[Qt-interest] Modification for faster coordinate clipping in QOutlineMapper

[Marco Hugentobler]

Hi

For applications that display geographic data (lakes), it is quite common to 
display large polygons (100000 vertices). 

One problem we have is that when zooming in very close, drawing polygons can 
take very long time (several minutes). Doing a bit of debugging, I found that 
most of the time is spent in the clipping code at 
src/gui/painting/qoutlinemapper.cpp, function QOutlineMapper::clipElements 
where the coordinates are clipped to +- 32767
 
In our GIS application, we already have code to do coordinate clipping based 
on Sutherland and Hodgman's polygon-clipping algorithm and I noticed that it 
runs much faster (but seems to me it is only suitable for straight line 
segments). So I extended QOutlinemapper to use this algorithm for large pathes 
without curves and it seems to work quite well for me.

If there is an interest in my modifications, the changed source files are 
available at:

www.hugis.net/qt/qoutlinemapper_p.h
www.hugis.net/qt/qoutlinemapper.cpp

I hope that this performance improvement (or any other improvement that 
enhances clipping speed) can be included in future versions of Qt.


Regards,
Marco




-- 
Dr. Marco Hugentobler
HUGIS - GIS Programmierung und Beratung
Honrainweg 5
CH-8038 Zürich
marco at hugis.net