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## Data Structures for 3D Searching

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**Data Structures for 3D Searching**partly based on: chapter 12 in Fundamentals of Computer Graphics, 3rd ed. (Shirley & Marschner) Slides by Marc van Kreveld**Data structures**• Data structures for representation • geometry (coordinates) • topology (connectivity) • attributes (anything else like material, …) • Data structures for searching • use the geometry to reduce the number objects that need be tested from all of them to some small subset • windowing queries, ray tracing, nearest neighbors 2**Data structures for searching**• Several steps in the algorithms we have seen until now can be sped up using data structures for efficient searching • Find the k nearest neighbors to each point(straightforward in O(n2) time; linear time per point) • Find the support of a plane tried in RANSAC (straightforward in O(n) time) • Test whether a mesh-changing operator causes self-intersections of the manifold (straightforward in O(n) time) • Also: for ray tracing, windowing 3**Data structures for searching**• Grid structure • Quadtree and Octree • Bounding Volume Hierarchy, R-tree • Kd-tree • BSP tree (Binary Space Partition) 4**Simple grid structure**• Also: cubical grid, cubic partitioning • Tile the space with equal-size cubes and store each object with every cube it intersects • The 3D grid is a 3D array of lists; list elements are pointers to the objects • Small grid cells: much storage overhead, but good for query time • Big grid cells: little storage overhead, worse query time 5**Simple grid structure**ray tracing query 7**Simple grid structure**k nearest neighbors query 8**Simple grid structure**RANSAC line support computation 9**Simple grid structure**windowing query 10**Simple grid structure**• A hierarchical version of a grid may give more efficient querying: one emptiness test at a higher level square may makeseveral tests at lower levelsquares unnecessary 11**Quadtrees and Octrees**• Tree version of the (hierarchical) square/cube grid • Storage requirements better than grid when large parts of the grid contain no objects • In a quadtree • the root corresponds to a bounding square • children correspond to four subsquares : NW, NE, SW, SE • nodes with 0 or 1 objects in their square are leaves • In an octree • the root corresponds to a bounding cube • children correspond to eight subcubes • nodes with 0 or 1 objects in their cube are leaves 12**Quadtrees and Octrees**NW NE NW NE SW SE root leaf leaf leaf storing one object SW SE 13**Quadtrees and Octrees**NW NE NW NE SW SE root leaf leaf leaf storing one object SW SE 14**Quadtrees and Octrees**NW NE This ray tracing query visits 10 nodes in the quadtree (12 squares in the original grid) SW SE 15**Quadtrees and Octrees**• Other queries are performed in the straightforward way • windowing query: easy • RANSAC support of line: determine all cells intersected by the strip, and test points in those cells only for inclusion in the strip, right normal, and therefore support • k nearest neighbors: explore cells further and further away from the query point until we know for sure that we have the k nearest neighbors explore all cells that intersect the circle centered at the query point and through the k-th closest point 16**Octree**18**Quadtrees and Octrees**• Often a quadtree or octree is not made deeper than a desired level of precision • E.g., given an object of 1 x 1 x 1 meters stored in a triangle mesh, store the triangles in an octree up to a cell size of 4 x 4 x 4 millimeters • Leaves that contain more than one object store them in a list (it is assumed that there would only be O(1) of them at this detail level) • Also possible: split a square/cube until at most some constant c objects or points intersect a cell 19**Bounding Volume Hierarchies**• Tree structure where internal nodes store bounding shapes of all objects in the subtree • If a query object intersects the bounding shape, it may intersect some object in that subtree, otherwise certainly not • Bounding shapes can be spheres, axis-parallel bounding boxes, or arbitrarily oriented bounding boxes; axis-parallel BB is most common 20**Bounding Volume Hierarchies**• In graphics usually binary trees • For huge data sets where the data must be on disk, usually higher-degree trees: R-tree 21**R-trees**• 2-dimensional version of the B-tree: B-tree of maximum degree 8; degree between 3 and 8 Internal nodes with k children have k – 1 split values 22**R-trees**• Can store: • a set of polygons (regions of a subdivision) • a set of polygonal lines (or boundaries) • a set of points • a mix of the above • Stored objects may overlap 23**R-trees**• Originally by Guttman, 1984 • Dozens of variations and optimizations since • Suitable for windowing, point location, intersection queries, and ray tracing • Heuristic structure, no order bounds ( O(..) ) • Example of a bounding volume hierarchy 24**Every internal node contains entries (rectangle, pointer to**child node) All leaves contain entries (rectangle, pointer to object) in database or file Rectangles are minimal axis-parallel bounding rectangles The root has 2 and M entries All other nodes have at least m and at most M entries All leaves have the same depth m > 1 and M > 2m(e.g. m = 200; M = 1000) R-trees 25**R-trees**• M is chosen so that a full node still (just) fits in a single block of disk memory • m is chosen depending on a trade-off in search time and update time • larger m: faster queries, slower updates • smaller m: slower queries, faster updates 26**Grouping of objects**Windowing query: the fewer rectangles intersected, the fewer subtrees to descend into 28**Grouping of objects**• Objects close together in same leaves small rectangles queries descend in only few subtrees • Group the child nodes under a parent node such that small rectangles arise 29**Heuristics for fast queries**• Small area of rectangles • Small perimeter of rectangles • Little overlap among rectangles • Well-filled nodes (tree less deep fewer disk accesses on each search path) 30**Searching in an R-tree**• Q is query object (point, window, object); we search for intersections with stored objects • For each rectangle R in the current node,if Q and R intersect, • search recursively in the subtree under the pointer at R (at an internal node) • get the object corresponding to R and test for intersection with R (at a leaf) 39**Nearest neighbor queries**• An R-tree can be used for nearest neighbor queries • The idea is to perform a DFS, maintain the closest object so far and use the distance for pruning pruned closest object so far queried 40**1**4 2 3 5 41**Inserting in an R-tree**• Determine minimal bounding rectangle of new object • When not yet at a leaf (choose subtree): • determine rectangle whose area increment after insertion of R is smallest • increase this rectangle if necessary and insert R • At a leaf: • if there is space, insert, otherwise Split Node 42**Split Node**• Divide the M+1 rectangles into two groups, each with at least m and at most M rectangles • Make a node for each group, with the rectangles and corresponding subtrees as entries • Hang the two new nodes under the parent node in the place of the overfull node; determine the new bounding rectangles (if the root was overfull, make a new root with two child nodes) • If the parent has M+1 children, repeat Split Node with this parent 48**Split Node, example**new bounding rectangles 49**Strategies for Split Node**• Determine R1 and R2 with largest bounding rectangle: the seeds for sets S1 and S2 • While |S1| , |S2| < M – m and not all rectangles distributed: • Take not yet distributed rectangle Rj, add tothe setwhose bounding rectangle increases least Linear R-tree of Guttman, 1984 50