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From: bullerj@handel.colostate.edu (Jon Buller)
Newsgroups: comp.graphics
Subject: Re: Pixar's noise function
Keywords: random numbers, Byte, elephants
Message-ID: <1648@ccncsu.ColoState.EDU>
Date: 14 Apr 89 14:34:23 GMT
References: <2553@ssc-vax.UUCP> <3599@pixar.UUCP> <97699@sun.Eng.Sun.COM> <1592@ccncsu.ColoState.EDU> <43327@tut.cis.ohio-state.edu>
Sender: news@ccncsu.ColoState.EDU
Reply-To: bullerj@handel.colostate.edu.UUCP (Jon Buller)
Organization: Colorado State University, Ft. Collins CO 80523
Lines: 34

In article <43327@tut.cis.ohio-state.edu> william r dirks <dirks@cis.ohio-state.edu> writes:
>Anyway, I rendered a polygon situated in the x-y plane with DNoise()
>added to the normal vectors to make a bumpy-looking surface.  But I
>noticed there were discontinuities in the "bumps."
>
>Studying the image, I noticed that where x and y were both positive
>the bumps were continuous, where x was negative and y was positive
>there were lines of discontinuity parallel to the y-axis, where x was
>positive and y was negative the lines were parallel to the x-axis, and
>where both x and y were negative the two sets of lines intersected to
>form squares.
>
>On a hunch I changed all the trunc()'s in the source to floor()'s.
>
>And presto!! Continuous bumps everywhere!

    I wish I could do that, my compiler doesn't have a built in Floor,
and I haven't had the time to hack one up from the Mac toolbox and SANE
(IEEE math pkg) calls, oh well, I'll have to settle for only using 
X, Y, and Z >= 0.0  One other thing you might notice, Noise is C1 continuous,
DNoise is only C0.  This means that DNoise will have creases in it (along
the planes of the random grid.  To see this, crank out a square: 0<X<5, 0<Y<5,
Z=0.  You will see smooth regions within each unit square, and creases btween
squares.  To aviod this, use a cubic B-Spline, or cubic Hermite (as hinted to
in the SIGGRAPH procedings) the problem there, is that you either need more
data points (64 instead of 27) for the B-Spline, or deravative info at each
point of the grid (a normal plane, 4 floats instead of 1).  This would take too
muh time for me to code up to be worth it, and would probably run too much
slower (10min for a 200x200 pixel picture now, ug.)  If somebody wants to give
me a cray-3 to play with, I'll write more accurate (and slower) code, until
then... 8-)
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Jon Buller                                      FROM fortune IMPORT quote;
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