[Qt-interest] QGraphicsOpacityEffect performance worse than QGraphicsItem::setOpacity()

[Josiah Bryan]

I'm incredibly excited about the new QGraphicsEffect "module" in Qt 4.6. 
Since the release of the beta last night (today?), I've integrated two 
of the effects (opacity and drop shadow) into the project I've been 
working on for the past two months (http://code.google.com/p/dviz/).

What I found, especially with regards to the opacity effect, was quite 
surprising to me. Specifically, the performance of using 
QGraphicsOpacityEffect to cross fade between QGraphicsItem's is 
*considerably* worse than just using QGraphicsItem::setOpacity() 
directly. A 30 frame, 250ms-long cross fade takes > 5 - 10 seconds due 
to painting using QGraphicsOpacityEffect. Whereas using straight 
QGraphicsItem::setOpacity(), we hit around 275ms - 500ms from start to 
completion of the crossfade. (The 250ms is the target time, the time 
between steps in the crossfade is calculated as 250 / 30 (QTimer is used 
to trigger the frames). The actual painting of the items  often causes 
the actual time to take a bit longer than 1ms to complete each frame, 
pushing it out past the 250ms length. When using QGraphicsOpacityEffect, 
we're hitting north of 150ms - 250ms *per repaint*. Under straight 
setOpacity(), its around 1 - 4ms per repaint. And this is a *simple* 
scene - two rectangular QGraphicsItems - and thats it!

An additional blow to my excitement was the performance of 
QGraphicsDropShadowEffect on a simple rectangular custom QGraphicsItem. 
Changing the blur radius, for example, from 1px to 5px takes around 
1/2second to re-render the scene. (Whereas, when rendering a "cheap" 
drop shadow as just using drawRect() with a different color and offset 
is imperceptibly fast.)

Those are the two major "problems" I've seen in this Qt 4.6 beta - and 
that is on both Windows XP Pro and Linux (FC 8), with both very high end 
graphics cards, dual / quad processor, 4 GB RAM. So the system isn't the 
lag - it seems the library code is (at least on this end.) Anyone else 
seeing similar problems?

Thanks for your time!

Cheers!
-josiah

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-=-=-=-=-=-=-=-=-=-=-=-=-
Josiah Bryan
Productive Concepts, Inc.
jbryan at pciint.com
(765) 964-6009, ext. 224