Conventional shadow mapping relies on uniform sampling for producing hard shadow in an efficient manner. This approach trades image quality in favor of efficiency. A number of approaches improve upon shadow mapping by combining multiple shadow maps or using complex data structures to produce shadow maps with multiple resolutions. By sacrificing some performance, these adaptive methods produce shadows that closely match ground truth. This paper introduces Rectilinear Texture Warping (RTW) for efficiently generating adaptive shadow maps. RTW images combine the advantages of conventional shadow mapping – a single shadow map, quick construction, and constant time pixel shadow tests, with the primary advantage of adaptive techniques – shadow map resolutions which more closely match those requested by output images. RTW images consist of a conventional texture paired with two 1-D warping maps that form a rectilinear grid defining the variation in sampling rate. The quality of shadows produced with RTW shadow maps of standard resolutions, i.e. 2,048×2,048 texture for 1080p output images, approaches that of raytraced results while low overhead permits rendering at hundreds of frames per second.

Paul Rosen. Rectilinear Texture Warping For Fast Adaptive Shadow Mapping. Proceedings of the ACM SIGGRAPH Symposium on Interactive 3D Graphics and Games, 2012.

@inproceedings{rosen2012rectilinear,
  title = {Rectilinear Texture Warping For Fast Adaptive Shadow Mapping},
  author = {Rosen, Paul},
  booktitle = {Proceedings of the ACM SIGGRAPH Symposium on Interactive 3D Graphics and
    Games},
  series = {I3D},
  pages = {151--158},
  year = {2012},
  keywords = {adaptive sampling, rendering, shadow algorithms},
  abstract = {Conventional shadow mapping relies on uniform sampling for producing hard
    shadow in an efficient manner. This approach trades image quality in favor of
    efficiency. A number of approaches improve upon shadow mapping by combining multiple
    shadow maps or using complex data structures to produce shadow maps with multiple
    resolutions. By sacrificing some performance, these adaptive methods produce shadows
    that closely match ground truth. This paper introduces Rectilinear Texture Warping (RTW)
    for efficiently generating adaptive shadow maps. RTW images combine the advantages of
    conventional shadow mapping - a single shadow map, quick construction, and constant time
    pixel shadow tests, with the primary advantage of adaptive techniques - shadow map
    resolutions which more closely match those requested by output images. RTW images
    consist of a conventional texture paired with two 1-D warping maps that form a
    rectilinear grid defining the variation in sampling rate. The quality of shadows
    produced with RTW shadow maps of standard resolutions, i.e. 2,048x2,048 texture for
    1080p output images, approaches that of raytraced results while low overhead permits
    rendering at hundreds of frames per second.}
}