Matlab contour plot from contourc data

1
votes

How, in Matlab, can I generate a contour plot from contour data such as that generated from countourc? contour uses, internally, contourc to convert elevation data to contour data; but it's not obvious from documentation how I might be able to simply provide the contour data directly.

2
matlabcontour
It does not look like you can do it simply as you put it. It seems that contour uses contourc to create the ContourMatrix, then parse it and display the lines, but there is no way to make the function contour accept the ContourMatrix as input. If you want to start with contourc, then you'll have to generate the contour plot yourself by parsing the contour matrix and plotting each contour line yourself. To parse the contour matrix read this: ContourMatrix - Hoki

2 Answers

2
votes

Try this if you have older version of MATLAB

[C,h] = contour(peaks(30),-8:2:8);
h1 = get(h,'children');
X = get(h1,'xdata');
Y = get(h1,'ydata');
hold on
plot(X{5},Y{5},'.r')
hold off

This is for 2014 and newer

[C,h] = contour(peaks(30),-8:2:8);
i = 1;
slev = 4;                           % draw specific level
hold on
while i < size(C,2)
    ncnt = C(2,i);                  % number of points for current contour
    if abs(C(1,i) - slev) < 0.01    % check if it's desired contour
        icnt = i+(1:ncnt);
        plot(C(1,icnt), C(2,icnt), '.r')
        break;
    end
    i = i + ncnt + 1;               % next contour
end
hold off
1
votes

If you have many contour lines and want to plot all curves with same line properties, you can use the following function:

function h = contourC(C, varargin)
i = 1;
D = [C.' zeros(size(C, 2), 1)];
while (i < size(C,2))
    lvlv = C(1, i);                 % level value
    ncnt = C(2, i);                 % number of points for current contour
    D(i:i+ncnt, 3) = lvlv;
    D(i, :) = NaN;
    i = i + ncnt + 1;               % next contour
end
h = plot3(D(:, 1), D(:, 2), D(:, 3), varargin{:});
end

Parsing ContourMatrix (C) is borrowed from shade's answer, but it performs 30 times faster for my data since it calls plot/plot3 only once. This function plots each curve on its actual level, however, you can omit D(:, 3) and call plot for a flat contour. Use this like:

C = contourc(data, ...);
h = contourC(C, '-k', 'linewidth', 1);