[yt-users] Stars on volume rendering

Greg Bryan gbryan at astro.columbia.edu
Fri Jan 7 08:24:07 PST 2011


Matt -- This seems like a reasonable way to do it.  I definitely agree with defining an effective radius for the star particles; in my limited experience, 1% is more than OK.

I worry that when the number of star particles is large, the delta_field will be almost all True, killing any speed-up due to that (although clearly for small numbers of star particles, this is still very efficient).

Can you explain in more detail why the search in 2b is going to be so slow?  I thought maybe you could do some clever sorting of the star particles to speed this step up; I suspect I just don't understand how the intersection calculation is done in the first place (and maybe I should actually look at the code).

Cheers,
Greg

On Jan 6, 2011, at 6:30 PM, Matthew Turk wrote:

> 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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