[Qgis-developer] Details of rendering GIS map data in QGis

[Marco Hugentobler]

Hi Eric

The class QgsMapToPixel  (src/core/qgsmaptopixel.cpp/.h) contains the 
transformation from map to pixel coordinates. Is this what you are looking 
for?

Regards,
Marco

Am Freitag, 30. September 2011, 16.32:13 schrieb Eric MSP Veith:
> Hello list,
> 
> first things first: This post is not strictly about QGis itself, but how
> about it renders the raw spatial data. If you happen to know a better
> place where to ask this question, please point me at it and ignore the
> rest of this e-mail. Thanks.
> 
> I am currently creating a simulation environment which lives of an
> interactive map. This map has its source in a PostGIS data base, from
> which I select only few general data (administrative boundaries, water
> areas, land, cities) to provide some points of orientation for the user.
> For the displaying itself I use Qt's drawing classes, i.e. QGraphicsView,
> QGraphicsScene, etc., which I subclass in cases needed.
> 
> I am currently trying to find a good (technical) way of displaying the data
> to the user. I first tried a simple approach: I fetched points from the
> PostGIS database, calculated a scale factor from the size of the
> QGraphicsView instance and the size of the layer fetched from the
> database, and multplied every value with that scaling factor. It worked
> (kind of), but of course now everything is upside down, since Qt's drawing
> begins from the top-left corner, whereas POINT(x, y) values from the
> database are actually POINT(lat, long).
> 
> I am wonderin how QGis does that, because it looks nice. I already checked
> out the source files from Git, but without knowledge of it I get lost
> fast.
> 
> I'd really appreciate some sentences of advice and/or a pointer to the
> relevant methods and classes in the QGis sources.
> 
> Thank you so much in advance.
> 
> 			Eric
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-- 
Dr. Marco Hugentobler
Sourcepole -  Linux & Open Source Solutions
Churerstrasse 22, CH-8808 Pfäffikon SZ, Switzerland
marco.hugentobler at sourcepole.ch http://www.sourcepole.ch
Technical Advisor QGIS Project Steering Committee