Hi Matt,

> After using the debugger as you suggested and looking more closely it seems
> that LE is still an empty list because self.bricks = None even after a call
> to self.partition_grids().  The latter is not working properly because
> self._brick_collection.source._grids is an empty ndarray.  For example, I
> get the following DEBUG message before the fatal error:
> yt         DEBUG      2010-12-30 16:40:49,932 Preloading ['Density'] from 0
> grids
> I now need to track down why _grids is not being filled appropriately; all
> the other grid-type-lists I have found are properly filled.

My guess is actually the width argument -- you're setting it to 1.0,
and the center to 0.5, 0.5, 0.5.  Can you try with:

W = pf['unitary']
c = 0.5*(pf.domain_right_edge + pf.domain_left_edge)

which should grab all the bricks?

Ok, that makes a bit more sense - I always thought the width and center values were relative coordinates.  Fixing c to be the physical center of the domain worked, but I had to modify W to be the physical width:

    W = pf.domain_right_edge - pf.domain_left_edge

to grab the bricks.  As far as I can tell with all the other frontends,

    pf['unitary'] = (1.0 / (pf.domain_right_edge - pf.domain_left_edge)).max(),

which is a a pretty small number.  With these fixes I'm at least getting a volume rendered image, but I'm not sure exactly what it is - it looks strange, but I'll play with the transfer functions and such to see if I can quantify what I mean by "strange."