Re: contour plot

[Paul Kienzle]

You can get a lot of the way there using imagesc.

First, construct some data to contour:

   [x,y] = meshgrid(linspace(-2,2,200));
   z = sinc(sqrt(x.^2 + y.^2));
   colormap(rand(9,3));
   imagesc(z);

That's unrealistic since real data is noisy.  With
the added noise, the contours aren't very pretty:

   Z = z + 0.05*randn(size(z));
   imagesc(Z);

We can clean up the boundaries by convoluting with a gaussian:

   [x,y] = meshgrid(2.5*linspace(-1,1,16));
   B = exp(-.5*(x.^2+y.^2));
   imagesc(filter2(B,Z,"same"))

It would be nice to be able to outline the individual
contours, so let's make every point black if it is
not the same as its neighbours.  A simple heuristic
is if the sum of the four points in a square divided by
four is an integer, then you are in the middle of a
contour, otherwise you are on the boundary.  We set the
boundary points to colour 0 which is before the beginning
of the colormap, then we add black to the start of the
colormap and redraw it:

   Q = filter2(ones(2)/4,imagesc(filter2(B,Z,"same")));
   Q(Q != fix(Q)) = 0;
   colormap([0,0,0; colormap]);
   image(Q+1);

Now that we have found the contour lines, let's do a gradient
fill on the original noisy data and plot the contours on top
of it:

   colormap(hsv(64));
   M = imagesc(Z);
   M(Q==0) = 0;
   colormap([0,0,0; colormap]);
   image(M+1);

You can use epstk to make a pretty plot with axis markers, etc.

Hope this helps.

Paul Kienzle
address@hidden

On Thu, Aug 29, 2002 at 05:47:12PM -0300, Afonso de Moraes Paiva wrote:
>       Hi
>       I am used to matlab but I am new to octave. Is there a way of making
> decent filled-contour plots with octave? I've got a fill3.m script from
> "somewhere" but can't make it work. Thanks for any help.
>       Afonso
> -- 
> 
>    Afonso de Moraes Paiva
>    Programa de Engenharia Oceanica (PEnO/COPPE)
>    Universidade Federal do Rio de Janeiro (UFRJ)
>    Cidade Universitaria, CT, sala C203
>    Rio de Janeiro, RJ, 21945-970, Brasil
>    phone: (021) 2562-8729
>    email: address@hidden
> 
> 
> 
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