[PD-dev] Pd's RNG.....

[Matju]

On Mon, 25 Oct 2004, chun lee wrote:
> On 24/10/04 12:59 pm, "Tim Blechmann" <TimBlechmann at gmx.net> wrote:
> >> Just wondering, does the methods used in Pd's [noise~] and [random]
> >> the implementation of minimal standard/Lehmer RNG method? Or something
> >> similar?
> > see doc/5.reference/help-random.pd ... random and noise are using the
> > same algorithm ...
> Thanks, I forgot to check the help files, my bad;(
> In addition,  I found the below info on RNG while I was search on net.
> http://www.sbc.su.se/~per/crng/seminar/rng5.html
> http://www.cs.virginia.edu/~jcc5t/classes/undergrad/simulation/random/rng.c

Some more info on the linear congruential method: the m can be anything
(in Pd it's 1<<32 exactly, because it's faster). To have the longest
possible period, where all numbers in the range are visited, i think the
conditions are that a,b,m are all coprime. Since m is a power of two this
just forces a,b to be odd, which they would have been anyway, and then the
only condition left is that a,b are coprime, that is, gcd(a,b)=1. However
you want to make it look random so you have to make a and b big enough
values that looks like nothing you already know. (else, you see, a=0 b=1
would be a good candidate ;-)

Then it only looks vaguely random. Obviously, if it guarantees to visit
all 1<<32 numbers before duplicates come, it's not very random. I once
filled a 256*256 grid (or something) using QuickBASIC (that was long ago)
and that algorithm, and I could see patterns in the filling process.
What's important to make it look random is to carefully avoid putting it
in a situation where the nonrandomness is exhibited (duh). In the case of
[noise~] no-one has an ear fine enough to find that it's
linear-congruential.

_____________________________________________________________________
Mathieu Bouchard -=- Montréal QC Canada -=- http://artengine.ca/matju