How can I get the zero crossing contour of a matrix

Hi,

I have a matrix of 201x201 size and it has a zero crossing. I am using contour3 to get the contour plot. but i actually need only the contour corresponding to zero values. the image it attached here.

any help would be highly appreciated.

Thank you.

Answer 1

[John D'Errico]

help contourc

 CONTOURC Contour computation.
    CONTOURC computes the contour matrix C for use by CONTOUR,
        CONTOUR3, or CONTOURF to draw the actual contour plot.
 
    C = CONTOURC(Z) computes the contour matrix for a contour plot
       of matrix Z treating the values in Z as heights above a plane.  A
       contour plot is the set of level curves of Z for some values V.  The
       values V are chosen automatically.
 
    C = CONTOURC(X, Y, Z), where X and Y are vectors, specifies the X-
       and Y-axes limits for Z.
 
    CONTOURC(Z, N) and CONTOURC(X, Y, Z, N) compute N contour lines,
       overriding the automatic value.
 
    CONTOURC(Z, V) and CONTOURC(X, Y, Z, V) compute LENGTH(V) contour lines
       at the values specified in vector V.  Use CONTOURC(Z, [v, v]) or
       CONTOURC(X, Y, Z, [v, v]) to compute a single contour at the level v.
 
    The contour matrix C is a two row matrix of contour lines. Each
    contiguous drawing segment contains the value of the contour,
    the number of (x, y) drawing pairs, and the pairs themselves.
    The segments are appended end-to-end as
 
        C = [level1 x1 x2 x3 ... level2 x2 x2 x3 ...;
             pairs1 y1 y2 y3 ... pairs2 y2 y2 y3 ...]
 
    See also CONTOUR, CONTOURF, CONTOUR3.

    Documentation for contourc
       doc contourc

For example:

v = linspace(-5,5,100);
[x,y] = meshgrid(v,v);
z = x.^2 - y.^2 + 1; % The contours will be hyperbolic
contour(x,y,z,[0,0])

C = contourc(v,v,z,[0 0])

C = 2×354

0 -4.8990 -4.7980 -4.7958 -4.6970 -4.6926 -4.5960 -4.5892 -4.4949 -4.4858 -4.3939 -4.3822 -4.2929 -4.2785 -4.1919 -4.1747 -4.0909 -4.0707 -3.9899 -3.9666 -3.8889 -3.8623 -3.7879 -3.7578 -3.6869 -3.6532 -3.5859 -3.5483 -3.4848 -3.4432 176.0000 5.0000 4.9011 4.8990 4.8022 4.7980 4.7034 4.6970 4.6048 4.5960 4.5062 4.4949 4.4077 4.3939 4.3094 4.2929 4.2112 4.1919 4.1131 4.0909 4.0152 3.9899 3.9174 3.8889 3.8198 3.7879 3.7224 3.6869 3.6252 3.5859

There will be two contour lines found, each of which have 176 points in the piecewise curves generated.

So if you want the points along that contour itself, contourc does it. If you want to draw the contour, then I contour will do it easily enough.

[harsha]

Thank you. It worked.