Horde3D

Occlusion Culling

Occlusion culling is an optimization technique that makes it possible to cull objects that are not visible since they are hidden/occluded by other objects. Horde3D uses hardware occlusion queries for its occlusion culling implementation. These queries have a delay of one frame to avoid stalls that would happen when waiting for the query result directly. The drawback of this approach is that some slight popping can occur when an object that was hidden in the previous frame becomes visible in the current frame. The implementation uses the depth buffer to check for occlusion of objects, so occlusion culling is only working if the rendering pass has proper scene depth information.

Horde3D supports multiple occlusion sets for different cameras. Enabling occlusion culling for a camera is basically very simple and works by setting a Camera node parameter. But in order that occlusion culling works efficiently, the scene needs to be sorted in front-to-back order. In Horde3D this sorting can be achieved through a special parameter in the pipeline drawing commands.

All renderable objects act automatically as occluders. Occludees (objects that can be culled because of occlusion) are currently models and particle emitters. Meshes of a model can occlude other models but a single mesh cannot occlude a mesh of the same Model node. With forward lighting, light sources can also be occluded by other geometry which can be very useful in indoor scenes. Occlusion of light sources is currently not supported for deferred shading.

<!-- Forward Shading Pipeline -->
<Pipeline>
   <CommandQueue>
      <Stage id="Geometry" link="globalSettings.material.xml">
         <ClearTarget depthBuf="true" colBuf0="true" />
         
         <DrawGeometry context="AMBIENT" class="~Translucent" order="FRONT_TO_BACK" />
         <DoForwardLightLoop class="~Translucent" order="FRONT_TO_BACK" />
         
         <DrawGeometry context="TRANSLUCENT" class="Translucent" order="FRONT_TO_BACK" />
      </Stage>
      
      <Stage id="Overlays">
         <DrawOverlays context="OVERLAY" />
      </Stage>
   </CommandQueue>
</Pipeline>