3 * Copyright 2012 Google Inc.
5 * Use of this source code is governed by a BSD-style license that can be
6 * found in the LICENSE file.
9 #ifndef SkRTree_DEFINED
10 #define SkRTree_DEFINED
13 #include "SkTDArray.h"
14 #include "SkChunkAlloc.h"
15 #include "SkBBoxHierarchy.h"
18 * An R-Tree implementation. In short, it is a balanced n-ary tree containing a hierarchy of
19 * bounding rectangles.
21 * Much like a B-Tree it maintains balance by enforcing minimum and maximum child counts, and
22 * splitting nodes when they become overfull. Unlike B-trees, however, we're using spatial data; so
23 * there isn't a canonical ordering to use when choosing insertion locations and splitting
24 * distributions. A variety of heuristics have been proposed for these problems; here, we're using
25 * something resembling an R*-tree, which attempts to minimize area and overlap during insertion,
26 * and aims to minimize a combination of margin, overlap, and area when splitting.
28 * One detail that is thus far unimplemented that may improve tree quality is attempting to remove
29 * and reinsert nodes when they become full, instead of immediately splitting (nodes that may have
30 * been placed well early on may hurt the tree later when more nodes have been added; removing
31 * and reinserting nodes generally helps reduce overlap and make a better tree). Deletion of nodes
32 * is also unimplemented.
34 * For more details see:
36 * Beckmann, N.; Kriegel, H. P.; Schneider, R.; Seeger, B. (1990). "The R*-tree:
37 * an efficient and robust access method for points and rectangles"
39 * It also supports bulk-loading from a batch of bounds and values; if you don't require the tree
40 * to be usable in its intermediate states while it is being constructed, this is significantly
41 * quicker than individual insertions and produces more consistent trees.
43 class SkRTree : public SkBBoxHierarchy {
45 SK_DECLARE_INST_COUNT(SkRTree)
48 * Create a new R-Tree with specified min/max child counts.
49 * The child counts are valid iff:
50 * - (max + 1) / 2 >= min (splitting an overfull node must be enough to populate 2 nodes)
54 * If you have some prior information about the distribution of bounds you're expecting, you
55 * can provide an optional aspect ratio parameter. This allows the bulk-load algorithm to create
56 * better proportioned tiles of rectangles.
58 static SkRTree* Create(int minChildren, int maxChildren, SkScalar aspectRatio = 1,
59 bool orderWhenBulkLoading = true);
63 * Insert a node, consisting of bounds and a data value into the tree, if we don't immediately
64 * need to use the tree; we may allow the insert to be deferred (this can allow us to bulk-load
65 * a large batch of nodes at once, which tends to be faster and produce a better tree).
66 * @param data The data value
67 * @param bounds The corresponding bounding box
68 * @param defer Can this insert be deferred? (this may be ignored)
70 virtual void insert(void* data, const SkRect& bounds, bool defer = false) SK_OVERRIDE;
73 * If any inserts have been deferred, this will add them into the tree
75 virtual void flushDeferredInserts() SK_OVERRIDE;
78 * Given a query rectangle, populates the passed-in array with the elements it intersects
80 virtual void search(const SkRect& query, SkTDArray<void*>* results) const SK_OVERRIDE;
82 virtual void clear() SK_OVERRIDE;
83 bool isEmpty() const { return 0 == fCount; }
86 * Gets the depth of the tree structure
88 virtual int getDepth() const SK_OVERRIDE {
89 return this->isEmpty() ? 0 : fRoot.fChild.subtree->fLevel + 1;
93 * This gets the insertion count (rather than the node count)
95 virtual int getCount() const SK_OVERRIDE { return fCount; }
97 virtual void rewindInserts() SK_OVERRIDE;
104 * A branch of the tree, this may contain a pointer to another interior node, or a data value
115 * A node in the tree, has between fMinChildren and fMaxChildren (the root is a special case)
118 uint16_t fNumChildren;
120 bool isLeaf() { return 0 == fLevel; }
121 // Since we want to be able to pick min/max child counts at runtime, we assume the creator
122 // has allocated sufficient space directly after us in memory, and index into that space
123 Branch* child(size_t index) {
124 return reinterpret_cast<Branch*>(this + 1) + index;
128 typedef int32_t SkIRect::*SortSide;
130 // Helper for sorting our children arrays by sides of their rects
131 struct RectLessThan {
132 RectLessThan(SkRTree::SortSide side) : fSide(side) { }
133 bool operator()(const SkRTree::Branch lhs, const SkRTree::Branch rhs) const {
134 return lhs.fBounds.*fSide < rhs.fBounds.*fSide;
137 const SkRTree::SortSide fSide;
141 bool operator()(const SkRTree::Branch lhs, const SkRTree::Branch rhs) {
142 return ((lhs.fBounds.fRight - lhs.fBounds.fLeft) >> 1) <
143 ((rhs.fBounds.fRight - lhs.fBounds.fLeft) >> 1);
148 bool operator()(const SkRTree::Branch lhs, const SkRTree::Branch rhs) {
149 return ((lhs.fBounds.fBottom - lhs.fBounds.fTop) >> 1) <
150 ((rhs.fBounds.fBottom - lhs.fBounds.fTop) >> 1);
154 SkRTree(int minChildren, int maxChildren, SkScalar aspectRatio, bool orderWhenBulkLoading);
157 * Recursively descend the tree to find an insertion position for 'branch', updates
158 * bounding boxes on the way up.
160 Branch* insert(Node* root, Branch* branch, uint16_t level = 0);
162 int chooseSubtree(Node* root, Branch* branch);
163 SkIRect computeBounds(Node* n);
164 int distributeChildren(Branch* children);
165 void search(Node* root, const SkIRect query, SkTDArray<void*>* results) const;
168 * This performs a bottom-up bulk load using the STR (sort-tile-recursive) algorithm, this
169 * seems to generally produce better, more consistent trees at significantly lower cost than
170 * repeated insertions.
172 * This consumes the input array.
174 * TODO: Experiment with other bulk-load algorithms (in particular the Hilbert pack variant,
175 * which groups rects by position on the Hilbert curve, is probably worth a look). There also
176 * exist top-down bulk load variants (VAMSplit, TopDownGreedy, etc).
178 Branch bulkLoad(SkTDArray<Branch>* branches, int level = 0);
180 void validate() const;
181 int validateSubtree(Node* root, SkIRect bounds, bool isRoot = false) const;
183 const int fMinChildren;
184 const int fMaxChildren;
185 const size_t fNodeSize;
187 // This is the count of data elements (rather than total nodes in the tree)
192 SkTDArray<Branch> fDeferredInserts;
193 SkScalar fAspectRatio;
194 bool fSortWhenBulkLoading;
196 Node* allocateNode(uint16_t level);
198 typedef SkBBoxHierarchy INHERITED;