how to plot contour for arbitrary shape (not rectangular) in matlab?

Hello guys, I have a question for you

In relation to drawing the contour of a non-rectangular shape, what method should be adopted?

For example, how can you draw the contour of the following figure?

If our data is the coordinates of each point (x,y) and the value of Z at each point.

Thank you all.

Answer 1

[Walter Roberson]

Use a rectangular array of Z, but set it to NaN outside of the area of interest.

[sajad sajad]

Hi Mr. Roberson, Oh right, thanks

But my mesh are not regular, so that the matrix can't be used!!

Answer 2

[William Rose]

@sajad sajad,

You can make a MxN grid of points that cover the X-Y extent of your shape, and define the Z values at those points,. Then you can use contour or surf.

Example:

M=10; N=20;
[X,Y]=meshgrid(0:N,0:M);
Z=exp(-((X-10).^2+(Y-5).^2)/8);
surf(X,Y,Z,EdgeColor='none');
xlabel('X'); ylabel('Y')
axis equal; hold on; view(0,90)
contour3(X,Y,Z,'-k',LineWidth=1)    

Now adjust the grid x and y coordinates (only adjust the Y values in this example):

for j=1:5,Y(:,j)=Y(:,j)/2; end
for j=6:10,Y(:,j)=Y(:,j)*j/10; end
for j=11:15,Y(:,j)=Y(:,j)*(21-j)/10; end
for j=16:21,Y(:,j)=Y(:,j)/2; end

Make another figure with the adjusted grid coordinates:

figure
surf(X,Y,Z,EdgeColor='none');
xlabel('X'); ylabel('Y')
axis equal; hold on; view(0,90)
contour3(X,Y,Z,'-k',LineWidth=1)

Good luck!

[William Rose]

@sajad sajad,

You could also reverse the order in the example above: first adjust the X,Y coordinates of the grid, then deifne the Z values on the adjusted X,Y grid.

M=10; N=20;
[X,Y]=meshgrid(0:N,0:M);
for j=1:5,Y(:,j)=Y(:,j)/2; end
for j=6:10,Y(:,j)=Y(:,j)*j/10; end
for j=11:15,Y(:,j)=Y(:,j)*(21-j)/10; end
for j=16:21,Y(:,j)=Y(:,j)/2; end
Z=exp(-((X-10).^2+(Y-4).^2)/20);  % define Z values on adjusted grid

Make figure:

figure
surf(X,Y,Z,EdgeColor='none');
xlabel('X'); ylabel('Y')
axis equal; hold on; view(0,90)
contour3(X,Y,Z,'-k',LineWidth=1)

OK

[sajad sajad]

Hi Mr. Rose, Thanks a lot.