Interp3 Producing Unexpected Output?

Hello,

When using the default trilinear interpolation of interp3, I'm getting an unexpected output:

>> Interpval = interp3(X,X,X,V,X(10),X(5),X(1))

Interpval =

     2.373878950731685e+04

>> V(10,5,1)

ans =

     2.423129551369272e+04

where X is an n x 1 vector, and V is an n x n x n matrix.

I expected the two results to match as it should notice that the interpolation point is a grid point.

Why do the results not match?

Hopefully I'm just tired and this is a trivial question.

Thanks for your time and help.

Answer 1

[Walter Roberson]

>> X = sort(randn(1,25));
>> V = rand(25,25,25);
>> Interpval = interp3(X,X,X,V,X(10),X(5),X(1))
Interpval =
         0.505428142457703
>> V(10,5,1)
ans =
         0.844855674576263
>> V(5,10,1)
ans =
         0.505428142457703

In other words, confusion over the fact that interp3's first coordinate is X, second coordinate is Y, whereas indexing in MATLAB has first coordinate corresponding to Y and second coordinate corresponding to X.

[John Chilleri]

This was the case - I wasn't thinking about grids, I was thinking about 3D space. Row and column formatting switch the X and Y...

Thank you!