[osg-users] How to check if pagedLod already exists in databasepager with same filename

Robert Osfield robert.osfield at gmail.com
Tue Nov 29 06:11:28 PST 2016

Hi Simone,

On 29 November 2016 at 12:37, Simone Rapposelli
<simone.rapposelli at gmail.com> wrote:
> thank you for your fast reply.
> My problem is that osgDB::ReaderWriter::ReadResult readNode(const std::string &fileName, const osgDB::ReaderWriter::Options *options) gets called even if a PagedLod with the same fileName has been previously loaded: for example, this happens if I move to any position on the viewer and then come back.
> Thus, inside this function I need to check if any of the PagedLod currently loaded in DatabasePager has the same fileName of the passed argument: in this case I could avoid to reload data already in memory.

The PagedLOD/DatabasePager paging scheme is designed to expire and
reload subgraphs, it *crucial* to load balancing.  If you cached all
loaded subgraphs your memory would rapidly be overwhelmed and your
system would grind to a halt.  The very scheme you are trying to
defeat is one of the best assets of the OSG, you *absolutely* do not
want to be breaking this mechanism.

Now, if you the defaults the paging scheme uses for load balancing is
too conservative w.r.t the number of PagedLOD it will aim for in
memory at one time you can adjust it to be higher simply by setting
the TargetMaximumNumberOfPageLOD parameter, from the DatabasePager
header you'll see:

        /** Set the target maximum number of PagedLOD to maintain in memory.
          * Note, if more than the target number are required for
rendering of a frame then these active PagedLOD are excempt from being
          * But once the number of active drops back below the target
the inactive PagedLOD will be trimmed back to the target number.*/
        void setTargetMaximumNumberOfPageLOD(unsigned int target) {
_targetMaximumNumberOfPageLOD = target; }

You can also set the default value using the env var OSG_MAX_PAGEDLOD
i,e under bash:

     export OSG_MAX_PAGEDLOD=2000
     osgviewer mypageddatabase.osgb

You can get a listing of the env vars supported by doing:

    osgviewer --help-env


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