Identifying Points in Triangulation - GNU Octave
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#### 29.1.2 Identifying Points in Triangulation

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 = sum (beta(1:N+1) * t(1:N+1),:)

where there are N`+ 1` values beta`(`i`)` that together as a vector represent the Barycentric coordinates of the point p. To ensure a unique solution for the values of beta`(`i`)` an additional criteria of

         sum (beta(1:N+1)) == 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.

— Loadable Function: 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: delaunay, delaunayn.

— Function File: [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: delaunay, delaunayn.

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, 1];

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.

— Function File: idx = dsearch (x, y, tri, xi, yi)
— Function File: idx = dsearch (x, y, tri, xi, yi, s)

> Return the index idx or the closest point in x`,` y to the elements `[`xi`(:),` yi`(:)]`. The variable s is accepted for compatibility but is ignored.

> See also: dsearchn, tsearch.

— Function File: idx = dsearchn (x, tri, xi)
— Function File: idx = dsearchn (x, tri, xi, outval)
— Function File: idx = dsearchn (x, xi)
— Function File: [idx, d] = dsearchn (...)

> Return the index idx or 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: dsearch, 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`.