[yt-users] Volume rendering rotation

Hi, I am generating volume renderings using a direct_ray_cast call. If I set L = [na.sin(2*na.pi*frame/rotframes),na.cos(2*na.pi*frame/rotframes), 0.0] (like Sam's python movie script) so that the consecutive images revolve, after 30 degrees of rotation the "up" vector of the output image suddenly rotates 90 degrees. The problem seems to be independent of the z coordinate of L. Is there something strange happening in the normalization of L within direct_ray_cast or am I missing something basic? I can rotate the png files back to the correct orientation before stringing them together but that is annoying. Thanks, Stella