Succinct Opacity Micromaps

Abstract

Alpha masked geometry such as foliage has long been one of the trickier things to render efficiently, both for rasterization based approaches and for hardware accelerated ray-tracing. Recently, a new type of primitive was introduced to the Vulkan® and DirectX® ray-tracing APIs that promises to alleviate this issue: Opacity Micromaps, a structure that uses a bit of extra memory as hints to the pipeline when it should actually call the AnyHit-shader. In this paper, we extend this primitive with a novel compression method that uses the concept of succinct 4-way trees to reduce the memory footprint by up to 110 times, including an algorithm for looking up micromap values directly from this compressed form. Further, we perform a comprehensive analysis of the generated micromaps to demonstrate their performance in terms of both memory footprint and frame render time compared to a number of similar structures. Finally, we highlight some aspects of the extension that developers and artists should be aware of to make the most out of it.