Apologies if this has been answered before; I suspect it should have been,

In my first-ever experimenting with iso-surfaces, I've run across this: <see
image>

There are some quite apparent "gridlines" appearing on the perturbed sphere;
they're ugly and I've no idea where they are coming from.  I've played with
the 'accuracy' setting with no success.

(POV v3.5 640x480noAA)

Scene file:

#include "functions.inc"
camera {location -1.1*z look_at <0,0,0>}
light_source {<5,5,-10> rgb 1}

#declare Sphere = function {pow(x,2) + pow(y,2) + pow(z,2)}
#local PETURB = 30;
#local FLATTEN = 30;
isosurface {
    function  { Sphere(x,y,z) +
f_snoise3d(x*PETURB,y*PETURB,z*PETURB)/FLATTEN }
    threshold 0.2
    max_gradient 3
    accuracy 0.0001
    pigment {rgb 1}
}

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Attachments:
Download 'isosurface.JPG' (32 KB)

Preview of image 'isosurface.JPG'

Canicus,
I cannot explain them, but I too have seen these on several iso-surfaces.   I
thought perhaps the artifacts might go away with AA as is the case
sometimes, but they did not.
Bill P.

> Apologies if this has been answered before; I suspect it should have been,
>
> In my first-ever experimenting with iso-surfaces, I've run across this: <see
> image>
>
> There are some quite apparent "gridlines" appearing on the perturbed sphere;
> they're ugly and I've no idea where they are coming from.  I've played with
> the 'accuracy' setting with no success

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On Wed, 4 Sep 2002 23:18:03 +1000, "Canicus" <d7p### [at] sneakemailcom>
wrote:

>There are some quite apparent "gridlines" appearing on the perturbed sphere;
>they're ugly and I've no idea where they are coming from.  I've played with
>the 'accuracy' setting with no success.

This is a well-known defect of the noise3d function, use
f_noise_generator(x,y,z,3) instead.


Peter Popov ICQ : 15002700
Personal e-mail : pet### [at] vipbg
TAG      e-mail : pet### [at] tagpovrayorg

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On Wed, 4 Sep 2002 23:18:03 +1000, "Canicus" <d7p### [at] sneakemailcom> wrote:
> There are some quite apparent "gridlines" appearing on the perturbed sphere;

http://web.wanadoo.be/wbrp0009/pov/qbugs/noisegrid/main.html

ABX

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The second derivative of all of the POV-Ray noise functions is discontinuous
at integer X, Y, or Z values. That causes the *first* derivative of the
*shading* of objects based off of the noise to be discontinuous there. This
creates a noticeable grid effect.

It's the way the noise is made to work internally. I've heard that there are
ways to fix or reduce the effect, but they haven't been implemented in
POV-Ray.

Try creating a pigment function based on the bozo pattern (which is really
just f_noise3d as a pattern, but be sure to add a color_map{[0 rgb 0][1 rgb
1]}), and then turbulating it, and using that function instead of f_noise3d.
The turbulence will at least move the grid around so it won't be quite so
noticeable.

 - Slime
[ http://www.slimeland.com/ ]

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