Frustum-Traced Raster Shadows: Revisiting Irregular Z-Buffers

"Frustum-Traced Raster Shadows: Revisiting Irregular Z-Buffers"
Chris Wyman (NVIDIA), Rama Hoetzlein (NVIDIA), Aaron Lefohn (NVIDIA), in Symposium on Interactive 3D Graphics and Games, February 2015
Research Area:
Author(s): Chris Wyman (NVIDIA), Rama Hoetzlein (NVIDIA), Aaron Lefohn (NVIDIA)
Date: February 2015

We present a real-time system that renders antialiased hard shadows using irregular z-buffers (IZBs). For subpixel accuracy, we use 32 samples per pixel at roughly twice the cost of a single sample. Our system remains interactive on a variety of game assets and CAD models while running at 1080p and 2160p and imposes no constraints on light, camera or geometry, allowing fully dynamic scenes without precomputation. Unlike shadow maps we introduce no spatial or temporal aliasing, smoothly animating even subpixel shadows from grass or wires.

Prior irregular z-buffer work relies heavily on GPU compute. Instead we leverage the graphics pipeline, including hardware conservative raster and early-z culling. We observe a duality between irregular z-buffer performance and shadow map quality; this allows common shadow map algorithms to reduce our cost. Compared to state-of-the-art ray tracers, we spawn similar numbers of triangle intersections per pixel yet completely rebuild our data structure in under 2 ms per frame.