[pgrouting-dev] Re: Network Layering support
Jay Mahadeokar
jai.mahadeokar at gmail.com
Fri Feb 18 15:53:47 EST 2011
Hi Anton,
On Fri, Feb 18, 2011 at 6:27 PM, Daniel Kastl <daniel at georepublic.de> wrote:
> Hi Jay,
>
> Thank you for taking a look at the source code!
> I think Anton can answer better to your questions, so I actually just want
> to send you two additional links I just found about now:
>
> - http://routingdemo.geofabrik.de/
> - http://sourceforge.net/projects/routed/
>
> It uses contraction hierarchies algorithm.
>
Thanks for the links. Are you certain that the above project uses
contraction hierarchies algorithm by Robert Geisberger? I went thru the
source, and they are using contraction, but I did not see where they have
used the node ordering step as proposed in the CH paper. I did not find
contraction hierarchies mentioned in their readme too:
http://sourceforge.net/apps/trac/routed/wiki/ReadMe
Anyways, they are using osm file as input and the output consists of two
files 'file.osrm' and 'file.osrm.names'. Which is again preprocessed to
generate file.osrm.hsgr' 'file.osrm.nodes' which are then used for
answering queries.
Again, a method to keep the data pre-processed in order to speed up
dijekstra.
> Regarding the license (AGPL) and the one of pgRouting (GPL) we probably
> need to make sure, that CH will become an optional component.
>
> From my level of understanding I agree with Steve, that CH would just
> provide another way of pre-processing the data than we do now. If this is
> possible and how it works best within a database, that's probably the big
> challenge.
>
I agree. So, basically if we want to add CH to pgRouting we will need
following components:
*
Preprocessing*
Tool like osmtopgrouting_CH that would preprocess the osm files to postgres
tables.
If we are using the code already made available thru AGPL (I dont
understand the intricacies of different licences) by Robert Geisberger,
then we can first have following:
A. Module that converts osm data to ddsg files.
B. Use the above code to convert ddsg files into .ch(contraction
hierarchy) files
C. Convert the .ch file into postgres_ch database table.
*Quering*
If you look at the CH algorithm, the main idea is to add shortcut edges
using node ordering such that the overall new edges added justify the extra
space used in terms of speed-up gained.
The .ch file produced retains original edges, but also contains the added
short-cut edges. Now, suppose we have converted this info into postgres
table. According to my intuition, the original shortest path query should
readily work on this table data and provide considerable speed up due to
shortcuts.
The CH paper also talks about various refinements in the shortest path algo
which will enhance the speed even more. Techniques include the concept of
searching on Upward and Downward graphs, pruning search space using
stall-on-demand technique etc. (I have just had an overview and not
understood the techniques in complete detail for now)
So, this can be a completely different module :
Shortest path query on the CH preprocessed table using the speed-up
techniques mentioned in CH paper.
I tried to put forward my understanding of the problem. Need to take a
deeper look into source code as well as paper before the above points can be
confirmed.
>
> @Anton ... did you catch this conversation?
>
> Daniel
>
>
>
> 2011/2/17 Stephen Woodbridge <woodbri at swoodbridge.com>
>
> On 2/16/2011 10:17 PM, Jay Mahadeokar wrote:
>>
>>> Hi,
>>>
>>>
>>> On Sat, Jan 29, 2011 at 6:13 AM, Stephen Woodbridge
>>> <woodbri at swoodbridge.com <mailto:woodbri at swoodbridge.com>> wrote:
>>>
>>> *Source Code *(Quote from their web-page)
>>>
>>> /The source code of our implementation of contraction
>>> hierarchies (CHs)
>>> is now available under the terms of the AGPL. If you are
>>> interested in
>>> getting the source code write an email to Robert Geisberger
>>> <http://algo2.iti.kit.edu/english/geisberger.php>.
>>> /
>>>
>>>
>>> I had requested Robert Geisberger for the source code of CH, and I went
>>> thru it (Grateful to him for making the source available under AGPL). It
>>> takes as input .ddsg files and produces contraction hierarchies in .ch
>>> file format.
>>>
>>> *.DDSG* *File Format:*
>>>
>>> * a /text/-file, whitespace-separated;
>>> * starts with single letter 'd';
>>> * followed by the number of nodes /n/ and the number of edges /m/
>>> * for each of the /m/ edges, we have
>>> o source node ID /s/, an unsigned 32-bit integer, 0 <= /s/ <
>>> /n/;
>>> o target node ID /t/, an unsigned 32-bit integer, 0 <= /t/ <
>>> /n/;Quering
>>> o edge weight /w/, an unsigned 32-bit integer; note that the
>>>
>>> length of the longest shortest path must fit into a 32-bit
>>> integer
>>> o the direction /d/: //
>>>
>>> + //0 = open in both directions //
>>> + //1 = open only in forward direction (from /s/ to /t/)
>>> //
>>> + //2 = open only in backward direction (from /t/ to /s/)
>>> //
>>> + //3 = closed //
>>> //Note that specifying an edge (s,t,w,1) is equivalent to
>>> specifying an edge (t,s,w,2). It does not matter which form
>>> is used. In the current implementation, 3 is interpreted as
>>> 0, i.e., closed roads are used in both directions. If you
>>> really want to completely close a road, just leave it away. //
>>>
>>
>> So this seems to define the minial requirements of the "ways" table in
>> that we need to easily be able to extract this data and pass it to the
>> pre-processor. And maybe the results need to be able to fetch some of this
>> data.
