On Wed, Dec 1, 2010 at 8:18 PM, Scott Crosby <[email protected]> wrote:
> On Wed, Dec 1, 2010 at 7:27 AM, Nic Roets <[email protected]> wrote: > > Hello Scott, > > > > How do you keep track of what bboxs each entity belongs to ? > > An Int2ShortMultiMap implemented by composing two underlying > Int2ShortMap implementations with different space efficiency > tradeoffs, a custom sparsearray > implementation based on > http://google-sparsehash.googlecode.com/svn/trunk/doc/implementation.html, > and one imported from a library of java collections specialized to > primitive types ('fastutils') that uses a standard hashtable. The > hybrid has a memory overhead of about 4 bytes per node output, storing > 750m keys with 800m vals in 3.2gb of heap. > > The approach for generating an Int2ShortMultiMap from several > Int2ShortMap's is by layering them. When put()ing keys, I store in > Int2ShortMap[0], but if the key is already there, I try > Int2ShortMap[1], ..... until I find one that is free. I create > additional maps as needed. For dense maps, sparsehash is the more > efficient implementation, and for non-dense maps at the higher layers, > a hashtable is the more efficient implementation. > > To avoid checking each bbox for each point, used a simpler design than > a quadtree or r-tree as andrzej suggests, some precomputing and binary > searches at a cost of (sqrt(n)) instead of O(log n) for mostly > non-overlapping-regions. > > > > > I'm not really asking a question, I'm just saying that I found a way to > > reduce the memory requirement for that considerably. Instead of a bit per > > bbox per entry, I store only 16 bits or 32 bits per entry. Here is the > > source. > > > http://trac.openstreetmap.org/browser/applications/rendering/gosmore/bboxSplit.cpp?rev=24484 > > Definitely. Can you explain how you track the information? > > You have a much more concise implememtation than mine, but I can't > easily figure it out what it is doing from the source code. > Let's say there are 80 bboxes, then you can use a 80 bit number for each entity to record which bboxes they fell into and which they didn't. That 2^80 space is extremely sparse. For my regular splits of 80 bboxes there are typically only 1000 combinations that actually occur. So I store 1000 "younions" and then a 16-bit index per entity. And my bbox are as chaotic as they come: http://dev.openstreetmap.de/gosmore/ > > Scott >
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