Confusion with interp3 and interpn

Hi,

According to my teacher, "for a matrix indexed as [M, N, P], the axis variables must be given in the order N, M, P" in relation to the swaping of variables at the interp3 line:

[X, Y, Z] = size(data);
ts= [20 30 20]
[x, y, z] = ndgrid((1:X)/X, (1:Y)/Y, (1:Z)/Z);       
[x2, y2, z2] = ndgrid((1:ts(1))/(ts(1)), (1:ts(2))/(ts(2)), (1:ts(3))/(ts(3)));
test1= interp3(y, x, z, data, y2, x2, z2, 'linear');  

However, I couldn't find this info on the function's page, nor it makes any sense to me (even though the computation seems correct!).

Any help welcome, thank you,

TS

[darova]

I don't understand the question. What are you asking?

[Tahariet Sharon]

Why the order of variables should be "y,x,z".

[darova]

Did you try 'x,y,z' order? I think the result should be the same

[Tahariet Sharon]

Yes, of course I did, but it outputs an error:

Error using interp3 (line 150)
Input grid is not a valid MESHGRID.

[darova]

Maybe it's the answer on your question

Answer 1

[Steven Lord]

I think reading through this documentation page may be of use to you, especially the examples in the Grid Representation section.

[Tahariet Sharon]

Still, Why the order of variables should be "y,x,z"?

Answer 2

[darova]

here is an example

[x,y] = meshgrid(1:5);
z = x.^2 + y.^2/2;
[x1,y1] = meshgrid(1:0.5:5);
z1 = interp2(x,y,z,x1,y1);      % standart interpolation
z2 = interp2(y',x',z,y1',x1');  % interpolation y x

surf(x,y,z,'facecolor','none')
hold on
plot3(x1(:),y1(:),z1(:),'.r')
plot3(x1(:),y1(:),z2(:),'ob')
hold off
legend('original surface',...
    'standard interpolation',...
    'y x interpolation','location','north')
axis vis3d

[Tahariet Sharon]

Thanks, however, you are using interp2, and not interp3. Do the same with interp3 and you will get an error, I would expect. See here: https://groups.google.com/forum/#!topic/comp.soft-sys.matlab/xlinA3xsxT8 "Does interp3 switch X and Y".

[Tahariet Sharon]

Also: https://stackoverflow.com/questions/56187508/any-differences-in-3d-interpolation-between-matlab-and-numpy-scipy