Probabilistically placing primitives.

Non-photorealistic rendering often requires placing drawing primitives onto a 2D canvas in such a way that the resulting tone approximates that of a greyscale reference image. Several iterative methods have been used where each stroke is tentatively placed on the canvas and the resulting tone is evaluated with respect to the reference image. [Salisbury et al. 1994; Salisbury et al. 1997; Praun et al. 2001] If the stroke over-darkens the output image it is rejected, otherwise it is accepted. While this back-and-forth iteration between the output and the reference image is capable of producing high-quality results, it is extremely costly in terms of computation and memory references.