Why such a fuss with ndgrid and meshgrid issues?

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Kristoffer Walker
编辑： Paul 2024-7-6
Folks,
In the world of artificial intelligence and more intelligent programming, I would like to see more intelligence put into place with how function like griddata, imagesc, pcolor, and surf interpret their inputs. There is an unfortunate degree of confusion that has persisted since the early 2000's at least with how to interpret arrays in terms of Cartesian axes. I am troubled today with trying to understand how griddata is working on input that I setup using ndgrid. I appear to have something wrong, and earth layers are not dipping in the direction that I expect. The documentation says griddata can work with either meshgrid or griddata input, which makes my head spin given my understanding of both meshgrid and ndgrid. What is really troubling is that the Mathworks documentation routinely contains only simple examples where the person that created the documentation made "quicky" non-realistic examples where they simply created input arrays that had exactly the same x and y dimensions, which does not help anyone in my situation who is dealing with realistic inputs that do not have the same dimension lengths. I request that future documentation efforts spend a little extra time making non-symmetric input x and y arrays as examples.
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Voss
Voss 2024-7-5
Ran in:
Don't use reshape for that; use permute:
% Using meshgrid approach for non-square slice example
x = -2:0.2:2; y = -5:0.1:5; z = -25:2:25;
[X,Y,Z] = meshgrid(x,y,z);
V = X.*exp(-X.^2-Y.^2-Z.^2);
xslice = [0.8]; yslice = []; zslice = [];
figure; slice(X,Y,Z,V,xslice,yslice,zslice)
% Using ndgrid approach for non-square slice example
x = -2:0.2:2; y = -5:0.1:5; z = -25:2:25;
[X,Y,Z] = ndgrid(x,y,z);
nX = length(x); nY = length(y); nZ = length(z);
xg = permute(X,[2,1,3]);
yg = permute(Y,[2,1,3]);
zg = permute(Z,[2,1,3]);
V = xg.*exp(-xg.^2-yg.^2-zg.^2);
xslice = [0.8]; yslice = []; zslice = [];
figure; slice(xg,yg,zg,V,xslice,yslice,zslice)
Kristoffer Walker
Kristoffer Walker 2024-7-5
Thank you, Voss. That fixed my issue with both splice and obliquesplice.

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Kristoffer Walker
Thank you everyone for your input. I will surely look back on this post in the future for the multiple, helpful contributions.

