In fact, I want to render the implicit iso-surface embeded in 3D volume data
set.

if there is no such feature in Povray, how about some other? such as
MegaRay, ...

thanks~

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lien <nomail@nomail> wrote:
> In fact, I want to render the implicit iso-surface embeded in 3D volume data
> set.

  Could you explain what that means?

-- 
                                                          - Warp

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Warp <war### [at] tagpovrayorg> wrote:
> lien <nomail@nomail> wrote:
> > In fact, I want to render the implicit iso-surface embeded in 3D volume data
> > set.
>
>   Could you explain what that means?
>
> --
>                                                           - Warp

Oh, maybe my description is not so clear.
what i want is the volume raytracing technique based feature in povray.
I know that we can render the implicit iso-surface by using the existing
keyword "isosurface" in povray, but it seems that it is based on some
technique like Marching Cube, which first extraction the geometry of the
iso-surface. But with volume raytracing technique, we can directly raytrace
the volume data and save the geometry extracting step.

who can tell me something about it? does any xx-ray can do such things
exists?

thanks

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> Warp <war### [at] tagpovrayorg> wrote:
>> lien <nomail@nomail> wrote:
>>> In fact, I want to render the implicit iso-surface embeded in 3D volume data
>>> set.
>>   Could you explain what that means?
>>
>> --
>>                                                           - Warp
> 
> Oh, maybe my description is not so clear.
> what i want is the volume raytracing technique based feature in povray.
> I know that we can render the implicit iso-surface by using the existing
> keyword "isosurface" in povray, but it seems that it is based on some
> technique like Marching Cube, which first extraction the geometry of the
> iso-surface. But with volume raytracing technique, we can directly raytrace
> the volume data and save the geometry extracting step.
> 

As far as I understand there is no extraction of geometry in POV-Ray's 
isosurfaces, it indeed uses what you call "volume raytracing". In other 
words, for each ray the intersection with the surface is computed, but 
the surface is not reconstructed as a whole.

Regards

-- 
Vincent

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> Oh, maybe my description is not so clear.
> what i want is the volume raytracing technique based feature in povray.
> I know that we can render the implicit iso-surface by using the existing
> keyword "isosurface" in povray, but it seems that it is based on some
> technique like Marching Cube, which first extraction the geometry of the
> iso-surface. But with volume raytracing technique, we can directly 
> raytrace
> the volume data and save the geometry extracting step.

Look up df3 files and media in the POV docs, it might do what you want 
(never used it myself).

Post a reply to this message

lien <nomail@nomail> wrote:
> what i want is the volume raytracing technique based feature in povray.
> I know that we can render the implicit iso-surface by using the existing
> keyword "isosurface" in povray, but it seems that it is based on some
> technique like Marching Cube, which first extraction the geometry of the
> iso-surface. But with volume raytracing technique, we can directly raytrace
> the volume data and save the geometry extracting step.

  Isosurfaces don't use anything like marching cubes, and I don't really
understand what is the relation between isosurfaces and volume raytracing.

-- 
                                                          - Warp

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Warp wrote:
> lien <nomail@nomail> wrote:
>> what i want is the volume raytracing technique based feature in povray.
>> I know that we can render the implicit iso-surface by using the existing
>> keyword "isosurface" in povray, but it seems that it is based on some
>> technique like Marching Cube, which first extraction the geometry of the
>> iso-surface. But with volume raytracing technique, we can directly raytrace
>> the volume data and save the geometry extracting step.
> 
>   Isosurfaces don't use anything like marching cubes, and I don't really
> understand what is the relation between isosurfaces and volume raytracing.

Volume data from medical imaging techniques measures densities. Everything
else you see, i.e. specific organs, is the result of clever post processing
in conjunction with contrast enhancing substances the human received. Thus,
if you want to extract specific information you can use isosurfaces by
defining their threshold to the desired density.

Regarding the original post, to use the data, it has to be converted to a
POV-Ray density file. This file can then be turned into a pattern, which can
be used in an isosurface for rendering. In fact, the original isosurface
patch website (iirc that page is no longer available) had a sample image of
a human head.

	Thorsten

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in news:web.45a4a43af46d0e7d1524ae1d0@news.povray.org lien wrote:

> In fact, I want to render the implicit iso-surface embeded in 3D
> volume data set.

You can render an isosurface from a data set as Thorsten mentioned, you 
can also use this data set with media. An old image that uses both 
techniques:

Newsgroups: povray.binaries.images
Subject: frog
Message-ID: <8FC995A5Cseed7@204.213.191.228>
Date: 10 Oct 2000 09:27:46 -0400
Xref: news.povray.org povray.binaries.images:41670

http://news.povray.org/povray.binaries.images/attachment/%
3C8FC995A5Cseed7%40204.213.191.228%3E/frog.jpg

Ingo

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Many thanks for your interests, everyone. Sorry, am pretty busy these days.

