[gdal-dev] Vector Tiles in OGR

Sun Jan 31 14:22:31 PST 2016

All,

The first thing that is going to be required likely is a good generic c++ library for encoding and decoding vector tiles IMO. 

I have spent a lot of time working on developing the mapnik vector tile library to be a solid implementation, especially around all the work done for 2.0 of Mapbox vector tile specification. I would love to eventually help make this a generalized implementation so that it is portable to a library like GDAL.

We have talked about doing something like this at Mapbox but will require a good deal of time, so it hasn't been pushed forward yet. The other issues that come to mind to me is that some of the libraries that mapnik vector tile depends upon most likely need good custom implementations if we wanted to make it more generic.

The reason for this is that I would prefer for it to be a header only library with no dependencies. Adding more dependencies for GDAL seems like it could be a mess.

Blake Thompson

> On Jan 31, 2016, at 10:10 AM, Stefan Keller <sfkeller at gmail.com> wrote:
> 
> Hi Petr, Blake, Even and everybody
> 
> I'm (besides Petr) the other maintainer of OSM2VectorTiles.
> I'd already asked Even a similar question.
> I'm advising now an intern to implement such a MVT Vector Tiles reader
> client in Python if possible as part of a QGIS plugin.
> And I'm interested not to do redundant implementations.
> Are there any news on this matter?
> 
> :Stefan
> Prof. at HSR and leader of Geometa Lab
> 
> @Even: Regarding OGR MVT development can you send to me (directly) a
> contact name of the group which won the bidding?
> 
> 
> 2016-01-06 15:24 GMT+01:00 Petr Pridal <petr.pridal at klokantech.com>:
>> Hi everybody,
>> 
>> thanks for the comments. Regarding the technical details:
>> 
>>> Each Tile is its own discrete dataset therefore the hardest part would be
>>> merging of features into the original features across tiles. You could use
>>> some complicated merging technique, but that could be very expensive.
>> 
>> 
>> The driver could require "id" parameter, which specifies name of an
>> attribute with unique identifier on the features shared across the tiles.
>> Both MapBox vector tiles and OSM2VectorTiles tiles have an unique non-zero
>> "osm_id" in place. The first version of driver could read only features with
>> such ID. The identifier does not have to be always filled, but often it is,
>> especially in deeper zoom levels derived purely from OpenStreetMap - but
>> yeh, a completely general stitching would be much harder.
>> 
>> The clipping + merging of shapes itself can be implemented with underlaying
>> GEOS library functions.
>> 
>> BTW It is possible to examine vector tiles data in pure JavaScript X-Ray
>> viewer (no WebGL support on browser required) made in OpenLayers3:
>> http://klokantech.github.io/ol3-sandbox/vector/xray.html
>> 
>> or with WebGL-enabled webbrowser with higher performance MapBox GL JS
>> powered X-Ray:
>> http://klokantech.github.io/mapbox-gl-js-offline-example/xray.html
>> 
>> A debug viewer for each vector tile (made in OL3) decoded in pure
>> JavaScript:
>> http://klokantech.github.io/ol3-sandbox/vector/tile-inspector.html
>> 
>> The OGR driver could be also practical for the unsimplified / ungeneralised
>> MBTiles with OSM data available at: http://osmlab.github.io/osm-qa-tiles/
>> 
>>> 
>>> If you wanted to simply view vector tile data it could get confusing as
>>> well since most vector tiles need a buffer when they are used for
>>> visualization programs. This is important for things like labeling on maps
>>> etc. Therefore, you could get a large amount of overlapping data on tiles if
>>> you loaded them all in at once.
>> 
>> 
>> Clipping on border of each tile must be applied before stitching the vector
>> features together  - so the buffer is removed. Buffer is there only for
>> visualisation and labeling.
>> 
>>> 
>>> As far as writing a simple decoder -- the mapnik-vector-tile decoder isn't
>>> currently able to decode vector tiles into a form not supported by mapnik.
>>> Therefore, OGR would not be able to easily use this.
>> 
>> 
>> Correct. It can't be used directly (GDAL can't link against), but the
>> relevant code can be extracted and reused directly. For basic decoding
>> critical is the Protobuf library (which is already in GDAL) and
>> https://github.com/mapbox/mapnik-vector-tile/blob/master/proto/vector_tile.proto
>> + the code for converting pixel coordinates in features to relevant geo
>> coordinates and GEOS-based merging of features.
>> 
>>> We plan to write a generalized encoder and decoder eventually so that any
>>> other library can plugin with out the mapnik requirement.
>> 
>> 
>> This would be amazing.
>> 
>>> If you have any questions specifically about Mapnik Vector Tile or the
>>> Mapbox Vector Tile Specification feel free to ask me. I would note that you
>>> guys will want to update your vector tiles once the V2 specification changes
>>> are into Mapnik-Vector-Tile.
>> 
>> 
>> Cool! Thanks. Our world tiles are generated via Docker containers on a
>> cluster of computers - using Mapnik via Tilelive directly - so upgrades on
>> Mapnik-Vector-Tile will propagate to the open-source rendering software.
>> I expect V2 spec will mean also new MapBox Streets V7, right?
>> 
>>> I see you mention MVT in MBTiles. Is it a recognized practice ? I don't
>>> see
>>> mention of that neither in the MVT spec (
>>> https://github.com/mapbox/vector-
>>> tile-spec/tree/master/2.0 ) nor the MBTiles one (
>>> https://github.com/mapbox/mbtiles-spec/blob/master/1.2/spec.md ), although
>>> I
>>> guess it is just a matter of storing a MVT blob in the tile_data column.
>> 
>> 
>> Yes. MapBox Studio Classic generates such MBTiles with Vector Tiles inside -
>> and Mapnik reads them directly when rasterising. Storing the vector tiles in
>> SQLite make sense - and it simplifies deployment, and you can then easily
>> have complete world on each node of a cluster of tileservers. If you don't
>> want to upgrade your base map too often, this approach is pretty efficient
>> and removes the centralised database as typical bottleneck.
>> 
>> I did some tests on vector tiles vs raster in MBTiles and documented these
>> in README.md at and https://github.com/klokantech/vector-tiles-sample before
>> we started to work on OSM2VectorTiles project.
>> There are also sample data and offline MapBox GL JS viewer in that repo.
>> Vector tiles can be hosted also "unpacked", just as files in folders on a
>> standard web server - exactly like GDAL2Tiles or MapTiler.com does by
>> default for raster tiles.
>> 
>> If OGR driver is implemented, the primary source should be probably reading
>> from an URL via curl.
>> 
>> Reading PBFs from an MBTiles (or another SQLite) is just the most practical
>> portable alternative to it.
>> 
>> Technically people may store the tiles in different ways - for example
>> Wikimedia guys (http://maps.wikimedia.org/ and
>> https://github.com/kartotherian/) use Cassandra for tile storage, I guess
>> MapBox internally is also on a different storage for production servers ...
>> for transfers and streaming tilesets there is the  - but tiles are ALWAYS
>> exposed via HTTP.
>> 
>> Regards,
>> 
>> Petr
>> --
>> Petr Pridal, Ph.D.
>> CEO
>> 
>> Klokan Technologies GmbH
>> Hofnerstrasse 98, 6314 Unterageri, Switzerland
>> Tel: +41 (0)41 511 26 12
>> Email: info at klokantech.com
>> Web: http://www.klokantech.com/
>> 
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