Hi(gh)!

I started to model the building blocks of a Swiss-made marble lane 
system... when I came to an elongated block with a serpentine-shaped 
marble track on top, I chose to use a cubic sphere to create the 
hollow... but it resulted in heavy artifacts (object on the extreme 
right of the scene attached). I tried various tolerance values - but 
nothing removed those artifacts! Here is the code:

#declare T_Beech =
texture
{
   pigment { color rgb <0.6, 0.55, 0.45> }
   finish { ambient 0.1 diffuse 1 brilliance 0.4 }
}

#declare Cube =
box
{
   0, 0.05
   texture { T_Beech }
}

#declare HalfCube =
object
{
   Cube
   scale <1, 0.5, 1>
}

#declare HalfCube_Triple =
object { HalfCube scale <1, 1, 3> }

#declare Serpentine =
sphere_sweep
{
   cubic_spline
   8
   <0.025, 0.025, -0.02>, 0.05/6
   <0.025, 0.025, 0>, 0.05/6
   <0.021, 0.025, 0.02>, 0.05/6
   <0.012, 0.025, 0.05>, 0.05/6
   <0.038, 0.025, 0.10>, 0.05/6
   <0.029, 0.025, 0.13>, 0.05/6
   <0.025,  0.025, 0.15>, 0.05/6
   <0.025, 0.025,  0.17>, 0.05/6
   texture { T_Beech }
}

#declare KL =
difference
{
   object { HalfCube_Triple }
   object { Serpentine }
}

See you in Khyberspace!

Yadgar

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Attachments:
Download '2017-04-30 cugolino, take 1.jpg' (41 KB)

Preview of image '2017-04-30 cugolino, take 1.jpg'

I tried all sorts of tricks to try and eliminate the problem, but no luck:

Scaling both cube and sphere-sweep (both before and after they are created), as
well as the sphere radii; changing the sphere-sweep 'tolerance' value by orders
of magnitude up and down (which seems to have no effect at all); rotating only
the sphere-sweep in space; changing the number of entries in the sphere-sweep;
even moving the camera around. Adding commas after the 'sweep entries (as
recommended in the docs) has no effect either.

Using a b_spline instead of cubic also shows the problem.

Using union (or merge!) instead of difference results in NO problem; those
operations work OK.

I assume that this is your only sphere-sweep that uses a CUBIC spline?

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"Kenneth" <kdw### [at] gmailcom> wrote:

> I assume that this is your only sphere-sweep that uses a CUBIC spline?

Oops, I was thinking of your closest block in the image, the one with the curved
indent. That's probably a differenced torus shape, not a sphere-sweep.

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On 2017-04-29 08:05 PM (-4), Jörg "Yadgar" Bleimann wrote:
> Hi(gh)!
>
> I started to model the building blocks of a Swiss-made marble lane
> system... when I came to an elongated block with a serpentine-shaped
> marble track on top, I chose to use a cubic sphere to create the
> hollow... but it resulted in heavy artifacts (object on the extreme
> right of the scene attached). I tried various tolerance values - but
> nothing removed those artifacts! Here is the code:

It is for just such a reason that I created the SphereSweep module in 
the Object Collection:

http://lib.povray.org/searchcollection/index2.php?objectName=SphereSweep&contributorTag=Cousin%20Ricky

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Hi(gh)!

Am 30.04.2017 um 06:06 schrieb Cousin Ricky:

> It is for just such a reason that I created the SphereSweep module in
> the Object Collection:
>
>
http://lib.povray.org/searchcollection/index2.php?objectName=SphereSweep&contributorTag=Cousin%20Ricky

Very good, I will try this out!

See you in Khyberspace!

Yadgar

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On 04/29/2017 08:05 PM, Jörg "Yadgar" Bleimann wrote:
> Hi(gh)!
>
> I started to model the building blocks of a Swiss-made marble lane
> system... when I came to an elongated block with a serpentine-shaped
> marble track on top, I chose to use a cubic sphere to create the
> hollow... but it resulted in heavy artifacts (object on the extreme
> right of the scene attached). I tried various tolerance values - but
> nothing removed those artifacts! Here is the code:
>
Thanks for posting code. I've opened a github issue for this:

https://github.com/POV-Ray/povray/issues/287

given it seems to render correctly in 3.6.1.

