I like it! Tricky trace work there. I just recently found out that using
just method 2 and eval with a very low accuracy (about .001) gives the best
results. I hope to work on my tutorial soon :)

Chris Huff wrote:

> Here is my first attempt at a cactus in MegaPOV(inspired by some
> discussions in the thread "A clock"), it uses trace() and an isosurface.
> The texturing still needs a lot of work...
>
> --
> Chris Huff
> e-mail: chr### [at] yahoocom
> Web page: http://chrishuff.dhs.org/
>
>  [Image]

--
Samuel Benge

E-Mail: STB### [at] aolcom

Visit the still unfinished isosurface tutorial:
http://members.aol.com/stbenge

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In article <38B5EA14.BC81C7D6@aol.com>, "SamuelT." <STB### [at] aolcom> 
wrote:

> I like it! Tricky trace work there. I just recently found out that 
> using just method 2 and eval with a very low accuracy (about .001) 
> gives the best results. I hope to work on my tutorial soon :)

Hmm, actually, that is a pretty high accuracy. :-)
Maybe this keyword should be changed to something like "allowable_error" 
in the official version...
I confess that I haven't tweaked the settings for this isosurface at 
all...it rendered as wanted and wasn't too slow, so I just didn't 
bother. I just typed in the equation, liked the results, gave it a plain 
green pigment, and began playing with trace().

-- 
Chris Huff
e-mail: chr### [at] yahoocom
Web page: http://chrishuff.dhs.org/

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Bravo!!!

Eric
--------------------
http://www.datasync.com/~ericfree
--------------------
"The whole problem with the world is that fools and fanatics are always so
certain of themselves, but wiser people so full of doubts."
--Bertrand Russel

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"Chris Huff" <chr### [at] yahoocom> wrote in message
news:chrishuff_99-00EEB8.15435524022000@news.povray.org...
>
> function {
>    sqrt(
>       sqr(x)
>       + sqr(y-sqrt(sqr(x/2)+sqr(z/2))*1.5)
>       + sqr(z)
>    ) - 1
>    - (sin(TH(x, y, z)*18)*0.1)//This is what makes the ridges
> }

How did you come up with this?  Luck?  Patience?  Or did you say, "hmm...
the formula for a cactus should be blah blah blah" and just whip it up?
Even tho I've had a couple semesters of calculus and physics (20 years ago)
I have no clue how to take an idea and make an iso-surface out of it.

Eric
--------------------
http://www.datasync.com/~ericfree
--------------------
"The whole problem with the world is that fools and fanatics are always so
certain of themselves, but wiser people so full of doubts."
--Bertrand Russel

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In article <38b7154e@news.povray.org>, "Eric Freeman" 
<eri### [at] datasynccom> wrote:

> How did you come up with this?  Luck?  Patience?  Or did you say, "hmm...
> the formula for a cactus should be blah blah blah" and just whip it up?
> Even tho I've had a couple semesters of calculus and physics (20 years 
> ago)
> I have no clue how to take an idea and make an iso-surface out of it.

No calculus or physics required. If there were, I wouldn't be able to 
accomplish anything. I am still in Algebra II.
Well, the cactus is approximately spherical, with an indentation in the 
middle and radial ridges.

I started from the equation for a sphere:
function {
   sqrt(
      sqr(x)
      + sqr(y)
      + sqr(z)
   ) - 1
}

To indent the top of the cactus and extend the bottom, I modified the y 
portion of the equation like this:
      + sqr(y-sqrt(sqr(x/2)+sqr(z/2))*1.5)
This subracts a certain amount from the y value which depends on the 
distance from the y axis. The /2 and *1.5 were just to "tune" it to the 
right proportions. Subtracting from the y value has the effect of 
raising that portion of the surface, since a higher initial value is 
required to reach the threshold value. This modification really raises 
the sides of the cactus, although a variant of it could be made to 
depress the middle.

