Hi All! I've been recently struck by roundoff errors in yt. AFAIR some variants of such bugs have been already reported from time to time on #yt or mailing lists and I'd like to propose a general solution that should solve this. The problem can be seen here: http://imgur.com/oIaarPR It's a dense blob (\rho = 2) moving diagonally across uniform medium (\rho = 1). As you can see there are blueish stripes on the few block boundaries. They're caused by yt thinking that blocks on n+1 refinement level overlap with those regions in blocks on "n" level. It all happens here: (yt/data_objects/object_finding_mixin.py) ObjectFindingMixin::get_box_grids ... grid_i = np.where( (np.all(self.grid_right_edge > left_edge, axis=1) & np.all(self.grid_left_edge < right_edge, axis=1)) == True ) The problem is that comparing floats is *not* accurate. What should be done: instead of "a > b", we should do e.g. "(a - b) > eps", where eps is small number grater than machine precision. I've fixed this particular bit as follows: ... dx = self.get_smallest_dx() * 0.1 grid_i = np.where( (np.all(self.grid_right_edge > (left_edge + dx), axis=1) & np.all(self.grid_left_edge < (right_edge - dx), axis=1)) == True ) 10% of the smallest cell size seems to be good approximation of eps at this point. As a result I've got: http://imgur.com/vCGQUKv Still one artifact remained. It was caused by exact the same issue in other place in the code (utilities/lib/geometry_utils.pyx:get_box_grids_level) Using similar trick there yielded: http://imgur.com/wpOCGaf \o/ Sorry for the long mail, but I wanted to share this so we could inspect other place that could suffer from similar issues too. Cheers, Kacper P.s. Nice animations are available here: http://tinyurl.com/aw2tbof http://tinyurl.com/amp7noc http://tinyurl.com/arnqmfw