From help-request at octave dot org Mon Mar  7 03:45:39 2005
Subject: Re: 0-1 function
From: Gorazd Brumen <brumen at hispeed dot ch>
To: help at octave dot org
Date: Mon, 07 Mar 2005 10:52:02 +0100

Thanks for the answer. I tried the image solution and it looks quite ok.
But is there any way of putting axes to the plot or I have to do it 
manually?
Can you do similar thing as with image command with some gnuplot command?

G.

Geordie McBain wrote:
> On Sun, Mar 06, 2005 at 10:03:40PM +0100, Gorazd Brumen wrote:
> 
>>Hi again,
>>
>>I have a function of 2 variables which takes only the values
>>0 and 1. What would be in your oppinion
>>the best way to present this
>>function graphically? If I draw a mesh plot or a contour
>>plot, gnuplot always wants to make graphs continuous and
>>they look weird.
>>
>>Gorazd
> 
> 
> Hello.  One way is to colour-in the region f=1.  For example
> 
> octave> function p = pred (x, y); p = y < sqrt (abs (x) + x.**2); endfunction
> octave> [XX, YY] = meshgrid (linspace (-2, 2, 100), linspace (3, 0, 100));
> octave> imagesc (pred (XX, YY))
> warning: in fopen near line 197, column 11:
> warning: fopen: default open mode is now binary
> 
> This draws a somewhat pixellated black-and-white plot of the function
> pred.  (Note that the y-coordinates are reversed.)
> 
> Another way to display a function f(x,y) -> {0,1} is to draw the
> margin separating the regions f=0 and f=1.  This involves a lot less
> function evaluations that the first method.  I came up with an
> algorithm to walk along such marginal curves, given two initial
> points, one on either side of the curve.  If this is of interest, see
> 
> 	http://www.aeromech.usyd.edu.au/~mcbain/stability/skirting.html
> 
> for the m-file, or
> 
> 	http://anziamj.austms.org.au/V45/CTAC2003/McBa/
> 
> for detailed description, theory, examples, etc.
> 
> This could also be useful for that question from someone else over the
> weekend about drawing tricky contours.  Say you wanted to draw the
> level-curve f(x,y) = z0, define a function p(x,y)=(f(x,y)<z0), then
> p(x,y) takes only two values and proceed as above.
> 
> Geordie McBain
> www.aeromech.usyd.edu.au/~mcbain
> 
> 
> 
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> 

-- 
Gorazd Brumen
Mail: brumen at isb dot unizh dot ch
WWW: http://valjhun.fmf.uni-lj.si/~brumen
PGP: Key at http://pgp.mit.edu, ID BCC93240



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