Matlab中球面函数的等高线图中的输入误差

Question

I am trying to plot a sphere function using contour plot, but I keep getting the following error.

??? Error using ==> Z must be size 2x2 or greater.

Error in is at this line of code:

contour(X1, X2, Z);

Here is the sphere function:

function ph = sphere(x)
         ph = sum(x.*x, 2);
end

Here is the code to plot the sphere using contour plot:

min = -25;
max = 25;
h = 25;
c= linspace(min, max, h); % Create the mesh
[X1, X2] = meshgrid(c, c); % Create the grid

Z = sphere(X1,X2);

figure;
icontour(X1, X2, Z);

I expect Z to return at least a 2x2 matrix. Z is supposed to be the height of the sphere and X1 is the x-axis and X2 is the y-axis.

How can I eliminate the error?

Answer 1

The problem is sum in your sphere function. While X1 and X2 are 25x25 matrices, ph is a 25x1 vector. Try this:

function ph = sphere(x)

ph = x.*x;  

In this case, your value ph will be of the same dimenion as your grid variables.

Edit

If you use your updated approach:

Z = arrayfun(@sphere, X1);

You will get constant lines because you just pass X1 to your spheres function. Which, in that case is the same as:

ph = sum(x.^2,2);  

To get a sphere, you want to pass both arguments, X1 and X2 :

function ph = sphere(x1,x2)
     ph = sum(x1.*x2, 2);
end  

And call:

Z = arrayfun(@sphere, X1, X2);

If you call your modified function like that, you'll get this result:

Edit 2

ATTENTION: If you are using sphere somewhere else in your script, you have to update each call to it to include two input arguments.

Edit 3

Depending on how carefully you implement this code, I would advise against using sphere as a name for a function, because of Matlab's own function with that name .