Note:

PI = nearestNeighbor(DT,QX)
PI = nearestNeighbor(DT,QX,QY)
PI
= nearestNeighbor(DT,QX,QY,QZ)
[PI,D] = nearestNeighbor(DT,QX,...)
PI = nearestNeighbor(DT,QX)
returns the
index of the nearest point in DT.X
for each query
point location in QX
.
PI = nearestNeighbor(DT,QX,QY)
and PI
= nearestNeighbor(DT,QX,QY,QZ)
allow the query points to
be specified in column vector format when working in 2D and 3D.
[PI,D] = nearestNeighbor(DT,QX,...)
returns
the index of the nearest point in DT.X
for each
query point location in QX
. The corresponding Euclidean
distances between the query points and their nearest neighbors are
returned in D
.
Note:

DT  Delaunay triangulation. 
QX  The matrix QX is of size mpts byndim , mpts being
the number of query points and ndim the dimension
of the space where the points reside. 
PI  PI is a column vector of point indices
that index into the points DT.X . The length of PI is
equal to the number of query points mpts 
D  D is a column vector of length mpts . 
Create a Delaunay triangulation:
x = rand(10,1); y = rand(10,1); dt = DelaunayTri(x,y);
qrypts = [0.25 0.25; 0.5 0.5];
pid = nearestNeighbor(dt, qrypts)