[Qgis-developer] comments on qgsvectorlayer.cpp

Albert Cervera i Areny albert at nan-tic.com
Fri Aug 31 15:44:48 EDT 2007

Thanks for your response Martin. 

A Divendres 31 Agost 2007, Martin Dobias va escriure:
> That clipping code has been added to overcome a nasty bug with Qt3 on
> X11 platform where we were getting freezes of the whole system. But
> also for Qt4 we had to leave it there - in Qt4.1.x QGIS was crashing
> sometimes at greater zooms during the rendering. Maybe it's not a
> problem anymore with Qt4.2 or 4.3 as Trolltech devs tried to make the
> drawing routines more roboust, but I don't know if this was fixed too.
> Anyway, the clipping amgorithm is pretty fast and IIRC removing it was
> just about 1-2% performance gain - not a big deal...

With Qt3 I've seen it documented but not with Qt4. I'd be nice to know if 
they've solved that, maybe not a problem for performance but quite ugly 

> I've been thinking about this - mostly about where to put the caching
> mechanism. There are generally two options:
> - cache objects ready for painting (as you suggest) - e.g. save
> ready-made QPolygonF instances
> - cache objects fetched from data provider - e.g. save QgsFeature instances

I've been thinking about this and I've found that maybe the most intuitive 
thing I had done is to consider every polygon or groups of them (that is the 
result of drawPolygon for example) a new item in the QGraphicsView scene. 
Indeed, Qt4 has the QGraphicsPathItem class which holds a path and its brush 
(i think nothing else is needed). So qgis should only worry to create those 
items on demand and leave them created once they have been shown once. (Some 
enhancements could be done by removing some with some cache heuristics, but 
you get the idea). This way, no real cache is created, indeed QGraphicsScene 
holds the item for you and draws them when needed, and can make it's internal 
optimizations which is stuff that QGis won't have to worry about.

In this case I don't quite see why zooming and moving should be a problem, as 
QGraphicsView has been designed exactly for this kind of things. Even for 
dragging and clicking the items if needed. You just create the paths 
(polygons, lines, ...) in QGraphicsPathItem's and Qt will handle translations 
and zooming etc. With the small view of all the architecture I have, it seems 
that QGis should only translate layer coordinates to scene coordinates. 
Rotation, zooming and translation should be handled in scene/view coordinates 
by Qt scene/view arquitecture.

Of course, I'm only talking about vector layers, not bitmaps.

Is all that reasoning feasable or am I missing something?

Albert Cervera i Areny

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