[OSM-dev] OpenStreetMap Cartographic: A client-side rendered OpenStreetMap Carto

Christoph Hormann osm at imagico.de
Tue May 26 14:43:58 UTC 2020


On Tuesday 26 May 2020, ndrw6 at redhazel.co.uk wrote:
>
> Rasterization is a data generalization technique too.

No it isn't.  Generalization is the most frequently abused term in 
cartography.  Calling ST_Simplify and way_area filtering on OSM 
landcover polygon data generalization is a bit like calling JPEG 
compression noise reduction.

But we are drifting off topic here.

> Yes, in some 
> cases it leaves no artifacts at all 

Who would say that?  In fact sampling artefacts (a.k.a. aliasing) are 
the mother of all artefacts in computer graphics.

The point is that sampling artefacts are very well understood and it is 
known how to minimize them.

> I've been looking into the code and I couldn't find anything related
> to polygon generalization. Where is it done?

Lossy geometry compression is happening in ST_AsMVTGeom - which is not 
just the inevitable coordinate rounding to the vector tile coordinate 
grid but also the subjective choice of per feature Douglas-Peucker line 
compression.  See

https://github.com/postgis/postgis/pull/463

for the well argued suggestion not to do that.  The way_area filtering 
is explicit in the SQL code.

-- 
Christoph Hormann
http://www.imagico.de/



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