I found this page on the Japanese povray webring.

http://www.cs-w.com/~earth/material/isopattern.htm

There is here the use of the cells pattern in a way that does not give
diffuse=0 on the edges of step functions.  I will study the source code,
but the reason why it looks so good hasn't hit me yet.  Any clues?

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"Greg M. Johnson" wrote:
> 
> I found this page on the Japanese povray webring.
> 
> http://www.cs-w.com/~earth/material/isopattern.htm
> 
> There is here the use of the cells pattern in a way that does not give
> diffuse=0 on the edges of step functions.  I will study the source code,
> but the reason why it looks so good hasn't hit me yet.  Any clues?

I'm not totally sure what you mean by diffuse=0 but i suspect you meant the
problems occuring with abrupt steps in the pattern.  

You can see in the source code, that the critical patterns all use higher
max_gradient values (20/50) and there are still errors occuring although not
very strong.  On 'isopattern2.htm' they are even better visible.  

Christoph

-- 
Christoph Hormann <chr### [at] gmxde>
Homepage: http://www.schunter.etc.tu-bs.de/~chris/

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In article <3A06F4C7.3AB43D80@my-dejanews.com>, 
gre### [at] my-dejanewscom wrote:

> There is here the use of the cells pattern in a way that does not give
> diffuse=0 on the edges of step functions.  I will study the source code,
> but the reason why it looks so good hasn't hit me yet.  Any clues?

As I have mentioned before: I think the problem is not that the diffuse 
is being set to 0(which sounds impossible), but that there is a problem 
calculating the normal for these functions. If the resulting normal is 
bad(containing NaN or something) or is just wrong, the lighting 
calculations will not work properly.

-- 
Christopher James Huff
Personal: chr### [at] maccom, http://homepage.mac.com/chrishuff/
TAG: chr### [at] tagpovrayorg, http://tag.povray.org/

<><

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Chris Huff wrote:

> In article <3A06F4C7.3AB43D80@my-dejanews.com>,
> gre### [at] my-dejanewscom wrote:
>
> > There is here the use of the cells pattern in a way that does not give


>
> As I have mentioned before: I think the problem is not that the diffuse
> is being set to 0(which sounds impossible), but that there is a problem
> calculating the normal for these functions. If the resulting normal is
> bad(containing NaN or something) or is just wrong, the lighting
> calculations will not work properly.

Thanks for the reply, but the point is that in this gentleman's (Makoto Mukui)
image, there is no such problem.
http://www.cs-w.com/~earth/material/isopattern.htm
Thus it is not a fundamental, inescapable problem with step functions in


to sleep.

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In article <3a08035b@news.povray.org>, "Greg M. Johnson" 
<"gregj;-()"@aol.c;-()om> wrote:

> Thanks for the reply, but the point is that in this gentleman's 
> (Makoto Mukui) image, there is no such problem.
> Thus it is not a fundamental, inescapable problem with step functions 
> in isosurfaces.? Perhaps his solution is simply a clever use of the 
> max_gradient at 50 that none of us thought of before.? Will 
> experiment after I put the baby to sleep.

I understand that...and I would like to see some experiments using the 
object pattern with this technique. He seems to have overlooked it...
However, if it is a problem with normal calculation(or self shadowing, 
another possibility), there might be a better way to solve it than using 
an extremely high gradient. If someone takes a closer look at what goes 
wrong and why his code gives a better image, they might find a faster 
rendering compromise.

-- 
Christopher James Huff
Personal: chr### [at] maccom, http://homepage.mac.com/chrishuff/
TAG: chr### [at] tagpovrayorg, http://tag.povray.org/

<><

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