[yt-users] Stars on volume rendering

Thu Jan 6 15:30:05 PST 2011

Hi John,

(As a quick comment, one can export to Sunrise from yt, so that could
also serve as a mechanism for rendering star particles.)

I have been thinking about this a lot lately, and I think you're
right: we need a proper mechanism for compositing star particles on
the fly during the traversal of rays across a homogenized volume.  I
had planned on this being one of my first yt projects this year.  The
current process of volume rendering (for more detail see the method
paper) is basically:

1) Homogenize volume, splitting fields up into uniquely-tiling bricks
2) Sort bricks
3) For every brick, for every ray:
  a) Calculate intersection
  b) Update all channels (typically 3) based on *local* emission and absorption
  c) Update ray position
4) Return image plane

This is true for both the old, homogenized volume rendering technique
and the new kD-tree technique.  To fit star particles into this, we
would regard them as exclusively sources of emission, with no impact
on the local absorption.  Nominally, this should be easy to do: for
every cell, simply deposit the emission from a star.  As you noted in
your email, this results in very, very ugly results -- I tested it
last summer with the hopes of coming up with something cool, but was
unable to.  Testing it today on an airplane showed it had bitrot a
bit, so I haven't attached it.  :)

I think we would need to move to, rather than cell-based emission (so
that the smallest emission point from a star is a single cell) to a
method where emission from star particles is calculated per ray (i.e.,
pixel).  This would require an additional step:

1) Homogenize volume, splitting fields up into uniquely-tiling bricks
2) Sort bricks
3) For every brick, for every ray:
  a) Calculate intersection
  b) Calculate emission from stars local to the ray
  c) Update all channels (typically 3) based on *local* emission and absorption
  d) Update ray position
4) Return image plane

This would enable both density-based emission *and* star emission.
This could be both density isocontours, for instance, and individual
stars.  The visual language in that would probably be very confusing,
but it would be possible, particularly for pure annotations.

The issue here is that step 2b is probably very, very slow -- even
using a (point-based) kD-tree it would likely add substantial run
time, because there's no early termination mechanism.  What I think we
could do, however, is execute a pre-deposition phase.  For the
purposes of rendering, we can describe a star particle by only a few
characteristics:

Emission(Red, Green, Blue)
Gaussian(Radius)
Position(x,y,z)

We should instead define an effective radius (ER), say at the 1%
level, at which we won't worry anymore.  We could then deposit delta
functions of size ER for every star particle.  This would give a cue
to the ray caster, and we could modify:

1) Homogenize volume, splitting fields up into uniquely-tiling bricks
2) Sort bricks
3) For every brick, for every ray:
  a) Calculate intersection
  b) If local delta_field == True, execute ball query and calculate
emission from stars local to the ray
  c) Update all channels (typically 3) based on *local* emission and absorption
  d) Update ray position
4) Return image plane

For the first pass, we would probably want all our stars to have the
same ER, which would then be the radius of our ball-query.  For
parallel rendering, we would still have to have all of the star
particles loaded on every processor; I don't think this is a problem,
since in the limit where the star particles are memory-limiting, you
would likely not suffer from pre-deposition.  This also solves the
grid-boundary issues, as each processor would deposit all stars during
its initial homogenization.

What do you think?  I think that the components external to the ray
tracer could be assembled relatively easily, and then the ray tracer
might take a bit of work.  As a post-processing step we could even add
lens flare, for that extra Star Trek look.

-Matt

On Thu, Jan 6, 2011 at 8:45 AM, John Wise <jwise at astro.princeton.edu> wrote:
> I forgot to mention that another way to do this is making a derived field
> that adds the stellar density to the gas density.  However this doesn't look
> good when particles are in coarse grids, when they should be point sources
> in the image.
>
> def _RelativeDensityStars(field, data):
>    return (data["Density"] + data["star_density"])/dma
> add_field("RelativeDensityStars", function=_RelativeDensityStars,
>          take_log = False)
>
> where dma is a scaling variable.
>
> I'm uploading my stand-alone script if you want to try to decipher it,
> although I tried to comment it some.
>
> http://paste.enzotools.org/show/1475/
>
> Also I uploaded the colormap based on B-V colors that I ripped from
> partiview to
>
> http://www.astro.princeton.edu/~jwise/temp/BminusV.h5
>
> John
>
> On 01/06/2011 11:14 AM, John Wise wrote:
>>
>> Hi Libby,
>>
>> I'm afraid that there isn't a good solution for rendering stars, at
>> least to my knowledge!
>>
>> You can add them as pixels after you've determined the pixel numbers (as
>> in Andrew's email) of the particles with the splat_points() routine in
>> the image_writer module.
>>
>> I wrote my own stand-alone splatter to put Gaussian splats for
>> particles, but I never incorporated it into yt. I meant to a few months
>> back when I wrote it but never did! It will produce these types of splats
>>
>>
>> http://www.astro.princeton.edu/~jwise/research/GalaxyBirth_files/combine.png
>>
>>
>> I had to manually blend the gas volume rendering and star splats
>> afterwards to produce that image.
>>
>> I hope I can make something that looks as good as partiview soon. This
>> is the same dataset but with partiview.
>>
>>
>> http://www.astro.princeton.edu/~jwise/research/GalaxyBirth_files/stars_only.png
>>
>>
>> I'll see if I can make time (first I have to find the code!) to
>> incorporate my splatter into yt.
>>
>> John
>>
>> On 01/06/2011 09:15 AM, Elizabeth Harper-Clark wrote:
>>>
>>> Hi all,
>>>
>>> Thanks for all your help over the last couple of days. One more question:
>>> - Can I plot particles on a volume rendered image?
>>> I have stars and I want to show where they are!
>>>
>>> Thanks,
>>>
>>> Libby
>>>
>>> --
>>> Elizabeth Harper-Clark MA MSci
>>> PhD Candidate, Canadian Institute for Theoretical Astrophysics, UofT
>>> Sciences and Engineering Coordinator, Teaching Assistants' Training
>>> Program, UofT
>>>
>>> www.astro.utoronto.ca/~h-clark <http://www.astro.utoronto.ca/%7Eh-clark>
>>> h-clark at cita.utoronto.ca <mailto:h-clark at cita.utoronto.ca>
>>> Astronomy office phone: +1-416-978-5759
>>>
>>>
>>>
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