Thank you all for the great suggestions and comments. As two of you pointed out, the problem was not well defined (who said a well-posed problem is a problem solved?), and also it seems to be a very wide topic. I've had an interesting reading discussing the similarities between half-toning in black and white printing, and the quasi Monte Carlo integration technique [1].
I'm trying the suggested ways plus a few other things, I'll put together a summary of my discoveries when I've clarified it all. Thanks again, baptiste [1] "Halftoning and Quasi-Monte Carlo," K. M. Hanson, Sensitivity Analysis of Model Output, K. M. Hanson and F. M. Hemez, eds., pp. 430-442 (Los Alamos Research Library, 2005) _____________________________ Baptiste Auguié Physics Department University of Exeter Stocker Road, Exeter, Devon, EX4 4QL, UK Phone: +44 1392 264187 http://newton.ex.ac.uk/research/emag http://projects.ex.ac.uk/atto ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.