On Thu, 5 Sep 2002 16:49:26 +0200, "Niki Estner" <nik### [at] freenetde>
wrote:
> There's one thing isosurfaces can't do: you can't wrap them around a
> height_field.

Perhaps I don't understand what you are trying to say but I think otherwise.

ABX

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In article <3d777099@news.povray.org>,
 "Niki Estner" <nik### [at] freenetde> wrote:

A similar idea, but much faster: use the normal perturbation pattern to 
perturb the intersection distance. The outline of the object and its 
shadow would be unaffected, but it would make a difference in CSG and 
when another object is partially penetrating it, like a sphere embedded 
in a plane. It would probably be too obviously fake to be worth 
anything, though.


> I was just thinking how media works: a ray hits an object, several samples
> along the ray are taken, and the light along the ray is calculated. This
> takes time, but with GHz PCs...
> Just thinking aloud: A ray hits an object (say, a sphere). A second sphere
> which is a little smaller is tested against the same ray. Now I know a
> finite segment of the ray that is inside the "boundary" of the sphere. I'd
> apply a root solver to e.g. the leopard pattern plus the distance to the
> inner sphere along that finite segment (I'm afraid this isn't even close to
> being mathematically correct, I hope you can see what I mean...). So now I
> have the intersection of a sphere with a "leopard" hyper-texture. Of couse
> this is slow, but currently I'm using millions of spheres, which is probably
> slower (at least as soon as windows starts swapping).

Hmm...how is this different from an isosurface? Aside from the more 
flexible container.


> Yes, close. Pigments like granite or ripples aren't interpreted either.
> There's one thing isosurfaces can't do: you can't wrap them around a
> height_field.

I think you might be able to warp the height field function in such a 
way to do this...maybe. But other objects are more of a problem: meshes, 
CSG, julia fractals, etc. Deforming an isosurface is just not the same 
thing as deforming a mesh: deforming the surface in a direction parallel 
to the normal is easy for spheres or planes, but much harder for other 
shapes, and some shapes just can't be represented as an isosurface (you 
could probably do some bezier patches, but it would take some very 
clever clipping, and won't work for all patches).


> I also think wraping an isosurface around the boundary of a primitive should
> be faster because the boundary is smaller than the whole object, so less
> space has to be tested against some function etc.

So just make a way to specify more complex container shapes. I don't 
think they have to be as limited as they are.

I just don't think the concept of a hypertexture applies to anything but 
a surface-based rendering engine, using meshes, spline surfaces, etc.
It seems to me that the best alternative for POV is to add tessellation 
capability for all objects. Add a general algorithm like marching 
tetrahedrons that will work for anything (though you will have to place 
limits on it for infinite shapes), and more refined methods for spheres, 
cones, etc. (for example, tesselating a triangle is a bit redundant and 
the general algorithms often just won't work well)
Then add support for performing complex operations on meshes: 
deformations, subdivision, etc. Maybe capability to do subdivision at 
render-time, so you don't have to store so many triangles but can stand 
the slower rendering (similar to the bezier patch primitive).

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: chr### [at] tagpovrayorg
http://tag.povray.org/

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In article <chr### [at] netplexaussieorg> , 
Christopher James Huff <chr### [at] maccom>  wrote:

<snip>

I have to admit I never actually read either the section in the book (I
happen to only have a German shortened and updated version of it at home)
nor the paper.  However, if you are interested I can try to remember looking
it up at the university library next time when I have to return books (that
would be on the 19th).  I could then scan it and upload a copy somewhere, if
you are interested.

    Thorsten

____________________________________________________
Thorsten Froehlich, Duisburg, Germany
e-mail: tho### [at] trfde

Visit POV-Ray on the web: http://mac.povray.org

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> Hmm...how is this different from an isosurface? Aside from the more
> flexible container.
The difference is: you can design a scene with "basic" objects, render them
fast until you get the right geometry, and then apply a texture to it that
deforms the surface.
Optimizations could be done where a ray is sure to hit the object (only the
normal has to be calculated in that case), and the slow isosurface algorithm
only has to be used at the borders of an object, where you could see the
difference between a faked (normal perturbation) and a real (surface
perturbation) texture.
I just don't have a clue how to do it...

> I think you might be able to warp the height field function in such a
> way to do this...maybe. But other objects are more of a problem: meshes,
> CSG, julia fractals, etc.
Well, I guess the feature I'm thinking of won't work for all kinds of
objects (nothing new: media doesn't work for meshes, for example)

> I just don't think the concept of a hypertexture applies to anything but
> a surface-based rendering engine, using meshes, spline surfaces, etc.
If you told me that media, smooth shadows or radiosity features weren't
possible with a raytracer a few years ago, I' probably have agreed.
I'm just trying to add realism to my images, and a good way to acchieve
realsim for certain scenes is to get rid of that smooth and polished look
raytracing images always used to have. isosurfaces are great, while loops
help a lot and I'd be lost without procedural textures. But I think lots of
things can still be done here.

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Niki Estner wrote:

I did something similiar to hypertextures in the my picture "bear" 
(http://www.grafik.willhalm.de/), so perhaps I should comment
on this. I more or less read the paper of Perlin and Hoffert, but didn't 
like the idea of numerically finding the gradient. That's why I used the
lighting model of Kajiya and Kay from the same proceedings. 

>> Hmm...how is this different from an isosurface? Aside from the more
>> flexible container.
> The difference is: you can design a scene with "basic" objects, render
> them fast until you get the right geometry, and then apply a texture to it
> that deforms the surface.

What prevents you from doing this with isosurfaces? They're not _that_ slow,
if you don't use fancy functions like noise (which would give you 
interesting
hypertextures). That's what I did in my picture: Model the geometry with
simple functions and apply the "fur" later. At the moment, I not sure
whether it's really worth the trouble to implement the hypertextures in
POVRay or whether we can stick with isosurfaces.

>> I think you might be able to warp the height field function in such a
>> way to do this...maybe. But other objects are more of a problem: meshes,
>> CSG, julia fractals, etc.
> Well, I guess the feature I'm thinking of won't work for all kinds of
> objects (nothing new: media doesn't work for meshes, for example)

Well, CSG is possible with Christoph Hormann's isocsg library
(http://www-public.tu-bs.de:8080/~y0013390/pov/ic/index.html), but
meshes and fractals might be indeed, mmh.., "difficult".
 
Thomas

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