[PyQt] QGraphisView performance

[Giovanni Bajo]

On 11/22/2008 6:35 AM, Nahuel Defossé wrote:
> Hi,
> I'm trying to display a map on a QGraphicsView widget having some scrolling 
> and zooming. I need very basic interaction, add points, remove points, maybe 
> a contextual menu. The issue I'm experiencing is that zooming and scrolling 
> when on high zoom values is SLOW. 
> For zooming I use view.setMatrix(QtGui.QMatrix().scale(factor, factor)) where 
> factor > 1)
> At first I tried to show directly the map I converted from a DFX file, a big 
> SVG, using QtSvg.QGraphicsSvgItem but it was big and I knew it was gonna take 
> a lot of CPU, it did. Then I tried with a pixmap, PNG 150k, which works 
> better when no zoom is applied but starts to sorely degrade as the zoom 
> factor raises.
> I don't understand which is the best approach to achieve acceptable 
> performance when you have to show big areas on this kind of canvas.
> I'd highly appreciate any hints

My understanding is that QGraphicsView is just slow for this kind of 
thing (displaying a zoomed big picture). I also understand that this has 
been fixed in Qt 4.5, but I am not hundred percent sure.

You could try changing the graphics backend of QGraphicsView from the 
default one (raster or native) to the OpenGL one. I think this can be 
achieved by manually setting a QGLWidget as viewport, or something like 
that. Again, in Qt 4.5 this is just a matter of passing "-graphicssystem 
opengl" on the command line, which makes it easier to play with.
-- 
Giovanni Bajo
Develer S.r.l.
http://www.develer.com