POV-Ray : Newsgroups : povray.bugreports : "mandel" as function in isosurface has unexpected results : Re: "mandel" as function in isosurface has unexpected results Server Time
28 Mar 2023 16:44:55 EDT (-0400)
  Re: "mandel" as function in isosurface has unexpected results  
From: Lars Rohwedder
Date: 15 Dec 2022 14:30:33
Message: <639b75d9@news.povray.org>
Am 15.12.22 um 19:36 schrieb Bald Eagle:
> Lars Rohwedder <rok### [at] gmxde> wrote:
>> I'd expected that the areas "far outside" of the Mandelbrot set (outside
>> of the red cylinder) would have "low" values, so they are far "outside"
>> of the isosurface. But they aren't, so the result is that the whole
>> contained_by box is filled.
>> Is it a bug how mandel pattern & isosurface work together or is the bug
>> in my scene file? :-/
>> Lars R.
> It looks like the pattern drops off to zero.


According to the documentation of isosurface, all points with function 
value  < threshold are "inside" the isosurface object.

The "mandel" pattern is "low" (near zero) far away from the Mandelbrot 
set, increases slowly near the border of the set and is 1 inside the 
"apple shape" set itself (with default values for "interior" and "exterior".

So I had expected that a formula like 0.9 - mandel would give the 
desired result:

   * the points inside the Mandelbrot set will be <0, so are consideded 
as "inside" the rendered shape.
   * the points far away of the Mandelbrot set will be >0, so are 
"outside" of the rendered shape.

But the rendering looks completely different than expected, see attached 

Changing the formula to 0.5-mandel2(x,y,z) it looks much more than a 
Mandelbrot set, but it is "inverse": the Mandelbrot set is "outside", 
the surrounding areay is inside. Hence I switched the sign, so the 
forumla becomes mandel(x,y,z)-0.5.

So the inside of the Mandelbrot set becomes the inside of the 
isosurface, as expected, but also the far away areas are "inside", which 
is _not_ what I expected.

What went wrong here?

Lars R.

pS: The "open" keyword just removes the visibility of the contained_by 
object, but I consider it helpful to see what is insided and what it is 
outside of the isosurface object.

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Download 'mandel-0.5.png' (25 KB) Download 'mandel-0.9.png' (20 KB)

Preview of image 'mandel-0.5.png'

Preview of image 'mandel-0.9.png'


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