|
 |
"bgimeno" <bgimeno[at]persistencia[dot]org> wrote in message
news:4a865de4@news.povray.org...
> While experimenting with the function rand () to simulate a random
> distribution of trees, (moving object at a random distance from origin and
> then doing it orbit on the y-axis, when I found this.
> I've noticed a pattern when attempting to distribute objects on a square
> grid. But why this circular distribution?
>
> ... snip ...
>
> #local Num_Tree = 0 ;
> #while (Num_Tree<1000)
> #local rnd_tree = seed (Num_Tree);
> #if (rand (rnd_tree)<.95 )
> object {Tree
> translate <50*rand(rnd_tree),0,0>
> rotate y*360*rand(rnd_tree)
> }
> #end
> #local Num_Tree = Num_Tree +1 ;
> #end
>
>
> pd. I found it while editing the code to upload the image. The key is in
> the use of the same rand value to control the density of generating
> objects. But interesting anyway.
> pd2 google searchs results: "the seven arms of Shiva", "the seven arms of
> the Menorah", "an Octopus with only seven arms"...
>
Take your pick:
Answer A: Yes it's controlled by ancient deities.
Answer B: You're using the seed function wrong.
Answer B continued:
Because the seed function is being called from within the #while loop you
are simply stepping sequentially through the seeds rather than taking
successive numbers from a random number stream. If you move the seed
definition outside the loop and use a single number as the seed you should
avoid this problem ie use:
#local rnd_tree = seed (0);
and move it so that it's set before starting the #while loop.
This characteristic of the seed/rand functions was discussed in some detail
in a long thread quite a while ago:
http://news.povray.org/povray.binaries.images/thread/%3Cweb.4a0d5729a7098c1c34d207310@news.povray.org%3E/
I don't remember seeing anything there that explains what it is that will
give you 7 arms rather than a different number of arms, so this is maybe
where Answer A and Answer B mysteriously come together.
The other thing to note is that even when you fix this, your algorithm still
gives a cluster at the centre of the circle, but the reason for this is
categorically mathematical :o)
Regards,
Chris B.
Post a reply to this message
|
|
