Contour not taking into account edge of domain

Hello eveyone,

I have an issue using contour (contourf or contourc as well) with convhull in order to get the convex polygon that includes the domain where Z=1.

When i try to retrieve the contour matrix, only the coordinates contained are those of strictly inside the domain, and the edges of the domain do not appear, then the polygon I get is way smaller than I expect.

I hope this is rather clear. Thank you in advance!

Here is my code

X = 1:0.2:2;
Y = 2:0.2:3;
[X_mesh,Y_mesh]=meshgrid(X,Y);
Z = [1 1 1 0 0 0;
     1 1 1 1 0 0;
     1 1 1 0 0 0;
     1 1 1 1 1 1;
     1 1 1 1 1 1;
     1 1 1 1 1 1];

figure,
[M,h]=contourf(X_mesh,Y_mesh,Z,[1 1]);

M

M = 2×9

1.0000 1.4000 1.6000 1.6000 1.6000 1.4000 1.6000 1.8000 2.0000 8.0000 2.0000 2.2000 2.2000 2.2000 2.4000 2.6000 2.6000 2.6000

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coords_x=M(1,2:end);
coords_y=M(2,2:end);
K=convhull(coords_x,coords_y);

figure,hold on,
contourf(X_mesh,Y_mesh,Z,[1 1]);
patch(coords_x(K),coords_y(K),'green', 'FaceAlpha', 0.5);

Answer 1

[lepolonais03]

I found a workaround. I added zeros all around the contour I wanted in order to virtually 'erase' the edges.

Z = [1 1 1 0 0 0;

1 1 1 1 0 0;

1 1 1 0 0 0;

1 1 1 1 1 1;

1 1 1 1 1 1;

1 1 1 1 1 1];

Z_extended = [zeros(1,size(Z,2)+2);

[zeros(size(Z,1),1),Z,zeros(size(Z,1),1)];

zeros(1,size(Z,2)+2)]

[Paul]

Doesn't this approach yiled the same polygon as in the original code?

Z = [1 1 1 0 0 0;
     1 1 1 1 0 0;
     1 1 1 0 0 0;
     1 1 1 1 1 1;
     1 1 1 1 1 1;
     1 1 1 1 1 1];
Z_extended = [zeros(1,size(Z,2)+2);
             [zeros(size(Z,1),1),Z,zeros(size(Z,1),1)];
             zeros(1,size(Z,2)+2)];
X = [0.8 , 1:0.2:2 , 2.2];
Y = [1.8 , 2:0.2:3 , 3.2];
[X_mesh,Y_mesh]=meshgrid(X,Y);
figure,
[M,h]=contourf(X_mesh,Y_mesh,Z_extended,[1 1]);

I confess that I don't understand why M has so many points, and some vertices are repeated.

M.'

ans = 28×2

1.0000 27.0000 1.0000 2.0000 1.0000 2.0000 1.2000 2.0000 1.4000 2.0000 1.4000 2.0000 1.6000 2.2000 1.6000 2.2000 1.6000 2.2000 1.4000 2.4000

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