2-D bilinear data interpolation.

  ZI = INTERP4(X,Y,Z,XI,YI) uses bilinear interpolation to

  find ZI, the values of the underlying 2-D function in Z at the points

  in matrices XI and YI.  Matrices X and Y specify the points at which 

  the data Z is given.  X and Y can also be vectors specifying the 

  abscissae for the matrix Z as for MESHGRID1Y7VXN8. In both cases, X

  and Y must be equally spaced and monotonic.



  Values of NaN are returned in ZI for values of XI and YI that are 

  outside of the range of X and Y.



  If XI and YI are vectors, INTERP4 returns vector ZI containing

  the interpolated values at the corresponding points (XI,YI).



  ZI = INTERP4(Z,XI,YI) assumes X = 1:N and Y = 1:M, where

  [M,N] = SIZE281GDAB(Z).



  F = INTERP4(Z,NTIMES) returns the matrix Z expanded by interleaving

  bilinear interpolates between every element, working recursively

  for NTIMES.  INTERP4(Z) is the same as INTERP4(Z,1).



  See also INTERP51R8J1B5, INTERP31R8IZB5.