(GPU-driven) occlusion culling with meshlet rendering would help a lot while being relatively straightforward to implement if you already have a GPU-driven engine like OP does. Occlusion culling techniques cull objects that are completely hidden behind other objects. Meshlets break up objects (at asset build time) into tiny meshlets of around 64 to 128 triangles, such that these meshlets can be individually occlusion culled. This would help a lot by allowing the renderer to skip not just individual parts of the dragons that are hidden behind other dragons, but even parts of each dragon that are occluded by the rest of the dragon itself! There's a talk on YouTube about the Alan Wake 2 team implementing these techniques and being able to cull complex outdoor scenes of (iirc) hundreds of millions of triangles down to around 10-20 million.

The basic idea is to first render as normal some meshes that you either know are visible, or are likely to occlude objects in the scene (say the N closest objects, or some large terrain feature in a real game). Then you can take the resulting depth buffer and downsample it into something resembling a mipmap chain, but with each level holding the max depth of the contributing pixels, rather than the average. This is called a hierarchical Z (depth) buffer, or HZB for short. This can be used to very quickly, with just a few samples of the HZB, test if an object's bounding box is behind the all the pixels in a given area and thus definitely not visible. The hierarchical nature of the HZB allows both small and large meshes to be tested at the same performance cost.

Typically, a game would track which meshlets were known to be visible last frame, and start by rendering all of those (with updated positions and camera orientation, of course). This will make up most of what is drawn to the scene, because typically objects and the camera change very little from frame to frame. Then all the meshlets that weren't known to be visible get tested against the HZB, and just the few that were revealed by changes in the scene will need to be rendered. Lastly, at some point the known visible meshlet set should be re-tested, so that it does not grow indefinitely with meshlets that are no longer visible.

The result is that the first frame rendered after a major camera change (like the player respawning) will be slow, as all the meshlets in the frustum need to be rendered. But after that, the scene can be narrowed down to just the meshlets that actually contributed to the frame, and performance improves significantly. I think this would be more than enough for a demo, but for a real game you would probably want to explore methods to speed up that first frame's rendering, like sorting objects and picking the N closest/largest ones so you can at least get some occlusion culling working.

user:	cullingculling
created:	May 21, 2025
karma:	4