MATLAB: Extracting data from Contour plots

dataobjsextracting data from figures

I am trying to extract data from a contour plot. I managed to get all the information about the dataObjs using

 open('colormapinhomo.fig');
 h = gcf;
 axesObjs = get(h, 'Children');
 dataObjs = get(axesObjs, 'Children')
 dataObjs{2}
Contour with properties:
    LineColor: [0 0 0]
    LineStyle: 'none'
    LineWidth: 0.5000
         Fill: 'on'
    LevelList: [1×62 double]
        XData: [1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40]
        YData: [1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20]
        ZData: [20×40 double]

but cant get the data itself. Any help ? And is this the only way to extract data, or can I somehow output a .dat file of the figure with the data whether it was a contour or a histogram or even a simple plot line ?

Best Answer

If you want the contour (x,y) values, you have to ask for them:

figure(1)
[C,h] = contour( ... );

The (2xN) ‘C’ array will have the (x,y) values of the contour lines. The first column in each has the contour value as the first row and the number of (x,y) values defining the contour as the second row. (It requires some coding to extract all of them effectively.) Use the LevelList property to find the values of the contours (these will match the first row and first column of each contour) so you can then use find to locate the beginning of each (x,y) contour matrix.

See the documentation for contour for details. There is a link to ‘Contour Properties’ that lists all of them.

Related Solutions

MATLAB: How to extract x,y,z coordinates from a contour

This works in R2017a, so it should also work for R2016a.

Example —

f = @(x,y) x.^2 - y.^2;                                     % Your Function
x = linspace(-10, 10);                                      % X-Range
y = linspace(-10, 10);                                      % Y-Range
[X,Y] = meshgrid(x,y);                                      % Create Matrix Arguments
figure(1)
[C,h] = contour(X, Y, f(X,Y));
grid
Lvls = h.LevelList;
for k1 = 1:length(Lvls)
    idxc{k1} = find(C(1,:) == Lvls(k1));
    Llen{k1} = C(2,idxc{k1});
    conturc{k1,1} = C(:,idxc{k1}(1)+1 : idxc{k1}(1)+1+Llen{k1}(1)-1);
    conturc{k1,2} = C(:,idxc{k1}(2)+1 : idxc{k1}(2)+1+Llen{k1}(2)-1);
end
figure(2)
plot(conturc{3,1}(1,:), conturc{3,1}(2,:))
grid

The contour data now are a (2xN) vector created by horizontally concatenating the (x,y) coordinates of each contour line. The segments are created each as:

C = [Level x-coordinates; Length y-coordinates]

so the first column of each segment are the ‘Level’ and the ‘Length’ of the segment. The second output are the properties of the plot.

My code first plots the contour function, returning both outputs. It then uses the ‘LevelList’ property to return the plotted levels, and then uses these in the loop to find the start indices of the segments in the ‘C’ matrix in the ‘idxc’ cell array, and the number of elements in each contour segment in the ‘Llen’ cell array. It then uses these to extract the x (first row) and y (second row) coordinates for each contour segment in the ‘conturc’ cell array. The z-coordinates are the levels. They do not appear in the cell arrays, but correspond to the values in the ‘Lvls’ vector.

The plot in figure(2) is not necessary for the code. It shows how to get the data from the cell arrays (in this instance the third cell in the first group), and that the extracted values are correct.

This turned out to be an interesting adventure!

MATLAB: Is there a way to extract the width/diameter of the contour lines

Get a binary image of your data where it's more than 1/e^2

binaryImage = data > 1 / exp(2);

Now get the boundaries:

boundaries = bwboundaries(binaryImage);
x = boundaries{:,2};
y = boundaries{:,1};

then use the FAQ to fit an ellipse: http://matlab.wikia.com/wiki/FAQ#How_can_I_fit_an_ellipse_or_other_shape_to_a_set_of_XY_data.3F to those x and y values.

Best Answer

Related Solutions

MATLAB: How to extract x,y,z coordinates from a contour

MATLAB: Is there a way to extract the width/diameter of the contour lines

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