更多回答（4 个）

Paul
Paul 2024-7-5
编辑：Paul 2024-7-6
Hi Kristoffer,
The inputs to griddata that can be in either meshgrid or ndgrid format are the query points in Xq and Yq. I think that their specific format doesn't matter because griddata processes each pair ( Xq(i,j),Yq(i,j) ) where the output vq(i,j) = f(Xq(i,j) , Yq(i,j)) is the interpolation based on Delaunay triangulation of scattered sample points and sample values (in contrast to, for example, interp2 that requires the sample points and values to be in meshgrid format).
What would matter is what's done with Xq, Yq, and vq after the call to gridddata. For example, surf prefers (requires?) the input data to be in meshgrid format, so a call to griddata should use query points in meshgrid format if to be followed by a call to surf with the results.
If having a problem with griddata with query points in ndgrid format, you're likely to get more help by posting a simple code example to show what the issue is.
Edit: As shown by @Star Strider in this Answer, surf is agnostic to whether or not the the input is in ndgrid/meshgrid format.
Edit: Use of surf with meshgrid/ndgrid was also explained by @Voss in this thread that I had forgotten about.
I think my confusion with surf (et. al.?) might be the result of a long-ago behavior change that still hasn't solidified with me.
In surf (2019a), the doc stated "To create a matrix for arbitrary domains, use the meshgrid function." No ambiguity there.
In surf (2019b) the doc states: "You can use the meshgrid function to create X and Y matrices." ("can" not "must") which suggests something changed from 2019a to 2019b, though there is no mention of such in the 2019b release notes.
David Goodmanson
David Goodmanson 2024-7-5
编辑：David Goodmanson 2024-7-5
Hi Kristoffer,
I totally agree that Mathworks should use nonsquare examples in the documentation for meshgrid, ndgrid, surf etc. It just increases awareness. And comparing [x y] = meshgrid(a,b) and [x y] = ndhgrid(a,b),
***** the meshgrid x matrix and ndgrid x matrix are transposes of each other. Same for y.
I think you meant to say that griddata works with either meshgrid or ndgrid input, and that's true. Griddata is agnostic on the topic, assuming of course that either meshgrid or ndgrid are used consistently throughout.
Star Strider
Star Strider 2024-7-5
Ran in:
I’m not certain what the problem is.
I generally prefer scatteredInterpolant to griddata.
The choce of meshgrid or ndgrid does not seem to be an issue, at least in this example —
xv = linspace(-2, 2, 15);
yv = linspace(-1, 4, 19);
[X1,Y1] = meshgrid(xv, yv);
Z1 = exp(-X1.^2) .* exp(-(Y1-2.5).^2*1.5)*2.5;
figure
surf(X1, Y1, Z1)
colormap(turbo)
axis('equal')
title('Original')
F = scatteredInterpolant(X1(:), Y1(:), Z1(:));
xv2 = linspace(-2, 2, 40);
yv2 = linspace(-1, 4, 30);
[X2,Y2] = ndgrid(xv2,yv2);
Z2 = F(X2,Y2);
figure
surf(X2,Y2,Z2)
colormap(turbo)
axis('equal')
title('scatteredInterpolant — ndgrid')
[X2,Y2] = meshgrid(xv2,yv2);
Z2 = F(X2,Y2);
figure
surf(X2,Y2,Z2)
colormap(turbo)
axis('equal')
title('scatteredInterpolant — meshgrid')
Do either of these deal with the issue you’re describing?
.
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Kristoffer Walker
Kristoffer Walker 2024-7-5
Thanks Star. I will try to recreate the problem with a different data set. It is very hard because I cannot share any data.
Kristoffer Walker
Kristoffer Walker 2024-7-5
编辑：Kristoffer Walker 2024-7-5
One solution that I tried was using "slice". I already have an array V that took a long time to grid. I do not want to have to regrid is using a meshgrid format. So I thought I could use reshape to convert my arrays from ndgrid to meshgrid format, because that is what splice seems to require. Below shows success using meshgrid format, but an issue arises when using ndgrid format that I do not understand how to fix without using meshgrid format.
clear all; close all
% Using meshgrid approach for non-square slice example
x = [-2:0.2:2]; y = [-5:0.1:5]; z = [-25:2:25];
[X,Y,Z] = meshgrid(x,y,z);
V = X.*exp(-X.^2-Y.^2-Z.^2);
xslice = [0.8]; yslice = []; zslice = [];
figure; slice(X,Y,Z,V,xslice,yslice,zslice)
% Using ndgrid approach for non-square slice example
x = [-2:0.2:2]; y = [-5:0.1:5]; z = [-25:2:25];
[X,Y,Z] = ndgrid(x,y,z);
nX = length(x); nY = length(y); nZ = length(z);
xg = reshape(X,[nY,nX,nZ]);
yg = reshape(Y,[nY,nX,nZ]);
zg = reshape(Z,[nY,nX,nZ]);
V = xg.*exp(-xg.^2-yg.^2-zg.^2);
xslice = [0.8]; yslice = []; zslice = [];
figure; slice(xg,yg,zg,V,xslice,yslice,zslice)

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William Rose
William Rose 2024-7-5
@Kristoffer Walker, I definitely agree with you. I use inputs that are non-square when i use surf() and related routines. Thus I use vectors X, Y with different lengths in surf(X,Y,Z), etc. If the inputs are square, I may flip things around by mistake, and I might not know it, because there's no error message.
I realize @John D'Errico is right that this forum is not the Suggestion Box for matlab. Try here.

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