Whiteout regions on Contour Plot

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laoliu102 2017-12-14

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评论： Walter Roberson 2023-3-31

I don't want to display certain regions of the contour plot. How can I whiteout these regions on the contour plot? The interpolated data fills the entire space while the real data has blank regions that physically don't contain anything. These blank regions are the regions that I don't want to display on the contour plot. I have attached the two plots.

I have the scatter plot data as x,y,z arrays and the interpolated data for the contour plot as x,y,z matrices.

Any help/suggestion is welcomed and appreciated. Thank you.

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Answer 1

Walter Roberson 2017-12-14

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My recommendation would be to set the interpolated data to nan in those regions before doing the contour plot.

However, since you have scattered data, you have the challenge of determining which points are "near enough" to explicit scattered data that they should validly be interpolated, and which points are instead to be interpreted as being too far from the scattered data and so should be interpreted as being outside the map.

You could consider a pdist2() approach but it appears you have a fair bit of data, so that would not be computationally feasible. You might be able to take advantage of kd-tree approaches; https://www.mathworks.com/help/stats/classification-using-nearest-neighbors.html

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Walter Roberson 2017-12-14

Are you doing something similar to

 qxvec = linspace(min(x), max(x), 76);
 qyvec = linspace(min(y), max(y), 96);
 [qx, qy] = ndgrid(qxvec, qyvec);
 qz = griddedInterpolant(x, y, z, qx, qy);

In such a case then by default you would be using bilinear interpolation, so the value at each gridpoint qx(I,J), qy(I,J) would be determined by the four points in the x, y vectors nearest to the query points, and no other data points would be used.

Or are you doing something like a density plot, or something like a maximum intensity projection plot? Or something like creating a grid and determining where each point fits on it, and averaging all of the points in any one grid location?

If you did happen to be using an approach involving gridding the points and then considering the properties of the subset of points within each grid location, then that would make it relatively easy to determine which grid locations were empty and to assign nan to them. The trade off in such a case is that there could well be an adjacent point "just outside" the grid location whose influence would not be considered at all. But that might be acceptable for your purposes.

Walter Roberson 2017-12-14

cutoff_distance = 6.28;   %set as appropriate
[xi,yi] = meshgrid(300:20:2200, 300:20:1800);
zi = griddata(xn, yn, zn, xi, yi);
DT = delaunayTriangulation(xn, yn);
[vi, d] = nearestNeighbor(DT, xi(:), yi(:));
mask = d > cutoff_distance;
zi(mask) = nan;

laoliu102 2017-12-17

Thank you so much! It works now!

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Answer 2

Muhammad Usman 2019-11-6

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I have got the same problem but mine was a little bit simple. I use the built-in MATLAB function "scatterInterpolant" to plot a contour. The plot is shown below.

It's s stress contour and the ellipse is an empty space and there are no nodes inside the ellipse. I want the ellipse to be whiteout. Somehow I managed to set the values inside the ellipse to be zero (because I can't delete them) and I got the following result

I want the ellipse to be whiteout. Any sort of help is highly appreciated. Thanks in advance.

P.S. I use the fill command to fill the ellipse with white color but that's not a proper way to do it.

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Jiandong 2023-3-31

Maybe you can use the inpolygon function

[in,on] = inpolygon(x_list,y_list,ellipse_x,ellipse_y);
    for i = 1:length(x_list)
        if(in(i))
            ellipse_x(i)=NaN;
            ellipse_y(i)=NaN;
        end
    end

Walter Roberson 2023-3-31

@Jiandong

That suggests to me that you could use the shorter

[in,on] = inpolygon(x_list,y_list,ellipse_x,ellipse_y);
ellipse_x(in) = NaN;
ellipse_y(in) = NaN;

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Whiteout regions on Contour Plot

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