Fast Planetary Shadows using Fourier-Compressed Horizon Maps

Shadows on large-scale terrains are important for many applications, including video games and scientific visualization. Yet real-time rendering of realistic soft shadows at planetary scale is a challenging task. Notably, many shadowing algorithms require keeping significant amounts of extra terrain geometry in memory to account for out-of-frustum occluders. We present Fourier-Compressed Horizon Mapping, an enhancement of the horizon mapping algorithm which is able to circumvent this requirement and render shadows in a single render pass. For a given digital elevation model, we create a compact representation of each pixel's horizon profile and use it to render soft shadows at runtime. This representation is based on a truncated Fourier series stored in a multi-resolution texture pyramid and can be encoded in a single four-channel 32 bit floating point texture. This makes this approach especially suitable for applications using a level-of-detail system for terrain rendering. By using a compact representation in frequency space, compressed horizon mapping consistently creates more accurate shadows compared to traditional horizon maps of the same memory footprint, while still running well at real-time frame rates.