I have been playing with functions in isosurfaces lately.  The attached picture
shows what amounts to a damped cosine wave overlayed on a hexagonal plate.  The
first image is the basic isosurface.  Note the peak in the center.

For the second image, I used a cylinder to cut a hole in the center of the
isosurface using a difference operation.  Note the "ghost" of the peak
appearing in the image.

For the third image, I added a mostly transparent view of the cutting cylinder
as a separate object to visualize and ensure the cylinder was the right size,
etc.  Note that the "ghost" has disappeared in this image.  The only difference
between second and third images is the addition of the view of the cylinder.

Question:  what is causing the "ghost" of the peak of the isosurface (the part
cut out by the difference operation?

Thanks,

Eric

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Download 'damped_cosine.png' (69 KB)

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Wasn't it EricG who wrote:
>I have been playing with functions in isosurfaces lately.  The attached picture
>shows what amounts to a damped cosine wave overlayed on a hexagonal plate.  The
>first image is the basic isosurface.  Note the peak in the center.
>
>For the second image, I used a cylinder to cut a hole in the center of the
>isosurface using a difference operation.  Note the "ghost" of the peak
>appearing in the image.
>
>For the third image, I added a mostly transparent view of the cutting cylinder
>as a separate object to visualize and ensure the cylinder was the right size,
>etc.  Note that the "ghost" has disappeared in this image.  The only difference
>between second and third images is the addition of the view of the cylinder.
>
>Question:  what is causing the "ghost" of the peak of the isosurface (the part
>cut out by the difference operation?

Odd things can happen with isosurface CSG operations if you don't set 
max_trace high enough.

-- 
Mike Williams
Gentleman of Leisure

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"EricG" <imn### [at] sbcglobalnet> wrote:
> Question:  what is causing the "ghost" of the peak of the isosurface (the part
> cut out by the difference operation?

A bug maybe? :P

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EricG wrote:
| Question:  what is causing the "ghost" of the peak of the
isosurface (the part
| cut out by the difference operation?
|
	From the docs:
By default POV-Ray searches only for the first surface which the ray
intersects. But when using an isosurface in CSG operations, the
other surfaces must also be found. Therefore, the keyword max_trace
must be added to the isosurface statement. It must be followed by an
integer value. To check for all surfaces, use the keyword
all_intersections instead.
With all_intersections POV-Ray keeps looking until all surfaces are
found. With a max_trace it only checks until that number is reached.

http://povray.org/documentation/view/3.6.1/300/

		Jerome
- --
mailto:jeb### [at] freefr
http://jeberger.free.fr
Jabber: jeb### [at] jabberfr
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Or an auto-bounding problem? (Are you using v3.6.1 or a 3.7 beta? They have
different auto-bounding schemes, AFAIK.)

I assume that the semi-transparent cylinder had to be added as another object,
to get the representational image. That would increase the number of objects to
three, the default auto-bounding threshold (in v.3.6.1.)

Purely out of conjecture, try changing Bounding_Threshold to 0 (and alternately
to some high value like 50) to see if that makes any difference.

KW

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Jérôme M. Berger nous illumina en ce 2009-05-09 05:12 -->
> EricG wrote:
> | Question:  what is causing the "ghost" of the peak of the
> isosurface (the part
> | cut out by the difference operation?
> |
>     From the docs:
> By default POV-Ray searches only for the first surface which the ray
> intersects. But when using an isosurface in CSG operations, the
> other surfaces must also be found. Therefore, the keyword max_trace
> must be added to the isosurface statement. It must be followed by an
> integer value. To check for all surfaces, use the keyword
> all_intersections instead.
> With all_intersections POV-Ray keeps looking until all surfaces are
> found. With a max_trace it only checks until that number is reached.
> 
> http://povray.org/documentation/view/3.6.1/300/
> 
>         Jerome
I go with this explanation and cure. In your case, a max_trace 3 should be 
correct: across the peak, you cross 2 hiden surfaces before you reatch the one 
you are interested in.

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Alain <ele### [at] netscapenet> wrote:

.....the keyword max_trace must be added to the isosurface statement.

I finally got back to this after being distracted by my real job.  It was the
max_trace that caused the "ghost".  I had originally set it to 3, but needed to
set it to 7 for this object.  I have attached an image showing the options
available in the macro I wrote, and a "full up" (all options on) isosurface.
Weird way to spend my time, but I learned a few things in the process.

Thanks all for your inputs.

Eric

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