On Thu, Apr 23, 2009 at 4:32 PM, Robert Osfield
<robert.osfi...@gmail.com> wrote:
> Thanks Evan, with your example I'm able to see the problem when
> browsing http://www.openscenegraph.org/data/earth_bayarea/earth.ive.
> I'll now dig into the DatabasePager to see what is up.
The culprit looks to be the following (normally valid) code in
DatabasePager::capped_removeExpiredSubgraphs(const osg::FrameStamp&
frameStamp):
int delta = frameStamp.getFrameNumber() -
plod->getFrameNumberOfLastTraversal();
if (delta>1)
{
if (_releaseDelay!=DBL_MAX)
{
plod->releaseGLObjects();
osg::notify(osg::NOTICE)<<"DatabasePager::removeExpiredSubgraphs(),
releasing gl objects"<<std::endl;
}
_inactivePagedLODList.push_back(plod);
itr = _activePagedLODList.erase(itr);
}
The only reason why this is a problem in this particular usage case is
that you have a viewer that is continue to increment the frame count
so the frameStamp.getFrameNumber() keeps getting incremented, but
since the scene isn't being rendering the
plod->getFrameNumberOfLastTraversal() is not updating so the frame
delta becomes > 1 and the PagedLOD gets assigned as expired.
The code in DatabasePager is correct for how the pager + PagedLOD were
designed to be used. The problem is that code is now being used in
way that is wasn't originally intended. Updating the frame count and
not rendering is the problem. Not updating the frame count and not
rendering would work, as does updating the frame count and rendering.
The next question is how to either get the viewer or DatabasePager to
do what is desired here - i.e. lazy updating of the graphics. It
might be possible to tell the DatabasePager that rendering of frame is
switched off for that frame and for it handle this case. Tweaking the
application code so that it doesn't do an viewer.advance() until a new
frame is actually required.
It's also worth noting that there is a potential bug in the supplied
viewer that does lazy update - it only will ever work if the pager
isn't doing precompile, if it does do precompile then you have to
render frames to enable the pager to get it's GL objects compiled,
otherwise the pager will just go to sleep waiting for the objects to
be compiled by the rendering thread(s), then you'd have stale mate.
A possible way to solve this would be to check the DatabasePager to
see if it has any database requests that are still active, and keeping
rendering while it does.
Robert.
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