Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!unmvax!indri!polyslo!usc!randvax!celia!peter
From: peter@celia.UUCP (Peter Farson)
Newsgroups: comp.graphics
Subject: Re: Pixar's noise function
Keywords: random numbers, Byte, elephants
Message-ID: <465@celia.UUCP>
Date: 22 Apr 89 00:57:10 GMT
References: <2553@ssc-vax.UUCP> <3599@pixar.UUCP> <97699@sun.Eng.Sun.COM> <1592@ccncsu.ColoState.EDU> <43327@tut.cis.ohio-state.edu> <1648@ccncsu.ColoState.EDU> <1771@skinner.nprdc.arpa> <1669@ccncsu.ColoState.EDU>
Reply-To: celia!peter@tis.llnl.gov (Peter Farson)
Organization: Rhythm & Hues, Inc., Hollywood
Lines: 46

In article <1669@ccncsu.ColoState.EDU> bullerj@handel.colostate.edu.UUCP (Jon Buller) writes:
>In article <1771@skinner.nprdc.arpa> malloy@nprdc.arpa (Sean Malloy) writes:
>>If I understand the problem correctly, writing a floor() function is
>>extremely simple:
>
>>int floor(x)
>>double x;
>>{
>>  if (x >= 0.0) return(trunc(x));
>>           else return(trunc(x)+1);
>>}
>
>
>Well, that almost works, if x = -1.0, floor = -1.0, your_func = 0.0
>(you meant return(trunc(x)-1); didn't you?) in this case, your_func = -2.0,
>this ---->  ^^^^^^^^    will work if x is NOT an integer, will break if it is,
>and it is admittedly a simple thing to fix, but as I said before, I've been
>LAZY.  I can't use the thing in a render quite yet, so I haven't bothered.
>
	
	How about:

int floor(x)
double x;
{
if (x >= 0.0) return(trunc(x));
    else return(trunc(x-.999999));
    }

The only problem with this is with the accuracy of the .999999 constant.
Ideally you would want a floating point binary constant with all bits set
to 1 in the mantissa, to the precision of the binary floating point format
of the machine.  Any error in this value should not be too big a deal though in
most cases - it just causes values that are very close to an integer to
be treated as though they are an integer (-1.00000001 being mistaken for
-1.0000000000, for example).  But there is one pitfall: if there are too
many decimal digits in this constant for the machine's precision,
it might get rounded up to 1.00, which would make the function return
incorrect results for negative integers.


-- 
Can a bee be said to be                Peter Farson
An entire bee if                       celia!peter@tis.llnl.gov
Half the bee is not a bee              ...{ihnp4,ames}!lll-tis!celia!peter
Due to some ancient injury?