(octave.info)Identifying Points in Triangulation

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30.1.2 Identifying Points in Triangulation
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It is often necessary to identify whether a particular point in the
N-dimensional space is within the Delaunay tessellation of a set of
points in this N-dimensional space, and if so which N-simplex contains
the point and which point in the tessellation is closest to the desired
point.  The functions ‘tsearch’ and ‘dsearch’ perform this function in a
triangulation, and ‘tsearchn’ and ‘dsearchn’ in an N-dimensional
tessellation.

   To identify whether a particular point represented by a vector P
falls within one of the simplices of an N-simplex, we can write the
Cartesian coordinates of the point in a parametric form with respect to
the N-simplex.  This parametric form is called the Barycentric
Coordinates of the point.  If the points defining the N-simplex are
given by N + 1 vectors ‘T(I,:)’, then the Barycentric coordinates
defining the point P are given by

     P = BETA * T

where BETA contains N + 1 values that together as a vector represent the
Barycentric coordinates of the point P.  To ensure a unique solution for
the values of BETA an additional criteria of

     sum (BETA) == 1

is imposed, and we can therefore write the above as

     P - T(end, :) = BETA(1:end-1) * (T(1:end-1, :)
                     - ones (N, 1) * T(end, :)

Solving for BETA we can then write

     BETA(1:end-1) = (P - T(end, :)) /
                     (T(1:end-1, :) - ones (N, 1) * T(end, :))
     BETA(end) = sum (BETA(1:end-1))

which gives the formula for the conversion of the Cartesian coordinates
of the point P to the Barycentric coordinates BETA.  An important
property of the Barycentric coordinates is that for all points in the
N-simplex

     0 <= BETA(I) <= 1

Therefore, the test in ‘tsearch’ and ‘tsearchn’ essentially only needs
to express each point in terms of the Barycentric coordinates of each of
the simplices of the N-simplex and test the values of BETA.  This is
exactly the implementation used in ‘tsearchn’.  ‘tsearch’ is optimized
for 2-dimensions and the Barycentric coordinates are not explicitly
formed.

 -- : IDX = tsearch (X, Y, T, XI, YI)
     Search for the enclosing Delaunay convex hull.

     For ‘T = delaunay (X, Y)’, finds the index in T containing the
     points ‘(XI, YI)’.  For points outside the convex hull, IDX is NaN.

     See also: Note: delaunay, *note delaunayn:
     XREFdelaunayn.

 -- : IDX = tsearchn (X, T, XI)
 -- : [IDX, P] = tsearchn (X, T, XI)
     Search for the enclosing Delaunay convex hull.

     For ‘T = delaunayn (X)’, finds the index in T containing the points
     XI.  For points outside the convex hull, IDX is NaN.

     If requested ‘tsearchn’ also returns the Barycentric coordinates P
     of the enclosing triangles.

     See also: Note: delaunay, *note delaunayn:
     XREFdelaunayn.

   An example of the use of ‘tsearch’ can be seen with the simple
triangulation

     X = [-1; -1; 1; 1];
     Y = [-1; 1; -1; 1];
     TRI = [1, 2, 3; 2, 3, 4];

consisting of two triangles defined by TRI.  We can then identify which
triangle a point falls in like

     tsearch (X, Y, TRI, -0.5, -0.5)
     ⇒ 1
     tsearch (X, Y, TRI, 0.5, 0.5)
     ⇒ 2

and we can confirm that a point doesn’t lie within one of the triangles
like

     tsearch (X, Y, TRI, 2, 2)
     ⇒ NaN

   The ‘dsearch’ and ‘dsearchn’ find the closest point in a tessellation
to the desired point.  The desired point does not necessarily have to be
in the tessellation, and even if it the returned point of the
tessellation does not have to be one of the vertexes of the N-simplex
within which the desired point is found.

 -- : IDX = dsearch (X, Y, TRI, XI, YI)
 -- : IDX = dsearch (X, Y, TRI, XI, YI, S)
     Return the index IDX of the closest point in ‘X, Y’ to the elements
     ‘[XI(:), YI(:)]’.

     The variable S is accepted for compatibility but is ignored.

     See also: Note: dsearchn, Note: tsearch.

 -- : IDX = dsearchn (X, TRI, XI)
 -- : IDX = dsearchn (X, TRI, XI, OUTVAL)
 -- : IDX = dsearchn (X, XI)
 -- : [IDX, D] = dsearchn (...)
     Return the index IDX of the closest point in X to the elements XI.

     If OUTVAL is supplied, then the values of XI that are not contained
     within one of the simplices TRI are set to OUTVAL.  Generally, TRI
     is returned from ‘delaunayn (X)’.

     See also: Note: dsearch, Note: tsearch.

   An example of the use of ‘dsearch’, using the above values of X, Y
and TRI is

     dsearch (X, Y, TRI, -2, -2)
     ⇒ 1

   If you wish the points that are outside the tessellation to be
flagged, then ‘dsearchn’ can be used as

     dsearchn ([X, Y], TRI, [-2, -2], NaN)
     ⇒ NaN
     dsearchn ([X, Y], TRI, [-0.5, -0.5], NaN)
     ⇒ 1

where the point outside the tessellation are then flagged with ‘NaN’.