Marc-Hendrik Bremer wrote:
> 
> max_gradient problems are far more likely, I agree. There poped up just
> another one in p.b.p-t. Perhaps Warp should also mention that it's a good
> idea to make the container object as small as possible, since you can lower
> max_gradient with a smaler container wuite a bit and things will speed up.
> 

I don't think a smaller container can reduce required max_gradient (unless
the high gradient only occurs outside the smaller container object).  

Making container objects as small as possible should be quite obvious. 
It's also mentioned in detail in the documentation, so i don't think it's
required to put it in the FAQ unless Warp wants to make a general
'isosurface speed' section.

> 
> You think, that's a bug? I would say it's a user error.
> 

I would not say it's a bug, but it could be that the isosurface code can
be modified to avoid it.

Christoph

-- 
Christoph Hormann <chr### [at] gmxde>
IsoWood include, radiosity tutorial, TransSkin and other 
things on: http://www.schunter.etc.tu-bs.de/~chris/

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Marc-Hendrik Bremer wrote:
> 
> But a bigger container can require a higher max_gradient (see p.b.b-t
> "isosurface contained_by or evaluation problem"). I'm not sure if this is an
> expectable behaviour, but it seems somewhat logical to me. I finally found
> the description of the isosurface search method from R. Suzuki in p.general:
> RE: Isosurface and function pattern in v3.5
> (Message-ID<3b984214@news.povray.org> if that helps anything). If the points
> d1 and d2 he mentions are further apart, isn't a greater max_gradient
> needed?

No. The gradient of a function is a local property:  <df/dx, df/dy,
df/dz>  The max_gradient value is used to determine if there's a possible
intersection between two points (of arbitrary distance) under the
condition that the gradient between the two evaluated points is maximum.

> But that's all just guessing of course. But it's clear that if the container
> is to big (for what reason ever), the surface will sometimes (but
> reproducible - no "random-number-thingie" as with those pigment-patterns)
> disappear unless you raise max_gradient.

Because of the things mentioned above changing the container size should
have no influence in whether a surface is visible or not (onless you use
'evaluate' of course) but the scene you mentioned really seems to prove
the opposite.

Christoph

-- 
Christoph Hormann <chr### [at] gmxde>
IsoWood include, radiosity tutorial, TransSkin and other 
things on: http://www.schunter.etc.tu-bs.de/~chris/

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Marc-Hendrik Bremer wrote:
> 
> Please have a look at Mike Williams reply to "isosurface contained_by or
> evaluation problem" which describes much better than I can, why the size of
> the container has an influence.
> 

Yes, saw it.  Thanks.

In fact i mentioned this possibility in

news://news.povray.org/3BBEE00B.2E13A766%40gmx.de

but then forgot about it myself. :-)

Christoph

-- 
Christoph Hormann <chr### [at] gmxde>
IsoWood include, radiosity tutorial, TransSkin and other 
things on: http://www.schunter.etc.tu-bs.de/~chris/

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On Fri, 5 Oct 2001 03:14:04 +0200, Lutz-Peter Hooge <lpv### [at] gmxde>
wrote:

>In article <3bbce60b.1409922@localhost>, PeterC@nym.alias.net.almost 
>says...
>
>> 	Been playing with isosurfaces in 3.5 and I am wondering
>> if the attached image is a bug or not. 
>
>This is a typical problem resulting from an insufficient max_gradient.
>Not a bug.
>
>Lutz-Peter

Thanks for the help.  Since I've never used Mega-POV, this whole
isosurface thing is completely new to me.

Pete

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On Fri, 05 Oct 2001 08:24:15 +0200, Christoph Hormann
<chr### [at] gmxde> wrote:

--snippage--

>
>Sorry, but sometimes i wonder why we put much work into the documentation
>if it's not read.  
>

Rather than playing dump-on-the-newbie as well as Oh-So-High-And
-Mighty, you *could* have referred me to the relevent section of the
documentation that I glaringly overlooked.

In other words, you expended the effort to reply to my posting
but contributed nothing.

Fortunately, others on these newsgroups with a less negative
frame of mind have posted helpful and informative posts.

Good day

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nospam wrote:
> 
> Rather than playing dump-on-the-newbie as well as Oh-So-High-And
> -Mighty, you *could* have referred me to the relevent section of the
> documentation that I glaringly overlooked.
> 
> In other words, you expended the effort to reply to my posting
> but contributed nothing.
> 

Sad you see it that way, looking for 'isosurface' in the help index will
bring you immediately it the relevant sections (isosurface
object/tutorial) and there you will find both the information needed to
solve the problem and the hints to understand it.

> Fortunately, others on these newsgroups with a less negative
> frame of mind have posted helpful and informative posts.
> 

I do not like this attitude.  If you don't appreciate the way i answered
your question that's ok, but you should not expect that everytime you have
a question someone will directly give a solution for it.  It was perfectly
possible to find a solution from what i answered.

Christoph

-- 
Christoph Hormann <chr### [at] gmxde>
IsoWood include, radiosity tutorial, TransSkin and other 
things on: http://www.schunter.etc.tu-bs.de/~chris/

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Christoph wrote:
>Marc-Hendrik Bremer wrote:
>> Wouldn't it be good to mention "accuracy" in this VFAQ-Answer, too?
>
>For what reason? I don't remember many people having problems with 
>that, and the meaning is pretty self explanatory.

There is a plobrem on "accuracy" if the thickness of the isosurface 
is thinner than the accuracy value.  For example, following isosurface 
will generate artifacts(artefacts). 

isosurface {
        function {-0.002+abs(-3.0+f_r(x,y,z))}
        contained_by { sphere { 0, 3.1 } } 
        accuracy 0.01                      
}

Thus if the isosurface has a fine-structure, the "accuracy" value 
should be less than the size of the fine-structure.

R. Suzuki

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"R. Suzuki" wrote:
> 
> There is a plobrem on "accuracy" if the thickness of the isosurface
> is thinner than the accuracy value.  For example, following isosurface
> will generate artifacts(artefacts).
> 
> isosurface {
>         function {-0.002+abs(-3.0+f_r(x,y,z))}
>         contained_by { sphere { 0, 3.1 } }
>         accuracy 0.01
> }
> 
> Thus if the isosurface has a fine-structure, the "accuracy" value
> should be less than the size of the fine-structure.
> 

Yes, it seems quite natural, same happens when tesselating such
isosurfaces.  I think there is nothing you can do about it.

BTW, you can create nice moire-like patterns with that method. :-)

Christoph

-- 
Christoph Hormann <chr### [at] gmxde>
IsoWood include, radiosity tutorial, TransSkin and other 
things on: http://www.schunter.etc.tu-bs.de/~chris/

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