Path: utzoo!attcan!uunet!dino!ux1.cso.uiuc.edu!uwm.edu!psuvax1!psuvm!brl102
From: BRL102@psuvm.psu.edu (Ben Liblit)
Newsgroups: comp.sys.apple
Subject: Re: AE, Mandelbrot generator
Message-ID: <90038.183056BRL102@PSUVM.BITNET>
Date: 7 Feb 90 23:30:56 GMT
References: <9002071850.AA29665@en.ecn.purdue.edu>
Organization: Penn State University
Lines: 51

Newsgroups: comp.sys.apple
Subject: Re: AE, Mandelbrot generator
References:  <9002071850.AA29665@en.ecn.purdue.edu>
Organization: Penn State University
Date: Wednesday, 7 Feb 1990 18:28:42 EST
From: Ben Liblit <BRL102@psuvm.psu.edu>
Message-ID: <90038.182842BRL102@PSUVM.BITNET>

In article <9002071850.AA29665@en.ecn.purdue.edu>, eldorado@EN.ECN.PURDUE.EDU
(David D Jansen) says:
>
>Could someone with fractal experience tell me why I get weird data.  It gets
>the basic shape but not quite right.

>type
>region = record {a rectangle on the imaginary plane}
>           amin : real;
>           bmin : real;
>           amax : real;
>           bmax : real;
>         end;

This declaration isn't the error source, but makes me cringe.  It seems that a
nicer-looking declaration, that better reflects what you're representing, would
be:

     TYPE
        region = RECORD
                    min, max : imaginary
                 END;

Of course, my format is different, but that doesn't matter -- it's the idea
that counts.

As for the actual source of your errors, your real variables are probably being
rounded off.  After a few hundred iterations, that can add up to some rather
large inaccuracies.  There are a few things you can try:

If your compiler has other, higher precision floating point types, use them.
Look for something called double or extended.  The tradeoff is reduction in
speed.

You could also junk the floating point altogether, and switch to straight
integer math.  It's blindingly fast, and I *think* it's supposed to be more
accurate.  Someone posted the core routines for Mandelbrot set iterating in C a
short while back.  Anyone remember where this appeared?  Major tradoff of going
all-integer:  you have to rewrite the multiplication and division routines.

                      Ben Liblit
                      BRL102 @ psuvm.bitnet -- BRL102 @ psuvm.psu.edu
                      "Fais que tes reves soient plus longs que la nuit."