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Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!turk
From: turk@Apple.COM (Ken Turkowski)
Newsgroups: comp.dsp,sci.math.num-analysis
Subject: Re: Looking for a fast exp(x) where -10.0<=x<=10.0
Keywords: exp fast functions lookup tables
Message-ID: <11086@goofy.Apple.COM>
Date: 2 Nov 90 08:28:26 GMT
References: <1990Nov1.184820.5796@walt.disney.com> <16021@netcom.UUCP>
Organization: Advanced Technology Graphics, Apple Computer, Cupertino, CA, USA
Lines: 24

In a previous article, scott@walt.disney.com (Scott Watson) writes:
>>Hi y'all -
>>
>>I have a need for an fast exp(x) function...
>>
>>x is a float and can range inside +-10.0.

You can calculate exp(x) in fixed-point with a series of shifts
and adds and a small lookup table using CORDIC iterations.
Convergence is linear, so you get about one bit of precision with
each iteration through a simple loop.  With 32-bit words, you can
probably get 26 bits of precision.  If you can get by with less
bits of precision, you can by with less iterations.

I don't have the references handy, but I just recently published
a paper in Graphics Gems (Academic Press, 1990) on CORDIC trigonometry.
In the references to that paper are pointer to papers by Walthers
and Chen which address exp, log, sqrt, and a host of other nonlinear
functions.
-- 
Ken Turkowski @ Apple Computer, Inc., Cupertino, CA
Internet: turk@apple.com
Applelink: TURK
UUCP: sun!apple!turk