[yt-users] volume rendering

Tue Apr 12 08:34:41 PDT 2011

Hi Sam,

I still don't understand this. The small ds's will only yield the
same integration as the large ds's if the physical width of the
isosurface is the same on all refinement levels. If the width is,
say, 10 cells on each refinement level, then the smallest ds's
will only emit 1/1000 of the contribution of the largest ds's.
In fact, if the emission is directly proportional to the RGB value,
then even a factor of 100 will make the color so dark that the
contribution to the final image is invisible. In this case you
would see the isosurface only on the largest scales.

Is this what would happen?

Cheers,
Maike

> Hi Maike,
> 
> I think there is some confusion here about how our transfer function
> works.
>  For the ColorTransferFunction, we are really specifying r,g,b
>  emissivities.  For example, let's say that we have chosen to specify that
> a
> density value of 5 corresponds to rgb = [0.5,0.5,0.5], whatever that color
> may be.  Now, that does not mean that when the rays pass through an area
> with a density of 5 that the rgb images will be equal to 0.5,0.5,0.5.
>  Rather, we will add j*ds to the image plane.  If the entire domain had a
> density of 5, then the resulting image would be (some
> factor)*[0.5,0.5,0.5]
> where the factor has to do with the integration length.  If instead the
> density=5 region was just in part of the volume, then it would add up the
> contribution to the final image, and it would  be blended along the line
> of
> sight with any other emission from the rest of the volume.
> 
> If you take your example of an AMR sim with 10 levels, then using the
> implementation we have written, while the high levels have much smaller
> ds's, there are many more of them, which will yield roughly the same
> integration as had it been down only using the lower resolution data.  It
> is
> a lot like an adaptive integration scheme in that sense.
> 
> Does this help?  Please let us know.
> 
> Best,
> Sam
> 
> On Wed, Apr 6, 2011 at 4:27 AM, Maike Schmidt <maikeschmidt2 at gmx.de>
> wrote:
> 
> > Hi Matt,
> >
> > many thanks for this really nice explanation! But I still
> > don't understand how the color transfer function works.
> > I thought that, if you add a Gaussian with peak (r,g,b),
> > then the intensity that arrives at the camera from cells
> > that contain the center of the Gaussian has exactly intensity
> > (r,g,b), when I am neglecting absorption. Otherwise I don't
> > understand how one can relate the color of the Gaussian
> > to the color at the camera.
> >
> > Now, say I want to visualize an isosurface of density
> > on an AMR grid with 10 refinement levels, then the actual
> > intensity contribution j * delta s will differ by 3 orders
> > of magnitude for cells with the same density but different
> > cell size. How do you make all these cells emit the same color?
> > This is what I don't understand.
> >
> > I guess it boils down to the question of how exactly you
> > calculate your j in the radiative transfer equation when
> > you have a color transfer function, say a single Gaussian
> > with peak (r,g,b).
> >
> > Many thanks,
> > Maike
> >
> >
> >
> >
> > -------- Original-Nachricht --------
> > > Datum: Tue, 5 Apr 2011 16:29:02 -0400
> > > Von: Matthew Turk <matthewturk at gmail.com>
> > > An: Discussion of the yt analysis package
> <yt-users at lists.spacepope.org>
> > > Betreff: Re: [yt-users] volume rendering
> >
> > > Hi Maike,
> > >
> > > (I think this is your first post -- welcome to yt-users!)
> > >
> > > On Tue, Apr 5, 2011 at 11:34 AM, Maike Schmidt <maikeschmidt2 at gmx.de>
> > > wrote:
> > > > Hi together,
> > > >
> > > > I would like to better understand how the volume rendering works.
> > > > It is explained here
> > > >
> > > > http://yt.spacepope.org/doc/visualizing/volume_rendering.html
> > > >
> > > > that the user defines transfer functions in terms of RGB values.
> > > > From the description of the add_gaussian function, I understand
> > > > that these RGB values describe the color value in the interval
> > > > [0,1]. Now, in the radiative transfer equation on the above
> > > > website, the emissivity gets multiplied by the path length
> > > > delta s. I am now wondering how this works: Depending on how
> > > > big the step size is, one could get extremely large or extremely
> > > > small intensities that are essentially unrelated to the RGB
> > > > values that were previously specified. How is it possible that,
> > > > for example, the color of a density isosurface depends on the
> > > > density only and not on the cell size? I guess I am missing
> > > > something.
> > > >
> > > > Cheers,
> > > > Maike
> > > >
> > >
> > > The short answer, I think, is that you are right, you can get very
> > > large intensities in large cells that are unrelated to what came
> > > before.  But, for the most part, this is not an issue because of how
> > > the weighting and color assignments are typically done.
> > >
> > > [begin long, rambly answer]
> > >
> > > There are a couple things in what you ask -- the first is that there
> > > are two primary methods for volume rendering.  The first is to tell a
> > > story using somewhat abstract visuals; this is typically what people