Then to add the "ridges", I subtracted a value depending on the sine of 
a multiple of the angle around the y axis from the total density:
   - (sin(atan2(x, z)*18)*0.1)

(note that while this is slightly different from the other version, it 
is really just a different way of calling the same function.)
The angle around the y axis can be calculated by atan2(x,z), which 
returns the angle in radians. This is ok, since the sin() function takes 
radians. I then multiply by 18 to get 18 "cycles" for a full revolution.
Since the sin() function returns values between -1 and 1, I multiplied 
it's result by 0.1 to get shallower ridges. This makes the ridges extend 
from about 0.1 units "below" the surface of the original sphere-like 
shape to about 0.1 units "above" it. Because the function returns both 
positive and negative values equally, I could just have easily used 
addition to incorporate it into the equation.

I probably did a very poor job of explaining it, but that is how I came 
up with this equation. I tend to have more success visualizing the 
isosurface as a density function with a "skin" at a certain density 
level(the threshold value). I start with a basic shape and progressively 
"sculpt" the density pattern by tweaking the function, adding 
characteristics to the function, and adding other density functions into 
the mix.

-- 
Chris Huff
e-mail: chr### [at] yahoocom
Web page: http://chrishuff.dhs.org/

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Chris Huff wrote:

>I probably did a very poor job of explaining it, ...

No you didn't, I can visualize the process.
Can you, or somebody else add this, and more like this, to a Iso-tutorial?

Ingo

-- 
Photography: http://members.home.nl/ingoogni/
Pov-Ray    : http://members.home.nl/seed7/

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Hey - that looks healthier than the one I got on the windowsill in front of me.
Great pic. Don't over-water it :-/

Alf

http://www.peake42.freeserve.co.uk/
http://ourworld.compuserve.com/homepages/Alf_Peake/


Chris Huff <chr### [at] yahoocom> wrote in message
news:chrishuff_99-9D37D5.05283324022000@news.povray.org...
> Here is my first attempt at a cactus in MegaPOV(inspired by some
> discussions in the thread "A clock"), it uses trace() and an isosurface.
> The texturing still needs a lot of work...
>
> --
> Chris Huff
> e-mail: chr### [at] yahoocom
> Web page: http://chrishuff.dhs.org/
>
>

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On 26 Feb 2000 03:53:35 -0500, ingo <ing### [at] homenl> wrote:
>Chris Huff wrote:
>
>>I probably did a very poor job of explaining it, ...
>
>No you didn't, I can visualize the process.
>Can you, or somebody else add this, and more like this, to a Iso-tutorial?
>
>Ingo
>

I'd like to second that. 

-- 
Cheers
Steve              email mailto:sjl### [at] ndirectcouk

%HAV-A-NICEDAY Error not enough coffee  0 pps. 

web http://www.ndirect.co.uk/~sjlen/

or  http://start.at/zero-pps

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Alf Peake wrote:

> Hey - that looks healthier than the one I got on the windowsill in front of me.
> Great pic. Don't over-water it :-/

My cacti said water every 2 weeks, but then my succulent in the same pot shrivelled
up. Now there's only 6 in the pot instead of 7 :-( But the six are doing quite
well. :-)

--
___     ______________________________________________________
 | \     |_                 <dav### [at] faricynet> <ICQ 55354965>
 |_/avid |ontaine               http://www.faricy.net/~davidf/

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"Chris Huff" <chr### [at] yahoocom> wrote in message
news:chrishuff_99-3B26BF.19203525022000@news.povray.org...
>
> I probably did a very poor job of explaining it, but
> that is how I came up with this equation.

Not at all.  This makes more sense than any other explanation of
iso-surfaces I've found.  Thanx.

Eric
--------------------
http://www.datasync.com/~ericfree
--------------------
"The whole problem with the world is that fools and fanatics are always so
certain of themselves, but wiser people so full of doubts."
--Bertrand Russel

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