[Qt-interest] Custom QGLWidget as viewport for QGraphicsView

[Ed Sutton]

>(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?


A crude way that will give you a *rough* idea:


 1.  Get a stop watch or timer running with hundreds of seconds accuracy or better. ( I think the iPhone stop watch only has 0.1 second resolution. )
 2.  Place stop watch next to your software running your video display and point your video camera at the running stop watch.
 3.  Take a picture with a camera with both the video display and the physical stopwatch in frame and compare the differences between physical and displayed.

Issues:
Frame rate of video camera, video screen refresh rate, etc.


I am also using a widget derived from QGLWidget that renders a video frame decoded to a bitmap as a 2-D texture.  This is very fast as it uses the NVidia hardware acceleration.  However, all I am simply displaying video with some text drawn on top.  It sounds like you are doing something more complicated.

-Ed


On Mar 4, 2010, at 12:49 PM, Josiah Bryan wrote:

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


--

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

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-Ed