[postgis-devel] Tiny WKB

Sandro Santilli strk at keybit.net
Wed Mar 6 02:29:44 PST 2013

I think it has a potential.
Not sure that "decimal digits" makes sense as a parameter as
the semantic of those digits really depends on the width/height
of the layer extent. Should the function maybe just take care
of the encoding leaving quantization to ST_SnapToGrid ?


On Wed, Mar 06, 2013 at 12:27:17AM +0100, Nicklas Avén wrote:
> Hallo 
> This question on gis.stackexchange started some thinking some time ago:
> http://gis.stackexchange.com/questions/15240/how-to-create-vector-polygons-at-the-same-amazing-speeds-giscloud-is-able-to-ren/15947#15947
> It is about not sending vektor-data to a web-client in text-format.
> In the post he discusses putting binary vektor data in an image.
> But that seems a little complicated I thought so I played around with
> parsing wkb in javascript. That works good. But to make it more slim I
> have tried to create a suggestion of a format that I have called TWKB
> (Tiny WKB)
> The idea is to handle relative coordinates, just like you can do with
> svg. But there was a problem. The relative distance from one coordinate
> to another can vary very much. So I have done some testing to have
> variable sizes in the binary stream. 
> The design right now is not optimal at all, but reduces the size from
> the original geometry quite a lot depending on the need of precision.
> I tested on a dataset with 10000 linestrings. With wkb the client
> recived 2.14 mb. With twkb and 1 decimal precision 670 kb and twkb with
> integer values 420 kb. 
> Then is also an integer32 id value included in the TWKB.
> I think it is a good idea for a web-optimized format to include the id
> for several reasons.
> One reason is that it would be possible to nest geometries with an
> aggregate variant of the function. With id preserved, that could be a
> way of creating vektor tiles in the database.
> I think there is many possibilities. This is just a test and some
> suggestions.
> I have created a spike with the code, both for the st_astwkb function
> and the javscript reader ( in extras):
> http://svn.osgeo.org/postgis/spike/nicklas/twkb/
> There is no sql-definitions of the function there, but this is the one
> with most arguments:
> CREATE OR REPLACE FUNCTION st_astwkb(geometry, integer, integer, text)
>   RETURNS bytea AS
> '$libdir/postgis-2.1', 'TWKBFromLWGEOM'
>   COST 1;
> ALTER FUNCTION st_astwkb(geometry, integer, integer, text)
>   OWNER TO postgres;
> where the arguments are in order:
> 1. the geometry
> 2. precision, how many decimals the output should have. Negative values
> gives integers with trailing zeros
> 3. the id to be included in the geometry
> 4. big or little endian, like ST_ASBinary
> So, the function is used like:
> SELECT ST_AsTWKB(geom,1,gid,'NDR') from my_table;
> to get a result with 1 decimal and gid as id.
> All feedback is welcome. Is it a waste of time or has it some potential.
> Regards
> Nicklas
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