Bill P.

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"Kenneth" <kdw### [at] gmailcom> wrote:
> I tried all sorts of tricks to try and eliminate the problem, but no luck...
>

I ran the scene in the latest(?) 3.7.1 beta 5, as well as in my original 3.70
install; same problems in both.

IIRC, sphere_sweeps using a cubic spline have been problematic for a *long*
time. If it were possible to do, I would suggest adding the 'natural_spline'
type to the sphere_sweep code. I don't know the math differences between natural
and cubic splines (or if one is mathematically 'simpler' than the other) but it
might give us at least one way to make curvy sphere_sweeps that can be
CSG-differenced from another object.

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Hi(gh)!

Am 30.04.2017 um 13:23 schrieb Jörg "Yadgar" Bleimann:
> Hi(gh)!
>
> Am 30.04.2017 um 06:06 schrieb Cousin Ricky:
>
>> It is for just such a reason that I created the SphereSweep module in
>> the Object Collection:
>>
>>
http://lib.povray.org/searchcollection/index2.php?objectName=SphereSweep&contributorTag=Cousin%20Ricky
>>
>
> Very good, I will try this out!

Now it looks correct: see attachment!

See you in Khyberspace!

Yadgar

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Attachments:
Download '2017-04-30 cugolino, take 2.jpg' (31 KB)

Preview of image '2017-04-30 cugolino, take 2.jpg'

"Kenneth" <kdw### [at] gmailcom> wrote:

> IIRC, sphere_sweeps using a cubic spline have been problematic for a *long*
> time.

I was unaware of this.

>If it were possible to do, I would suggest adding the 'natural_spline'
> type to the sphere_sweep code. I don't know the math differences between natural
> and cubic splines (or if one is mathematically 'simpler' than the other) but it
> might give us at least one way to make curvy sphere_sweeps that can be
> CSG-differenced from another object.

Maybe for the time being you could use:

http://www.econym.demon.co.uk/isotut/splines.htm

it may be slower, but would give good end results straightaway.

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Le 30/04/2017 à 17:06, William F Pokorny a écrit :
> On 04/29/2017 08:05 PM, Jörg "Yadgar" Bleimann wrote:
>> Hi(gh)!
>>
>> I started to model the building blocks of a Swiss-made marble lane
>> system... when I came to an elongated block with a serpentine-shaped
>> marble track on top, I chose to use a cubic sphere to create the
>> hollow... but it resulted in heavy artifacts (object on the extreme
>> right of the scene attached). I tried various tolerance values - but
>> nothing removed those artifacts! Here is the code:
>>
> Thanks for posting code. I've opened a github issue for this:
> 
> https://github.com/POV-Ray/povray/issues/287
> 
> given it seems to render correctly in 3.6.1.
> 
> Bill P.
> 
Thanks for logging.

I tested with hg-povray and it is fine there (at least the tmp372.pov file)

Actually, it is also ok on my master head, but both stable and official
master have problem.

Which leads me to conclude: read the note about the patch of my master,
for sphere_sweep:
* sphere sweep: less noise under some angles
* sphere sweep: UV mapping
* sphere sweep: removed the preliminary workaround for FS#81 (not
effective, was adding noise)
* sphere sweep: added the suggested fix for FS#243 (negative uniform
scaling of untransformed object should have effect on radius as absolute
factor)


From what I would keep in the sphsweep.cpp, previously in
source/backend/shape (now spheresweep.cpp in source/core/shape ):
const DBL ZERO_TOLERANCE        = 1.0e-1;// less noise when near 1 than
near 0 [JG]

instead of

const DBL ZERO_TOLERANCE        = 1.0e-4;

Good luck.

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Attachments:
Download 'offmaster.png' (51 KB) Download 'hg.png' (52 KB) Download 'master.png' (52 KB) Download 'stable.png' (51 KB)

Preview of image 'offmaster.png'

Preview of image 'hg.png'

Preview of image 'master.png'

Preview of image 'stable.png'