[osg-users] Offscreen rendering of large scenes

[Philipp Wagner]

Hi,

I want to perform an offscreen rendering of a scene that is too large to 
fit into the GPU memory at once (lots of big textures).
My idea was to render parts of the scene each in its own frame, and disable 
the clearing of the depth and color buffer.
This does work for the color buffer, but the depth buffer seems to be 
cleared after each frame, which leads to incorrect results (The later parts 
are drawn over the previous ones, even though they are behind the other 
parts).

The algorithm:
1. Create camera/context/...
2. Clear the buffers (i.e. Render an empty scene with a clear mask of 
(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT)
3. Disable buffer clearing
4. Render each part in its own frame
 a. Add part to the scene
 b. Render
 c. Remove part from the scene
5. Read back color buffer


osg::ref_ptr<::osg::Camera> camera = new osg::Camera();

//Here we set the camera orientation/projection matrix

camera->setClearColor(...);
camera->setGraphicsContext(context);

osg::ref_ptr<osg::Image> img = new osg::Image;
img->allocateImage(textureWidth, textureHeight, 1, GL_RGBA, 
GL_UNSIGNED_BYTE);
camera->attach(osg::Camera::COLOR_BUFFER0, img, numSamples);
camera->attach(osg::Camera::DEPTH_BUFFER, GL_DEPTH_COMPONENT32);
camera->setRenderOrder(osg::Camera::PRE_RENDER);
camera->setRenderTargetImplementation(osg::Camera::RenderTargetImplementation::FRAME_BUFFER_OBJECT);
camera->setReferenceFrame(osg::Camera::ABSOLUTE_RF);
camera->setProjectionResizePolicy(osg::Camera::FIXED);
camera->setViewport(0, 0, img->s(), img->t());
viewer->addSlave(camera);

camera->setClearMask(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);
viewer->frame();
camera->setClearMask(0);
for (auto& item : parts)
{
   //Here we create and add the item to the scene...
       viewer->frame();
   //Here we remove item from the scene and delete it
}

Thanks in advance
Phil

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