Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!ucsfcgl!pixar!aaa
From: aaa@pixar.UUCP (Tony Apodaca)
Newsgroups: comp.graphics
Subject: Re: Pixar's noise function
Summary: Ken Perlin's 1985 Siggraph paper explains it all.
Keywords: random numbers, Byte, elephants
Message-ID: <3599@pixar.UUCP>
Date: 29 Mar 89 23:27:39 GMT
References: <2553@ssc-vax.UUCP>
Reply-To: aaa@pixar.UUCP (Tony Apodaca)
Organization: Pixar -- Marin County, California
Lines: 47

In article <2553@ssc-vax.UUCP> coy@ssc-vax.UUCP (Stephen B Coy) writes:
>	Last night I saw the article in Byte about the Renderman
>standard.  In the sample source code the displacement function, dent(),
>calls a function called noise()...
>          ...My question:  Does anyone out there know what this
>noise function really is?

	Three-dimensional simulated natural textures using pseudorandom
functions were simultaneously and independently developed by Darwyn
Peachey and Ken Perlin in 1984-5.  Both have papers in the 1985
Siggraph Proceedings describing their systems.  Perlin, in particular,
describes in detail how noise() is implemented and can be used creatively
to describe large classes of natural phenomena.  Conceptually, noise()
is a "stochastic three-dimensional function which is statistically
invariant under rotation and translation and has a narrow bandpass
limit in frequency" (paraphrased from [Perlin1985]).  This means that
you put three-space points in, and you get values back which are basically
random.  But if you put other nearby points in, you get values that are
very similar.  The differences are still random, but the maximum rate of
change is controlled so that you can avoid aliasing.  If you put a set
of points in from a different region of space, you will get values out
which have "the same amount" of randomness.
	If you have ever been interested in realistic computer graphics, do
whatever it takes to get a look at Perlin's paper.  In 1985, his pictures
were absolutely astounding.  In 1989, they are STILL astounding.

>ps  Anyone have any decent marble texture functions you'd like to
>    share?

	Perlin's paper has a simple marble texture function:

/* assume marble_color() is a color lookup table function */
float turbulence(point p) {
	float t=0, scale=1;
	while(scale > pixelsize) {
		t += abs(noise(p / scale)*scale);
		scale /= 2;
	}
	return t;
}
color marble(point p) {
	float x = xcomp(p) + turbulence(p);
	return marble_color(sin(x));
}
-- 
UUCP:		{sun,ucbvax}!pixar!aaa		Tony Apodaca
ARPA,BITNET:	aaa%pixar.uucp@sun.com		Pixar, San Rafael, CA, USA