> > > think of when they think of volume rendering, and it is supported by
> > > (and possibly the primary application of) yt.  This would be what's
> > > used when the ColorTransferFunction is used.  The other is designed to
> > > perform a meaningful line integral through the calculation; this is
> > > what's done with the ProjectionTransferFunction and the
> > > PlanckTransferFunction.  In all cases, the RT equation *is*
> > > integrated, but what varies between the two mechanisms is where the
> > > emission and absorption values come from.
> > >
> > > In all cases, while the code may call things RGB, they need not be RGB
> > > explicitly.  In fact, for the ProjectionTransferFunction, they are
> > > not.  For the ProjectionTransferFunction, the code simply integrates
> > > with the emission value being set to the fluid value in a cell and the
> > > absorption value set to exactly zero.  This results in the
> > > integration:
> > >
> > > dI/ds = v_local
> > >
> > > Where v_local is the (interpolated) fluid value at every point the
> > > integration samples, which defaults to 5 subsamples within a given
> > > cell.  So the final intensity is equal to the sum of all
> > > (interpolated) values along a ray times the (local-to-a-cell) path
> > > length between samples.  For the PlanckTransferFunction, the local
> > > emission is set to some approximation of a black-body, weighted with
> > > Density for emission, and the absorption is set to an approximation of
> > > scattering, which we then assign to RGB.  The PTF also utilizes a
> > > 'weighting' field inside the volume renderer, which I discuss briefly
> > > below, to allow it to utilize multiple variables at a given location
> > > to calculate the emission and absorption.  (i.e., Temperature governs
> > > the color, density governs the strength of the emission -- sliding
> > > along x and scaling along y, in the plot of the transfer function.)
> > >
> > > When integrating a ColorTransferFunction, the situation is somewhat
> > > different.  I've spent a bit of time reviewing the code, and I think I
> > > can provide a definite answer to your question.  For reference, the
> > > code that this calls upon is defined in two source files:
> > >
> > > yt/visualization/volume_rendering/transfer_functions.py
> > > yt/utilities/_amr_utils/VolumeIntegrator.pyx
> > >
> > > Specifically, in the class ColorTransferFunction and in the
> > > FIT_get_value and TransferFunctionProxy.eval_transfer functions.
> > >
> > > The ColorTransferFunction, which is designed for visualizing abstract
> > > isocontours, rather than computing an actual line integral that is
> > > then examined or modified, sets a weighting *table*.  (For the
> > > PlanckTransferFunction, a weight_field_id is set; this means to
> > > multiply the value from a table against a value obtained from another
> > > field.  This is how density-weighted emission is done.)  The
> > > weight_table_id for CTF is set to the table for the alpha emission.
> > > Functionally, this means that we accentuate the peaks and spikes in
> > > the color transfer function, because alpha is typically set quite high
> > > at the gaussians included.
> > >
> > > So in essence, with a color transfer function we accentuate only the
> > > regions where we have isocontours.  I think it's easiest to speak
> > > about this in terms of a visualization of Density isocontours.  If you
> > > place contours in the outer regions, if your emission value is too
> > > high, it will indeed obscure completely the inner regions.  I have
> > > experimented with this and have found that it is extremely easy to
> > > create a completely visually confusing image that contains only the
> > > outer contours and wispy hints of the inner contours.
> > >
> > > However, even if you do have outer isocontours, if you set the
> > > emission and color values lower, you can indeed provide glimpses into
> > > the inner regions.  The inner regions are likely generating *higher*
> > > emission values (this is certainly how it is done in yt, with the
> > > add_layers method on ColorTransferFunction.)
> > >
> > > Anyway, I hope that helps clear things up a little bit -- but please
> > > feel free to write back with any further questions about this or
> > > anything else.
> > >
> > > Best,
> > >
> > > Matt
> > >
> > > >
> > > > --
> > > > GMX DSL Doppel-Flat ab 19,99 Euro/mtl.! Jetzt mit
> > > > gratis Handy-Flat! http://portal.gmx.net/de/go/dsl
> > > > _______________________________________________
> > > > yt-users mailing list
> > > > yt-users at lists.spacepope.org
> > > > http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
> > > >
> > > _______________________________________________
> > > yt-users mailing list
> > > yt-users at lists.spacepope.org
> > > http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
> >
> > --
> > NEU: FreePhone - kostenlos mobil telefonieren und surfen!
> > Jetzt informieren: http://www.gmx.net/de/go/freephone
> > _______________________________________________
> > yt-users mailing list
> > yt-users at lists.spacepope.org
> > http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
> >

-- 
NEU: FreePhone - kostenlos mobil telefonieren und surfen!			
Jetzt informieren: http://www.gmx.net/de/go/freephone