sahkdtree2.h

Go to the documentation of this file.

/*
    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_ */