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function [F,mask] = m_interp4(z,s,t)
%INTERP4 2-D bilinear data interpolation.
% ZI = INTERP4(Z,XI,YI) assumes X = 1:N and Y = 1:M, where
% [M,N] = SIZE(Z).
%
% Copyright (c) 1984-93 by The MathWorks, Inc.
% Clay M. Thompson 4-26-91, revised 7-3-91, 3-22-93 by CMT.
%
% modified to
[nrows,ncols] = size(z);
if any(size(z)<[3 3]), error('Z must be at least 3-by-3.'); end
if size(s)~=size(t), error('XI and YI must be the same size.'); end
% Check for out of range values of s and set to 1
sout = find((s<1)|(s>ncols));
if length(sout)>0, s(sout) = ones(size(sout)); end
% Check for out of range values of t and set to 1
tout = find((t<1)|(t>nrows));
if length(tout)>0, t(tout) = ones(size(tout)); end
% Matrix element indexing
ndx = floor(t)+floor(s-1)*nrows;
% Compute intepolation parameters, check for boundary value.
d = find(s==ncols);
s(:) = (s - floor(s));
if length(d)>0, s(d) = s(d)+1; ndx(d) = ndx(d)-nrows; end
% Compute intepolation parameters, check for boundary value.
d = find(t==nrows);
t(:) = (t - floor(t));
if length(d)>0, t(d) = t(d)+1; ndx(d) = ndx(d)-1; end
d = [];
% Now interpolate, reuse u and v to save memory.
F = ( z(ndx).*(1-t) + z(ndx+1).*t ).*(1-s) + ...
( z(ndx+nrows).*(1-t) + z(ndx+(nrows+1)).*t ).*s;
mask = ones(size(z));
% Now set out of range values to zeros.
if length(sout)>0, F(sout) = zeros(size(sout));mask(sout)=zeros(size(sout));end
if length(tout)>0, F(tout) = zeros(size(tout));mask(tout)=zeros(size(tout));end
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