[OSM-talk] Mapnik Lowzoom TIles - proof of concept

Frederik Ramm frederik at remote.org
Sun Jun 30 13:17:43 UTC 2013


Hi,

On 30.06.2013 10:27, Christoph Hormann wrote:
> what method did you use for resizing the tiles?  It seems to be
> different in the new and the old tiles emphasizing the line features
> now.

Yes, I simply didn't manage to reproduce last year's version exactly.

What I do is precisely the following:

* Start from a modified z9 rendering (with thicker linework, no rivers, 
and lighter area colouring) - these are my "raw" tiles for z9.

* For tiles on zoom "z-1", use tiles from "z" and ImageMagick's "Scale" 
method to downscale them to 50%; merge labels onto these tiles; the 
result is the "cooked" tiles on z-1.

* To produce the "raw" tiles for z-1 (which are never visible but only 
used as an input for z-2 tiles later), I not only downscale the tiles on 
z, but after that, use ImageMagick's "Compose" function with a mode of 
"Darken" to repeat the placement of downscaled tiles three times, with 
an offset of (0,1), (1,0) and (1,1). This step is crucial because 
otherwise the roads would thin out to a "hint of road" after 2 
scale-down steps, but at the same time this is also resposible for the 
darkening of tiles as you get to lower zooms. (The "Darken" method 
chooses the darker of the two inputs.)

I would be happy to hear ideas about how to do this differently, and try 
them out.

> And when rendering the various area features at z=9 it would probably be
> best to turn off the area patterns like on the glaciers which just
> result in artefacts when scaling down.

Yes, I've currently commented out a large amount of areas but should 
maybe look more closely.

Bye
Frederik

-- 
Frederik Ramm  ##  eMail frederik at remote.org  ##  N49°00'09" E008°23'33"



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