[Tile-serving] Real time tile merging strategy

[Jon Burgess]

On Thu, 2013-03-28 at 19:41 +0100, sly (sylvain letuffe) wrote:
> Hi again,
> 
> Is anyone aware of a renderd or tirex option (or other software ?) able to 
> real time merge 2 or more (meta-)tiles themself generated by renderd before 
> serving it to client ?
> 
> The use case is this :
> I have a never changing hillshaded/colored + contours background who takes 
> ages to generate and a permanently updated osm layer on top of that, much 
> faster to generate.

I was given a patch by Andy Allan which implemented this feature. It has
been sitting in my inbox for a very long time and probably needs some
clean up to apply to the current code. 

Andy's description follows:

=====

Attached. It should be fairly self-explanatory, I think, and I'm happy
to release it to upstream if you want to incorporate it into mod_tile.

The foo.layerset files are a bit strange, since there's two formats on
my side. One is a nice XML format that my custom versions of
generate_tiles.py et all can read fine, but I don't think Dave wanted to
write an XML parser in C so there's another pre-processing step to
create a plain layerset file which is just

map-file-1.xml 1.0
map-file-2.xml 0.3

etc. The end result is the same.

I know that various people have talked about adding layer-level opacity
to mapnik, but if it comes up at the code sprint then I would say the
best place is one step further up at what we call "layerset" level.
Obviously each of these "layersets" could, if needed, have only one
<Layer> in them, but from a cartography point of view it's much better
having multiple <Layer>s in a "layerset" and applying an global opacity
to that.

Mind you, talk with Mike Migurski and I'm sure he'll come up with other
composition modes rather than just alpha-blending!

Cheers,
Andy








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