MATLAB: Meshgrid provides wrong dimensions

MATLABmesh

Hello,

I am using the meshgrid function to try and make a grid for my surface plots. My x,y, and z arrays are length 102, 42, and 102 respectively. But when I run [X,Y,Z] = meshgrid(x,y,z) the output has the wrong dimensions. X, Y and Z are 42 x 102 x 102 not 102 x 42 x 102. Can someone help me understand what is going wrong with this?

I tried playing around with the order and if I have [X,Y,Z] = meshgrid(y,x,z) it gives the correct output size of 102 x 42 x 102. For now I can run the code using this alternative order but I would like to know why it is making the wrong size.

Thanks

Best Answer

Why do you assume that the output is "wrong"? It is exactly how defined in the documentation:

 [X,Y,Z] = meshgrid(x,y,z) returns 3-D grid coordinates defined by the
 vectors x, y, and z. The grid represented by X, Y, and Z has size
 length(y)-by-length(x)-by-length(z).

See: doc meshgrid.

Well, length(y)-by-length(x)-by-length(z) sounds strange, I agree. But the function works as advertised. Note: Never trust your expectations, what a function returns, but trust only the documentation. Intuition is fine, but programming languages have been designed by human. ;-)

A similar problem is gradient:

[FX,FY] = gradient(F)

Now FX is along the "horizontal direction", FY the "vertical direction". Sounds okay. But in

[FX,FY,FZ] = gradient(F)

the 1st output concerns the 2nd dimension, the 2nd output the 1st one and the 3rd output the 3rd one. Brrr.

Related Solutions

MATLAB: Slice function starting from 1 and leaving empty space

Plotting in MATLAB always plots against the index of the element unless otherwise specified. In this particular case, the volume slice is being plotted starting at (1,1,1) because the first index is (1,1,1). This is similar to the "plot" function which plots plot([4,6,3]); as the points (1,4), (2,6), (3,3).

To specify the indices of the volume elements, use the meshgrid function.

https://www.mathworks.com/help/matlab/ref/meshgrid.html

Note the definition for the size of the grid:

[X,Y,Z] = meshgrid(x,y,z) returns 3-D grid coordinates defined by the vectors x, y, and z. The grid represented by X, Y, and Z has size length(y)-by-length(x)-by-length(z).

For the slice function, use the following code to start the volume at the origin:

>> s = size(V);
>> [X,Y,Z] = meshgrid(0:s(2)-1, 0:s(1)-1, 0:s(3)-1);
>> slice(X,Y,Z,V,10,10,10);

MATLAB: Cell array of coordinates from two vectors

No loops required, just use ndgrid and indexing:

>> X = randi(999,119,1); % must be column vector

>> Y = randi(999,102,1); % must be column vector
>> [idx,idy] = ndgrid(1:numel(X),1:numel(Y));
>> M = [X(idx(:)),Y(idy(:))];
>> size(M)
ans =
   12138       2

Here is a simpler example:

>> X = (1:5).';
>> Y = (6:9).';
>> [idx,idy] = ndgrid(1:numel(X),1:numel(Y));
>> M = [X(idx(:)),Y(idy(:))];
>> M
M =
   1   6
   2   6
   3   6
   4   6
   5   6
   1   7
   2   7
   3   7
   4   7
   5   7
   1   8
   2   8
   3   8
   4   8
   5   8
   1   9
   2   9
   3   9
   4   9
   5   9

Best Answer

Related Solutions

MATLAB: Slice function starting from 1 and leaving empty space

MATLAB: Cell array of coordinates from two vectors

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