>>
>> *CH File Format:*
>>>
>>>
>>> * a /binary/ file, 32-bit-interger organized
>>> * layout:
>>> o "|CH\r\n|" (|0x32 0x48 0x0d 0x0a|)
>>> o unsigned int: version (currently "|1|", shold be |==|
>>> compared)
>>> o unsigned int: number of nodes (= n)
>>> o unsigned int: number of original edges (= m_1 )
>>> o unsigned int: number of shortcut edges (= m_2 )
>>> o n times, for each node 0..(n-1):
>>> + unsigned int: level
>>> o m_1 times, original edges:
>>>
>>> + unsigned int: source node
>>> + unsigned int: target node
>>> + unsigned int: weight
>>> + unsigned int: flags
>>> o m_2 times, shortcut edges:
>>>
>>> + unsigned int: source node
>>> + unsigned int: target node
>>> + unsigned int: weight
>>> + unsigned int: flags
>>> + unsigned int: shortcut middle node
>>> o unsigned int: |0x12345678| as terminator
>>> * possible (bit) flags are:
>>> o |1| = forward edge
>>> o |2| = backward edge
>>> o |4| = shortcut edge
>>> o Note that not all edges of the original graph are listed as
>>>
>>> original edges. Edges that are not on any shortest path may
>>> be removed or replaced by shortcuts.
>>>
>>
>> Seem like this data could be put in tables or blob(s) depending on how
>> much there. Obviously get data in/out of the database instead of these files
>> will have an impact on the code, but hopefully that is manageable.
>>
>> *Few queries:*
>>>
>>>
>>> 1. If pgRouting has to support the routing using contraction
>>> hierarchies, what exactly is the idea? The postgres database table like
>>> "ways" will be taken as input and the output would be another postgres
>>> database table that will include data in form of contraction hierarchies?
>>>
>>
>> I would envision that we take a table like "ways" as input. I could see
>> additional columns or tables that might contain other data like turn
>> restrictions, or whatever might be needed for input. But to keep it simple
>> something similar to the current table which has edges, geometry, costs,
>> etc. I might have nodes assigned or not if you process did that.
>>
>> Today we prep the "ways" table by creating the "source" and "target" node
>> number columns and run assign_node_id() process to create nodes for each
>> edge. I see some kind of process the reads this data and creates the
>> contraction hierarchies data that then gets stored back into the database
>> however you deem appropriate.
>>
>>
>> 2. The queries will be same like shortest_path() just that at backend
>>> the contraction hierarchies pre-processed data be used?
>>>
>>
>> Correct. The input might change because the requirements for contraction
>> hierarchies might be different, but a route request would be started by a
>> call to a stored procedure. The result should be a record set where each
>> represents traversing a single segment in the original "ways" table. For
>> example the results might be simply a list of gid in the order of traversal
>> or something more. Some example results might be:
>>
>> Each of these represents a set of record:
>>
>> 1. gid
>> 2. gid, cost
>> 3. gid, reversed, cost
>> etc
>>
>> gid - unique edge id from the "ways" table
>> cost - cumlative cost to get to the end of this segment along the path
>> reversed - Y|N flag to indicate if traversed backwards or forwards
>>
>>
>> 3. What should be the exact format for representing data for contraction
>>> hierarchies?
>>>
>>
>> I'm not sure what you are asking here. If this is the internal contraction
>> hierarchies data created by the preprocessing step, then I think this is up
>> to you. You might want to store it as a blob, if data can be spatially
>> organized so you can fetch it as needed, then some brainstorming on how to
>> do that might be appropriate.
>>
>> To some extent, performance of this process in the database will probably
>> be limited to how efficiently you can access the data need to process the
>> request.
>>
>> In the current process, we give a bounding box for the data we need,
>> because we have to read that data and then build a boost graph structure and
>> then solve that graph. Since the data is preprocessed, I'm not sure that is
>> need for this so we might just have something like:
>>
>> select start_node from CH_pnt_to_node(start_point);
>> select end_node from CH_pnt_to_node(end_point);
>> select * from CH_solve("table", start_node, end_node);
>>
>> or something like that. Basically if there are helper function needed then
>> that is appropriate.
>>
>> Best regards,
>> -Steve
>>
>>
>>> Right, but there are also a few other implementations of the
>>> contraction hierarchies in the wild that might not be based on their
>>> code, like the MONAV implementation, which is released on GPLv3, and
>>> at least one person was looking at it in conjunction with Boost
>>> (google: contraction hierarchies boost)
>>>
>>>
>>>
>>> --
>>> Regards,
>>> -Jay Mahadeokar
>>>
>>>
>>>
>>> _______________________________________________
>>> pgrouting-dev mailing list
>>> pgrouting-dev at lists.osgeo.org
>>> http://lists.osgeo.org/mailman/listinfo/pgrouting-dev
>>>
>>
>> _______________________________________________
>> pgrouting-dev mailing list
>> pgrouting-dev at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/pgrouting-dev
>>
>
>
>
> --
> Georepublic UG & Georepublic Japan
> eMail: daniel.kastl at georepublic.de
> Web: http://georepublic.de
>
> _______________________________________________
> pgrouting-dev mailing list
> pgrouting-dev at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/pgrouting-dev
>
>
--
Regards,
-Jay Mahadeokar
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.osgeo.org/pipermail/pgrouting-dev/attachments/20110219/8dcc8251/attachment-0001.html
More information about the pgrouting-dev
mailing list