[Opensg-users] OpenSG and VRJuggler multi-threading

[Marco Gillies]

Hi, I am setting up OpenSG to work with VRJuggler on a multiprocessor CAVE system and I'd like to have a bit of a better understanding of how the multi-threading features of OpenSG and VRJuggler interact. There is a fair bit of documentation I can find of the individual multi-threading features of each, but none that I can find on their interaction, sorry if I've missed some or not found and existing thread on this.

Maybe I should explain what I am trying to do. I have a  character animation engine based on Cal3D (not VJAvatar). This engine works in 3 stages:

* do internal updates (animation etc)
* write out a new set of vertex position for the character mesh to a buffer
* use some external system (i.e. opensg) to render the buffer.

If I were to use VRJuggler and GL, as I understanding, it I would use 2 buffers and do something like this:

void intraFrame()
{
  // do update
  // write verteces to the update buffer
}

void preFrrame()
{
 // swap update and render buffers
}

void draw()
{
  // draw the render buffer
}

This allows the updates to proceed entirely in parallel with the renderer, the only thing that they need to block on is a pointer swap. In OpenSG I would like to be able to copy the update buffer into an OpenSG geometry object. The simplest way would be to something like this (what my code does now):

void preFrame()
{
           OSG::GeoPositions3fPtr pos =  
                    OSG::GeoPositions3fPtr::dcast(mGeometries[i][j]->getPositions());
            beginEditCP(pos, OSG::GeoPositions3f::GeoPropDataFieldMask);
                for (v = 0; v < vertexCount; v++)
                    pos->setValue(OSG::Pnt3f(updateBuffer[3*v],
                                updateBuffer[3*v+1],
                                updateBuffer[3*v+2]), v);
            endEditCP(pos, OSG::GeoPositions3f::GeoPropDataFieldMask);
}

But I don't want to do a copy of all the vertex data, particularly in preFrame, which blocks the render thread. I'd like to do something like:

void intraFrame()
{
  // internal update
  OSG::GeoPositions3fPtr pos =  
                    OSG::GeoPositions3fPtr::dcast(mGeometries[i][j]->getPositions());
  WriteVertecesToBuffer(pos->GetData());
}

But I'm not sure how to make this thread safe.

So here are the questions:

1) As I understand it in OpenSG each thread has its own copy of all the objects so that it is safe to do something like:

WriteVertecesToBuffer(pos->GetData());

in parallel to the renderer if I call:

animationThread->getChangeList()->applyAndClear();

at the end of each frame to sync the two copies. Does this still work when I write vertex values directly using getData?

2) If so, how does it interact with the VRJuggler mutli-threading? Does the above method still apply if do the update in a VRJuggler intraFrame call and the rendering in a preFrame call? Do I still call applyAndClear on the change list or is that handlled by the VRJuggler plugin? Or are the two thread systems fundamentally different? If so, any suggestions.

thanks a lot

Marco