Poisson-disk sampling patterns are of interest to the graphics community because their blue-noise properties are desirable in sampling patterns for rendering, illumination, and other computations. Until now, such patterns could only be generated by slow methods with poor convergence, or could only be approximated by jittering individual samples or tiling sets of samples. We present a simple and efficient randomized algorithm for generating true Poissondisk sampling patterns in a square domain, given a minimum radius R between samples. The algorithm runs in O(N log N) time for a pattern of N points. The method is based on the Voronoi diagram. Source code is available online.