Shadow height maps for terrain shadowing

Shadowing terrains is a challenging problem. Mainstream shadowing algorithmslike shadow volumes and shadow maps, do not work very well with height map geometry.Shadow volumes need silhouette extraction which is a very expensive operationfor a height map tessellated into thousands of triangles. Shadow mapping is a betteralternative, but it suffers from aliasing and self-shadowing artifacts. Additionally,shadow mapping needs two rendering passes to function, this means rendering theheight map twice in each frame. In this thesis, we propose a new specialized terrainshadowing algorithm to generate shadows caused by the directional light of the Sun.We propose the use of shadow height map (SHM), which is a discretized version of theshadow volume of the height map. A pixel in the SHM texture represents the height ofthe shadow on the current height map location. It is a hybrid method which borrowselements from shadow mapping, shadow volumes and ray casting. For each sunlightdirection we generate a new SHM, by casting parallel sunlight rays onto the terrain.We rotate the SHM texture, such that each ray corresponds to a single column of theimage and we apply a simple, iterative 2D shadow volume extraction algorithm on eachcolumn, storing the current height of the shadow to the current pixel in the column,as the algorithm progresses. A pixel of an arbitrary object or to the terrain comparesits world height with the sampled shadow height from the SHM and marks itself inshadow if it is below the shadow surface. We define two models to simulate realisticsoft shadows, first, by using a constant penumbra and second, by using an adaptivelyvarying penumbra height. According to our tests, our algorithm performed better thanthe state-of-the-art shadow mapping variant, Parallel Split Shadow Mapping (PSSM),both in the quality and performance aspects.