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Re: [Maposmatic-dev] Re: [cairo] Cairo & libpng/zlib parameters

From: Thomas Petazzoni
Subject: Re: [Maposmatic-dev] Re: [cairo] Cairo & libpng/zlib parameters
Date: Tue, 2 Feb 2010 21:45:54 +0100


On Tue, 19 Jan 2010 10:13:04 +0000
Chris Wilson <address@hidden> wrote:

> > Indeed, these overall rendering times also include the time it
> > takes to perform our own queries /before/ mapnik does its job. And
> > these queries can be very loooong, much longer than the time it
> > takes for mapnik/cairo to do the actual rendering. All in all, I do
> > believe that cairo is not the bottleneck for us. And if mapnik were
> > still considered too slow, I'd say it's more because of the sql
> > queries on a loaded postgis DB than because of cairo, but for this
> > I'm not totally sure, not at all.

Definitely, the rendering times given by David Mentré are the total
rendering times, that include both the rendering per se, but also
running all the DB requests we need to perform. And as David Decotigny
said, these requests are currently the bottleneck, not the rendering
through Mapnik/Cairo.

> If anyone ever does find a case where Cairo is a bottleneck, please
> don't hesitate in letting me and the cairo mailing list (or #cairo)
> know. In the meantime, I'll continue following the suggestions given
> above for identifying some interesting test cases to add to Cairo.

The thing that is limiting today is the size of the area that Cairo can
render in PNG mode. Some large French cities (such as Arles) cannot be
rendered because of this, the rendering fails with an out-of-memory.
Rendering the PDF and/or the SVG for the same area works fine, though.

Moreover, since the PNG output of Cairo is said to be "for debugging
only", I think we should investigate the idea of generating only SVG
and PDF with Cairo, and then convert one of SVG or PDF to a PNG image
using another tool. Maybe this other tool will be more capable than
Cairo of generating very large PNG pictures.


Thomas Petazzoni               
Promouvoir et défendre le Logiciel Libre
Logiciels Libres à Toulouse    

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