Art often provides valuable hints for technological innovations especially in the field of Image Processing and Computer Graphics. In this paper we survey in an unified framework several methods to transform a raster input image into good quality mosaics. The common and different ideas among the methods are reported and described. Finally we try to address the evaluation problem of an NPR algorithm. Categories and Subject Descriptors (according to ACM CCS): J.5 [Arts and humanities]: Architecture, Fine Arts ______________________________________________________________________________________________________