contourc - plotting contour matrix

I would like to find the contours, manipulate them, and then plot them.

This plots contours

x = 0:0.1:1; y = 0:0.1:1;
[X,Y] = meshgrid(x,y);
Z = sin(X).*cos(Y);
[con_mat, h] = contour(x, y, Z);

To get the contour information without plotting, can use contourc.

con_mat = contourc(x, y, Z);

However, there appears to be no built-in way to plot this "contour matrix".

Any thoughts?

John

Answer 1

[Darin]

Try this: it uses cplot, which you can find on Matlab central.

If you find something more elegant, please let us all know... it MUST be buried in the Matlab library function SOMEWHERE.

function[] = plot_contourmatrix(C);
% plot_countourmatrix - Plots a contour matrix c as returned from contour
%
%  plot_contourmatrix(C)  
  % Rev History:
  % 06-04-12 Created  
figure(gcf);
holdstate = ishold;
hold on;
i = 1;
while i<=length(C)
    lev = C(1,i);
    cnt = C(2,i);
    cplot(C(1,i+(1:cnt)),C(2,i+(1:cnt)),repmat(lev,1,cnt)); 
    i = i+cnt+1;
end;
if ~holdstate
    hold off;
end;
return

test code - execute as cell

figure(1);
[x,y] = meshgrid(-3:.1:3);
z = sinc(hypot(x,y));
[C,h] = contour(x,y,z);
axis equal

figure(2)
plot_contourmatrix(C);
axis equal

[John Mahoney]

Thanks Darin. I was using something similar, but was checking for integer values in C(2,:). This fails (almost never) when the y value is an integer. Yours does not have this problem.

It would be ideal to have a Matlab function not only for plotting this matrix, but for extracting the points in these contours (would use the same code as above.) as folks often want to process these contours before plotting etc.

[Johannes Korsawe]

Where to find cplot? it is not longer on the FEx ?

Answer 2

[Liviu Ivanescu]

Here is a way to plot contourc data containing several contours of the same value.

cnt = contourc(matrix,[-2 -2]);
szc = size(cnt);
idz = 1;
while idz<szc(2)
    izi = cnt(2,idz);
    cnt(2,idz) = nan;
    idz = idz+izi+1;
end
plot(cnt(1,:),cnt(2,:))

[Nikola Stan]

your example was useful to me. I added some code that allows it to keep different isolines separate, which is what I needed:

cnt = contourc(matrix,[-2 -2]);
szc = size(cntr);
idz = 1;
contourNo = 1;
while idz<szc(2)
    izi = cntr(2,idz);
    cntr(2,idz) = nan;
    contourXY{contourNo} = cntr(:,idz+1:idz+izi);
    idz = idz+izi+1;
    contourNo = contourNo+1;
end
figure()
hold on
for k = 1:contourNo-1
    plot(contourXY{k}(1,:), contourXY{k}(2,:));
end

[Eduardo Vicente Wolf Trentini]

in the first line did you mean "cntr = contourc(matrix,[-2 -2]);"?

i think you forget the "r" in cntr

thanks

[Liviu Ivanescu]

you are correct