Subdivided Shadow Maps

Brandon Lloyd, Sung-Eui Yoon, David Tuft, Dinesh Manocha
University of North Carolina at Chapel Hill


Comparison between trapezoidal shadow maps (TSMs) and subdivided shadow maps. Left) TSM with 1Kx1K shadow map. Middle) TSM with 4Kx4K shadow map. Right) 1Kx1K subdivided shadow map with subdivisions marked in white. This configuration with a small angle between the light and view directions is difficult for prior methods. Even with the largest shadow map that can be allocated on current hardware, TSMs are not able to match the quality of subdivided shadow maps for this view.


We present a technique for reducing perspective aliasing error in shadow maps. From the viewpoint of the light, the scene is first split into subdivisions defined by the visible faces of the camera frustum. The frustum subdivisions may be further subdivided along their corresponding faces. We apply a separate shadow map warp to each resulting subdivision. This produces significantly less error than applying a single shadow map warp to the whole scene We layout the subdivisions in rectangular regions within a single shadow map, using the maximum error of each subdivision to assign larger regions to subdivisions with higher error. Our method runs well on commodity graphics hardware and is easy to integrate into existing shadow map systems. We are able to achieve interactive performance (8-25 FPS) on a power plant model (12M triangles), a double eagle tanker model (82M triangles), and the St. Matthew model (370M triangles) running on a PC with a GeForce 7800 GTX. We observed significantly less aliasing compared to prior shadow map warping algorithms.

Technical Report (PDF 4 MB)


Video (20 MB - QuickTime 6 MPEG-4)

Click on any of the following images to enlarge:

Standard Shadow Maps Trapezoidal Shadow Maps Subdivided Shadow Maps
Standard Shadow Maps Trapezoidal Shadow Maps Subdivided Shadow Maps

In these images the light is placed nearly directly overhead. Trapezoidal shadow maps show an improvement over standard shadow maps. Subdivided shadow maps show further improvements. The shadow map is the same size for all three images.

Power Plant Double Eagle Oil Tanker St. Matthew
Power Plant Double Eagle Oil Tanker St. Matthew

Test models used for the paper. We integrated our system with Quick-VDR, a view dependent rendering system designed for large models.