Warp schrieb in Nachricht <3bbdba92@news.povray.org>...
>http://www.students.tut.fi/~warp/povVFAQ/languageVFAQ.html#isosurfacebug
>


Wouldn't it be good to mention "accuracy" in this VFAQ-Answer, too?

Marc-Hendrik

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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.

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 Hormann schrieb in Nachricht <3BBDE65F.62377969@gmx.de>...
>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.


Because in my opinion those artefacts are not always caused by an inaccurate
max_gradient. Esp. those where big parts are missing or disappearing seems
to me more like an inaccurate accuracy then a max_gradient problem (which
are those black spots like in the scene the thread started with). I may be
wrong in this, but that's how I experienced it in the past.
In addition max_gradient and accuracy seems not to be independent if I
remember an article of the original isosurface-patch writer some weeks ago
correct (unfortunately it was in one of those "Hey folks!"-threats and I
can't find it). While max-gradient seems to determine the "step-width" of
the algorithm, accuracy tells when to stop to look for an ray-intersection.
Is this wrong?
Of course, max-gradient is far more often the problem, which leads to those
artefacts.

Marc-Hendrik

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Marc-Hendrik Bremer wrote:
> 
> Because in my opinion those artefacts are not always caused by an inaccurate
> max_gradient. Esp. those where big parts are missing or disappearing seems
> to me more like an inaccurate accuracy then a max_gradient problem (which
> are those black spots like in the scene the thread started with). I may be
> wrong in this, but that's how I experienced it in the past.

Accuracy just influences how small details are visible, lower values lead
to more detailed surfaces and usually require higher max_gradient.  Just
try it, the default value of 0.001 is quite low, if you use for example
0.1 on a RMF function isosurface, you will get a not very detailed
surface.  

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 Hormann schrieb in Nachricht <3BBE0827.BE2E41F3@gmx.de>...
>Accuracy just influences how small details are visible, lower values lead
>to more detailed surfaces and usually require higher max_gradient.  Just
>try it, the default value of 0.001 is quite low, if you use for example
>0.1 on a RMF function isosurface, you will get a not very detailed
>surface.
>


While you are right that higher accuracy values decrease the level of
detail, it has other influences, too.
I just tried it with an old scene and attached the resulting image. I
increased the accuracy value by far (0.1 instead of 0.000001) and left
anything else the way it was (after converting to 3.5 language of course).
Pov reports, that I should decrease max_gradient to lower rentertime  (5.141
instead of 6). In my opinion that black part is one of those isosurface
artefacts.

Marc-Hendrik

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Attachments:
Download 'landschaft low acc.jpg' (12 KB)

Preview of image 'landschaft low acc.jpg'

Marc-Hendrik Bremer wrote:
> 
> While you are right that higher accuracy values decrease the level of
> detail, it has other influences, too.
> I just tried it with an old scene and attached the resulting image. I
> increased the accuracy value by far (0.1 instead of 0.000001) and left
> anything else the way it was (after converting to 3.5 language of course).
> Pov reports, that I should decrease max_gradient to lower rentertime  (5.141
> instead of 6). In my opinion that black part is one of those isosurface
> artefacts.
> 

From just the picture i can't conclude much, but Povray's suggestion that
max_gradient is too high does not necessarily need to be correct.  

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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Wasn't it nospam who wrote:

>       Been playing with isosurfaces in 3.5 and I am wondering
>if the attached image is a bug or not.  It happens using
>f_noise3d and with f_noise_generator using generator 0 or 1.
>Using f_noise_generator with a generator of 2 does not produce
>the problem.
>       .pov source posted on povray.scenes.text-files

I guess it's way too late to suggest that max_gradient ought to be made
mandatory for isosurfaces.

Or warn "Warning: isosurface has no max_gradient set. The isosurface may
contain holes! Set a correct max_gradient value to get a proper
rendering of the isosurface."

Or let the default value of max_gradient be changed to a value that's
likely to look OK for simple surfaces. I'd suggest a default value of
10.

As things currently stand we're likely to be seeing an *awful* lot of
occurrences of this question in the future.



-- 
Mike Williams
Gentleman of Leisure

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Marc-Hendrik Bremer wrote:
> 
> That's undoubted (now Pov found a max_gradient of 6.294), but my point was,
> that setting the accuracy value to high, can lead to artefacts. I'll post a
> simplified code for this in p.t.s-f (under "Low accuracy values in
> Isosurfaces"). Output changes a bit (because of a simplified texture), but
> the behaviour should be reproducible. To exclude that this is a max_gradient
> problem I just rendered it with max_gradient 60. Those black parts remained.
> Perhaps it's another problem, but I can't imagine which.
> 

I can see what you mean.  But these artefacts are different from those
resulting of insufficient max_gradient.  I don't think they are something
new users have to struggle with very often.  

I don't know much about the isosurface algorithm, but maybe that's even
something that could be fixed.

BTW, i recognize some of my textures in that scene...

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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