[Qgis-developer] QgsGeometry

[Marco Hugentobler]

Hi Martin,

Thanks for the feedback.

> In my opinion, we should
> only support WKB import and export, but represent the geometries as a
> GEOS geometry

I think that with data sources that provide only wkb geometries (e.g. ogr or 
postgis), there will be some overhead for drawing if we store geometries only 
in geos format internally.
Currently, it is:
postgis -> wkb -> drawing in vectorlayer
and with internal storage as geos:
postgis -> wkb -> geos -> drawing in vectorlayer

>My big
> wish about it is to stop completely doing the operations on geometries
> with WKB and start fully using GEOS for geometry manipulations

You are right that they shouldn't be done in WKB. But my experience with geos 
is that manipulation code can have similar length as with WKB, in some cases 
even more. In my opinion, the classes QgsPoint, QgsLinestring & co. are very 
well suited for manipulations. It is possible to just change one coordinate 
value, e.g. for a vertex move, without the need to do the whole geometry 
construction again (as in geos).
It may not be very performant to convert QgsPolyline to geos and than geos to 
wkb, as it is currently done with asPolyline(). But, unlike the drawing code, 
the manipulation methods are not performance critical at all since only few 
features are manipulated at once. 
So my preferred solution would be to move manipulations into separate classes 
which get and set geometry from QgsGeometry with 
QgsPoint/QgsLinestring/QgsPolygon & co. 
Would that make sense?

> If the class would be still too huge, I'm for splitting
> it into some smaller ones.

I think it would still be too huge only with geos representation. Splitting 
sounds good.



Regards,
Marco 



-- 
Dr. Marco Hugentobler
Institute of Cartography
ETH Zurich
Technical Advisor QGIS Project Steering Committee
marco.hugentobler at karto.baug.ethz.ch