[osg-users] Qt + non-continuous draw + pagedLOD: Random loss of detail??

Glenn Waldron gwaldron at gmail.com
Wed Jul 30 05:54:50 PDT 2008


John,

I have run into the same issue. PagedLOD children "expire" after a certain
period of time (default is something like 5 seconds?) If a PagedLOD child
has not been visited in the last N seconds, the pager will discard it and
revert to a lower LOD. OSG updates the "time-last-visited" during the cull
traversal in order to check for expiration. So, if you are not continuously
running the frame loop, PagedLOD's will think their children have expired
and will drop to the lowest LOD.

I used to refer to the technique you're using as "lazy rendering" (i.e. only
render a frame when something has changed) ... try searching the archives
... but at the time I never did find a good way to do this in OSG without
confusing the pager.

Glenn

-- 
Glenn Waldron : Pelican Mapping : http://pelicanmapping.com :
+1.703.652.4791

On Wed, Jul 30, 2008 at 8:39 AM, John Vidar Larring
<larring at weatherone.tv>wrote:

> Hi all,
>
> Background:
> -----------
> In our application the user can set up scenes ( or sequences as we call
> them) for playback on a timeline (e.g. seq.1 - transition - seg.2 -
> transition ... seq.N). Some scenes are too complex to render real-time and
> are recorded to movies before playback. Hence, when scenes are edited by the
> user all scenes are redrawn as needed rather than continuously to avoid
> sluggish GUI response.
>
> Problem:
> --------
> _Sometimes_ when the scene is redrawn in edit mode (i.e. redraw only if
> needed), the pagedLOD DB (osgTerrain) drops to the lowest detail level (Ref.
> osg_original.jpg and osg_problem.jpg). This behavior seems to appear at
> randomly. In playback mode (i.e. when frame rate is constant) the problem
> never occurs.
>
> Reproduce / Code example:
> ----------
> The problem has been reproduces in a small test program
> (osgtest_source_code.tgz) derived from the osgviewerQT example (since we are
> using Qt 4.x in our application). To reproduce: run the compiled
> application: 'osgtest -t <your osgTerrain db>' . To trigger refreshes,
> occlude parts of the window with another window. The problem typically
> reveals itself when at a redraw or when the window looses focus (be patient,
> since this happens randomly it may take some time before the problem
> occurs).
>
> Since I'm still quite new to OSG, I'm not sure where to look for a
> solution. Hopefully someone will spot the obvious error in the sample code.
> Any advice is highly appreciated.
>
> Versions:
> ---------
> OSG svn rev 8700
> Qt 4.2.3
>
> Best regards,
> John Larring
>
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