MAILBOX_ENABLED)
                                if (mailbox.contains(primIdx))
                                        continue;
                                #endif

                                bool result;
                                if (!shadowRay)
                                        result = this->cast()->intersect(ray, primIdx, mint, maxt, t, temp);
                                else
                                        result = this->cast()->intersect(ray, primIdx, mint, maxt);

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

                                #if defined(MTS_KD_MAILBOX_ENABLED)
                                mailbox.put(primIdx);
                                #endif
                        }

                        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 /* __SAH_KDTREE4_H */