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

[Eric MSP Veith]

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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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