Irregular Z-Buffers:

https://research.nvidia.com/publication/frustum-traced-raster-shadows-revisiting-irregular-z-buffers

Summed-Area Variance Shadow Maps:

http://http.developer.nvidia.com/GPUGems3/gpugems3_ch08.html

ATI:

http://amd-dev.wpengine.netdna-cdn.com/wordpress/media/2012/10/Isidoro-ShadowMapping.pdf

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