As realtime raytracing is slowly, but steadily, gaining traction, a range of opportunities to mix rasteration-based rendering systems with raytracing are starting to become available: hybrid raytracing where rasterisation is used to provide the hit points for the primary rays, hybrid shadows where shadowmaps are combined with raytracing to achieve smooth or higher detail shadows, hybrid antialiasing where raytracing is used to antialias the edges only, hybrid reflections, where raytracing is used to fill-in the areas that screenspace reflections can’t resolve due to lack of information.
Of these, I found the last one particularly interesting: how well can a limited information lighting technique like SSR be combined with a full-scene aware one like raytracing, so I set about exploring this further.
A few weeks ago I documented the experiments I made with hybrid raytraced shadows and reflections, describing how raytracing can be set up and used in the context of a deferred rendering architecture. It was great fun and I managed to produce some nice images.
I soon came to realise though that this simplistic approach was mostly suitable for simple models (such as spheres and cubes) as the bounding volume hierarchy (BVH) I created to accelerate scene traversal stored full meshes in the leaves. This reduced the opportunity to accelerate traversal further when a leaf was reached, which is especially bad for large meshes, and complicated the shader a lot by creating many paths through it, potentially increasing thread divergence and reducing occupancy (by increased register allocation). Also the raytracing pass was heavily memory bound, making it scale less well with more complex, and higher polygon, content. The current approach would easily break down when used with more representative game environments/meshes. Continue reading “Hybrid raytraced shadows part 2: performance improvements”→
Unless you’ve been hidden in a cave the past few months, doing your rendering with finger painting, you might have noticed that raytracing is in fashion again with both Microsoft and Apple providing official DirectX (DXR) and Metal support for it.
Of course, I was curious to try it but not having access to a DXR capable machine, I decided to extend my toy engine to add support for it using plain computer shaders instead.
I opted for a hybrid approach that combines rasterisation, for first-hit determination, with raytracing for secondary rays, for shadows/reflection/ambient occlusion etc. This approach is quite flexible as it allows us to mix and match techniques as needed, for example we can perform classic deferred shading adding raytraced ambient occlusion on top or combine raytraced reflections will screen space ambient occlusion, based on our rendering budget. Imagination has already done a lot of work on hybrid rendering, presenting a GPU which supports it in 2014. Continue reading “Hybrid raytraced shadows and reflections”→