[Qgis-developer] QGIS Multi-threaded Rendering

G. Allegri giohappy at gmail.com
Fri Dec 13 00:56:33 PST 2013

> I believe GeoServer (well, GeoWebCache) uses "metatiling" for that
purpose within its WMTS/TMS. My understanding is that rather than rendering
a single 256*256 pixel tile like it was asked to, it renders a grid of 4*4
(adjustable; but that's the default) of those tiles (so 1024*1024 pixels)
and then clips that to get the requested tile. The result is that labels
look quite good crossing tile borders. Maybe something similar could work
for QGIS.

Metatiling is a trick emplyed by almost every caching/tiling system. It's
on the client side. While buffering is made by the server.
Using both of them give the best results, but they're not alternatives.


> Jonathan
> On 12 December 2013 16:23, Bernhard Ströbl <bernhard.stroebl at jena.de>
>> Hi Martin,
>> Am 12.12.2013 16:50, schrieb Martin Dobias:
>>> Hi Tim!
>>>> That all sounds absolutely brilliant! Thanks for such a nice clear
>>>> description of how it all fits together. I know you are only
>>>> layer-by-layer rendering but does your design accommodate further
>>>> optimisations easily? I'm thinking of things like:
>>>> * predictive / off screen  rendering of 3x3 canvas dimensions after the
>>>> initial render so that any pan is near instantaneous (and would
trigger a
>>>> new off-screen render)
>>> I have had this idea in my mind while working on the project. In
>>> theory map canvas can spawn several renderer jobs (instead of just
>>> one) and let them render just one tile of the map. Some special
>>> handling of the labeling would be necessary if we wanted labels that
>>> are allowed to cross the border of tiles.
>> that reminds me that I needed some way to keep a stripe around the edge
free of labels in QGIS server for tiling purposes (labels were cut)
>> if you address this issue it would be nice to have this somehow
customizable for QGIS server.
>> Bernhard
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