I uploaded an image to povray.binaries.images,
http://news.povray.org/povray.binaries.images/thread/%3Cweb.45ac7028a8a7f8c81524ae1d0%40news.povray.org%3E/.

the image was rendered by using the isosurface object just like Thorsten
described.

to Vincent:
Am not sure how does povray implement the feature of "isosurface", if it
really does as what you said, that is exactly what I need. I was told that
this is also a kind of volume raytracing technique by a friend, but am not
sure. However, from the figure I uploaded, we can see patches, so I guess
that maybe povray firstly extracts the implicit surface geometry with some
tessellation technique. But this is just a guess. Another possibility is
that indeed it does with the method you said, but the normal for each
intersection is computed not so good by povray, so that the surface looks
not so smooth and appears like patches.

to Warp:
Sorry for my unclear description. According to my current limited knowledge,
the scattering, emission and absorption media are the typical feature of
volume raytracing technique, but do you think the method described by
Vincent is also a kind volume raytracing technique?

to Ingo:
Thanks for your image. I guess the white islands on the frog's skin are the
results of isosurface, am I right? :). But the image's resolution is not
large enough to determine whether there are quad patches or not.

to Thorsten:
Can you give me some comments towards the patches in my image? In fact, I
want to get a smooth surface from the volume data set. Maybe I used the
isosurface object incorrectly. the code for the isosurface object is:

#declare dens=function{pattern {density_file df3 densityfile interpolate 1}}
#declare interface=object{
  isosurface{
    function{dens(x,y,z)}
    threshold iso_value
    contained_by { box { 0.03, 0.97 } }
    open
  }
  pigment{rgbt <1., 1., 0.1, 0.5>}
  finish{
    ambient  0.3
    diffuse  0.3
    roughness 0.5
    brilliance 1
  }
  scale 1.06
  translate -0.03
}

The resolution for the volume data I used is 32x32x32, but i think finer
data can only alleviate the problem to some degree. Yet, I should try finer
data...

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lien0n nous apporta ses lumieres en ce 16-01-2007 01:36:
> Many thanks for your interests, everyone. Sorry, am pretty busy these days.

> I uploaded an image to povray.binaries.images,
>
http://news.povray.org/povray.binaries.images/thread/%3Cweb.45ac7028a8a7f8c81524ae1d0%40news.povray.org%3E/.

> the image was rendered by using the isosurface object just like Thorsten
> described.

> to Vincent:
> Am not sure how does povray implement the feature of "isosurface", if it
> really does as what you said, that is exactly what I need. I was told that
> this is also a kind of volume raytracing technique by a friend, but am not
> sure. However, from the figure I uploaded, we can see patches, so I guess
> that maybe povray firstly extracts the implicit surface geometry with some
> tessellation technique. But this is just a guess. Another possibility is
> that indeed it does with the method you said, but the normal for each
> intersection is computed not so good by povray, so that the surface looks
> not so smooth and appears like patches.

> to Warp:
> Sorry for my unclear description. According to my current limited knowledge,
> the scattering, emission and absorption media are the typical feature of
> volume raytracing technique, but do you think the method described by
> Vincent is also a kind volume raytracing technique?

> to Ingo:
> Thanks for your image. I guess the white islands on the frog's skin are the
> results of isosurface, am I right? :). But the image's resolution is not
> large enough to determine whether there are quad patches or not.

> to Thorsten:
> Can you give me some comments towards the patches in my image? In fact, I
> want to get a smooth surface from the volume data set. Maybe I used the
> isosurface object incorrectly. the code for the isosurface object is:

> #declare dens=function{pattern {density_file df3 densityfile interpolate 1}}
> #declare interface=object{
>   isosurface{
>     function{dens(x,y,z)}
>     threshold iso_value
>     contained_by { box { 0.03, 0.97 } }
>     open
>   }
>   pigment{rgbt <1., 1., 0.1, 0.5>}
>   finish{
>     ambient  0.3
>     diffuse  0.3
>     roughness 0.5
>     brilliance 1
>   }
>   scale 1.06
>   translate -0.03
> }

> The resolution for the volume data I used is 32x32x32, but i think finer
> data can only alleviate the problem to some degree. Yet, I should try finer
> data...


In a DF3 file, each value apply to a cube, so, you obviously get a blocky aspect.
POV-Ray don't use any tessellation. It samples along a ray and evaluate the 
isosurface value at a point, if it evaluate to less than the therhold, you are 
outside the object, it it evaluate at the threshold, you are ON the surface, if 
larger, then you are inside.
DF3's are mostly used for media dencity where the blockyness is less visible.

-- 
Alain
-------------------------------------------------
The strongest reason for the people to retain the right to keep and bear
arms is, as a last resort, to protect themselves against tyranny in
government.
Thomas Jefferson

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