Generate (X,Y) grid coordinates from within contour boundary

Hi Everyone

I have the (X,Y,Z) coodinates for a contourf plot.

I want to extract sample (X2,Y2) coordinates for a given size within the contour area under the input Z value threshold.

E.g. I only want Z values for Z_threshold = 10. If I input grid_size = 5, I want to get (X2,Y2) points spaced at 5m x 5m within the threshold contourf(X,Y,Z) area.

I can create the right size grid by using linspace, but I don't know how to restrict these coordinates to be only from within the desired contour area though?

load('contour_data.mat','X','Y','Z');

Z_threshold = 10;
grid_size = 20;

Z(Z > Z_threshold )= NaN;
contourf(X,Y,Z);
hold on

x_grid = linspace(min(X(:)), max(X(:)), range(X(:))/grid_size);
y_grid = linspace(min(Y(:)), max(Y(:)), range(Y(:))/grid_size);
[X2,Y2] = meshgrid(x_grid,y_grid);

plot(X2,Y2,'.r');
hold off

I only want (X2,Y2) from within the green circled zone below. Note that (X2,Y2) don't correspond to (X,Y) points.

Thanks for any advice :)

Answer 1

[KSSV]

1. Use boundary to get the boundary of your required region.
2. Use inpolygon with the generated grid and boundary points to get the points lying inside the area of interest.
3. Useing the indices generated from inpolygon, you can get your required points.

Refer this link: https://in.mathworks.com/matlabcentral/answers/499017-interpolation-with-non-zero-data-only-when-using-griddata for the same question.

[mackhina]

Loads slow for my 8 million point plot, but the results are just what I needed. Thanks for the advise!

I found this guide very helpful to do what I wanted for anyone else trying to do the same thing:

https://au.mathworks.com/matlabcentral/answers/443090-find-boundary-of-meshgrid