Wasn't it R. Suzuki who wrote:
>"Mike Williams" <mik### [at] nospamplease> wrote.
>>>> Isosurface *disappears* at high resolution (fwd)
>>>> http://news.povray.org/3bb0b351@news.povray.org
>>>
>>>Cannot duplicate.
>>
>>I can duplicate this (although the quick version that I wrote now runs
>>to completion). The point at which the quick version collapsed into
>>blankness has varied from beta to beta, and now manages to go all the
>>way through. The original scene file posted by Warp still fails when
>>rendered with the specified size and antialliassing.
>>
>>The question is whether this is a bug, or whether isosurfaces that use
>>crackle can legitimately be expected to disappear.
>
>This is not a bug.  That behavior is due to the wrong max_gradient value.
>
>The function of the original scene file posted by Warp has extremely 
>large gradient values.  See the warning message.
>
>Try the following function for the isosurface of that scene,
>
>function{min(1,max(-0.5,(1+Blob_threshold)-Blob_threshold^Funki1(x,y,z)
>          -Blob_threshold^Funki2(x,y,z)))}
>
>The maximum gradient in the warning message is much lower than that of 
>the original function.  Then, it gives us better result.

That explanation was proposed in the original thread, but it didn't
explain the fact that decreasing the max_gradient, or changing the
amount reflection or removing three blank lines caused the scene to
render correctly. I've got a nagging suspicion that for such very subtle
changes to cause the scene to go from rendering correctly to being so
badly wrong there may be something nasty deep down that might possibly
come back and bite us later.

(One, irrelevant, side effect of this scene is that the scene will
render completely differently on a render farm than on a single machine.
The machine that renders the bottom third of the image will see the
isosurface, but a single machine that starts at the top and works all
the way down will not see any isosurface in the bottom third of the
image.)

It may well not be a bug (that's why it's not in the confirmed bug
list). If it isn't a bug, then The reason it is not a bug is the one
stated by Thorsten in.

http://news.povray.org/3bb928b1$1@news.povray.org

        "Not a bug, but a limit in the crackle function when used with 
        isosurfaces."

-- 
Mike Williams
Gentleman of Leisure

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