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J. Boksansky, M. Wimmer, J. Bittner:
"Ray Traced Shadows: Maintaining Real-Time Frame Rates";
in: "Ray Tracing Gems: High-Quality and Real-Time Rendering with DXR and Other APIs", Springer, 2019, ISBN: 978-1-4842-4426-5, S. 159 - 182.

Efficient and accurate shadow computation is a long-standing problem in computer graphics. In real-time applications, shadows have traditionally been computed using the rasterization-based pipeline. With recent advances of graphics hardware, it is now possible to use ray tracing in real-time applications, making ray traced shadows a viable alternative to rasterization. While ray traced shadows avoid many problems inherent in rasterized shadows, tracing every shadow ray independently can become a bottleneck if the number of required rays rises, e.g., for high-resolution rendering, for scenes with multiple lights, or for area lights. Therefore, the computation should focus on image regions where shadows actually appear, in particular on the shadow boundaries.

We present a practical method for ray traced shadows in real-time applications. Our method uses the standard rasterization pipeline for resolving primary-ray visibility and ray tracing for resolving visibility of light sources. We propose an adaptive sampling algorithm for shadow rays combined with an adaptive shadowfiltering method. These two techniques allow computing high-quality shadows with a limited number of shadow rays per pixel. We evaluated our method using a recent real-time ray tracing API (DirectX Raytracing) and compare the results with shadow mapping using cascaded shadow maps.

http://dx.doi.org/10.1007/978-1-4842-4427-2_13