[Qt-interest] Custom QGLWidget as viewport for QGraphicsView

[Josiah Bryan]

Hey all -

How can I do my own performance sensitive painting as the background for 
a QGraphicsView? What, drawBackground() you say? Ummm...performance is 
terrible with that. Specifically, low-latency updates are very difficult 
to achieve. For example, when trying to render a video stream from a 
live camera (from a PCI capture card) in drawBackground(), the latency 
is quite noticeable - I'd say anywhere from 100ms - 300ms latency (e.g. 
for lip sync with audio or eye blinking.)

Now, the same video stream rendered to a dedicated widget derived from 
QGLWidget renders with virtually no perceptible latency - I'd say less 
than 100ms if I had to guess. (How *does* one quantifiable measure real 
time latency like that - e.g. event happens in front of camera to the 
time it displays on screen? Right now, I test it by seeing if I can see 
my self blink on screen - silly, I know - but it's fairly good judge of 
latency.)

However, trying to render in drawBackground() gives terrible performance 
- less than 1 frame every several seconds! I've optimized QGraphicsView 
using every possible optimization I can think of - setCachdeMode to 
none, render hints, viewport update mode, optimization flags, and of 
course, setting a plain QGLWidget as a viewport - nothing speeds up 
rendering. Even a plain repaint() or update() does NOTHING - I have to 
call scene()->update() to get the drawBackground() method to be called 
again.

My last thought is to try a custom QGLWidget (the one that works fine) 
as the viewport for the QGraphicsView so that QGV can draw the items on 
top of the QGLWidget - but that doesn't seem to do anything. The custom 
QGLWidget never gets to paint anything.

Any ideas or advice?

Thanks!
-josiah


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