[osg-users] Asynchronous file loading via DatabasePager questions

[Lionel Lagarde]

Tim Moore wrote:

> The other thing to keep in mind is that readNodeFile and friends aren't necessarily
> thread safe, but you don't have access to the lock that the pager uses to serialize
> access to those functions. Either you stop doing any file loading calls from your main
> thread once the pager has started, or you need to do the locking yourself by installing
> read callback functions that do the locking. The bugs you're seeing sound like random
> corruption of this kind.


I believed that osgDB::Registry operations were thread safe. The plugins may
not be thread safe but the registry has to be thread safe.
No ?


Tim Moore wrote:
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> Colin Branch wrote:
>   
>> Hi all,
>>
>> What I'm trying to do:
>> Asynchronous model loading using OSG 2.2
>>
>> How I'm currently doing it:
>> I'm trying to use the Database Pager's requestNodeFile.
>>     
> I just did this for the FlightGear project and it works well there.
>   
>> I created a new node class that I can add (as well as place temporary 
>> objects) that I can add and will stay in the scene.
>>
>> class DtDelayedModelNode : public osg::Group
>> {
>> ...
>> DtDelayedModelNode(const std::string& filename, osgDB::DatabasePager* 
>> pager) : osg::Group()
>> {
>>     osg::FrameStamp fs;
>>     fs.setFrameNumber(INT_MAX);
>>     pager->requestNodeFile(filename,this,1.0,&fs);
>> }
>> ...
>> };
>>     
>
> For this application you probably do want to check out ProxyNode as Bryan Thrall
> suggested. If I had known about ProxyNode I might have used it instead of calling
> requestNodeFile directly.
>
> What you've done here won't reliably work. You need to call requestNodeFile every frame
> until your request is satisfied. You will only find this out by looking closely at the
> implementation of ProxyNode and PagedLOD or by googling in the mailing list :)
>
>   
>> Problems:
>> #1 Very limited documentation of the Pager class and no example on how 
>> to use it.   I've searched the osgusers archive but haven't found 
>> anything too helpful, the 1.x examples don't seem relevant anymore.
>>     
> See above about ProxyNode and PagedLOD.
>   
>> #2 Occasional texture corruption.  This is difficult to reproduce 
>> however it appears that sometimes the width of the texture is 1px 
>> creating a streaking effect.
>> #3 Certain actions cause the main app to lock, for example bringing up 
>> the Stats HUD.
>>     
> The other thing to keep in mind is that readNodeFile and friends aren't necessarily
> thread safe, but you don't have access to the lock that the pager uses to serialize
> access to those functions. Either you stop doing any file loading calls from your main
> thread once the pager has started, or you need to do the locking yourself by installing
> read callback functions that do the locking. The bugs you're seeing sound like random
> corruption of this kind.
>   
>> Questions:
>> Is the Database Pager the proper way to be doing this?
>>     
> It works for the FlightGear flight simulator project. I created a very small subclass
> of DatabasePager in order to have access to the _childrenToDeleteList of the pager.
>
>   
>> What if any calls should I make to the pager around frame() (ie it 
>> doesn't appear that startFrame endFrame are needed, any other calls)?
>>
>>     
> None if you're using osgViewer. Otherwise look closely at the osgViewer::Viewer source
> code.
>
>   
>> The value of the frame stamp appears to control deletion of requests. Ie 
>> if I pass in the current frame stamp, the model is actually deleted in 
>> compileGLObjects. Unless I set it to some value in the future (currently 
>> using INT_MAX) I can't guarantee it will load. Could someone explain the 
>> purpose of the framestamp and the proper usage?
>>     
> See above. Normally you check every frame if a request has been satisfied; if not, you
> make it again with the current frame stamp. The OSG clients of DatabasePager, ProxyNode
> and PagedLOD, make these requests from the cull traversal, so this usage gives a way
> to cancel a request if the object drops out of the field of view.
>   
>> Is there anyway to get status feedback while a file loads?
>>     
> You can tell if a load is completed by checking if an object has been added to the group
> you passed to requestNodeFile. Otherwise no, though you could arrange for the readNodeFile
> callback you're going to write :) to return an empty error object of some kind.
>
> Tim
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