Samuli Laine Timo Aila Ulf Assarsson Jaakko Lehtinen Tomas Akenine-Möller Soft shadow volumes for ray tracing. 1156-1165 2005 24 ACM Trans. Graph. 3 http://doi.acm.org/10.1145/1073204.1073327 db/journals/tog/tog24.html#LaineAALA05