Hi Max, 'factor' is the number of the data points skipped while 'scale' is the data units per 'scale_units'. Try the following: p.modify["velocity"](factor=13, scale=1e8, scale_units='inches'). If you have a velocity of 1000 km/s, it should then produce an arrow 1 inch long. Let me know if it works. Cheers, JC On Aug 1, 2012, at 8:04 AM, Massimo Gaspari wrote: Hi guys, how can I set a given normalization for the velocity field superposed to a slice? p.modify["velocity"](factor=13) I am a bit puzzled by the "scale" keyword. E.g., I want to set 1000 km/s as the normalization for all the slices of a simulation. Thanks, Max _______________________________________________ yt-users mailing list yt-users@lists.spacepope.org<mailto:yt-users@lists.spacepope.org> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org