AMD researchers have developed a novel technique called 'tetrahedral cages' that significantly reduces the memory overhead associated with rendering complex, animated foliage in real-time ray tracing applications. The technology promises to make highly detailed natural environments more feasible for current and future hardware.
Traditionally, rendering a scene with approximately 25,000 independently animated plants, comprising hundreds of thousands of triangles, could consume up to 80 GB of GPU memory. Furthermore, calculating ray intersections with such geometry, managed by a data structure known as a Bounding Volume Hierarchy (BVH), could take a powerful GPU like the Radeon RX 9070 XT around 300 milliseconds per frame. This level of resource demand would result in significant performance degradation and choppy visuals.
Holger Gruen, a Fellow of AMD's Advanced Graphics Program, explained on GPUOpen that their tetrahedral cage demonstrations dramatically improve these figures. In comparison, the new method uses only 1.7 GB of BVH memory and approximately 3.3 ms for all BVH updates per frame. This efficiency is achieved by wrapping a simplified tetrahedral cage around the dense mesh of plant matter. Instead of deforming and tracing rays against every individual triangle of potentially thousands of animated leaves, only the simplified cage deforms.
"The dense geometry of an animated object and its BVH structures remain static and reusable," Gruen stated. "During runtime, only the cage deforms. Rays are transformed back into a static reference frame before intersecting them with the dense set of triangles. This means animation cost only scales with cage complexity, not with triangle count. This comes at the expense of losing animation control over every vertex, but this is fine for many animations."
This approach is particularly effective for scenes where the overall impression of lush foliage is more important than the precise animation of individual leaves. For elements like trees in games such as Alan Wake 2, where the focus is on the collective appearance of the forest rather than individual frond movement, tetrahedral cages offer a substantial performance benefit without a noticeable visual compromise.
While the tetrahedral cage technology is currently a research demonstration, AMD is actively working on developing it further. The company is creating DirectX Raytracing (DXR) samples and a C++ library to enable the construction of high-quality tetrahedral cages for various types of animated objects. Further details on the research can be found in the open-access paper 'Ray Tracing Massive Amounts of Animated Geometry'.