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@node Geometry
@chapter Geometry

Much of the geometry code in Octave is based on the Qhull
library@footnote{@nospell{Barber, C.B., Dobkin, D.P., and Huhdanpaa, H.T.},
@cite{The Quickhull Algorithm for Convex Hulls}, ACM Trans. on Mathematical
Software, 22(4):469--483, Dec 1996, @url{http://www.qhull.org}}.
Some of the documentation for Qhull, particularly for the options that
can be passed to @code{delaunay}, @code{voronoi} and @code{convhull},
etc., is relevant to Octave users.

@menu
* Delaunay Triangulation::
* Voronoi Diagrams::
* Convex Hull::
* Interpolation on Scattered Data::
@end menu

@node Delaunay Triangulation
@section Delaunay Triangulation

The Delaunay triangulation is constructed from a set of
circum-circles.  These circum-circles are chosen so that there are at
least three of the points in the set to triangulation on the
circumference of the circum-circle.  None of the points in the set of
points falls within any of the circum-circles.

In general there are only three points on the circumference of any
circum-circle.  However, in some cases, and in particular for the
case of a regular grid, 4 or more points can be on a single
circum-circle.  In this case the Delaunay triangulation is not unique.

@c delaunay scripts/geometry/delaunay.m
@anchor{XREFdelaunay}
@deftypefn  {Function File} {@var{tri} =} delaunay (@var{x}, @var{y})
@deftypefnx {Function File} {@var{tetr} =} delaunay (@var{x}, @var{y}, @var{z})
@deftypefnx {Function File} {@var{tri} =} delaunay (@var{x})
@deftypefnx {Function File} {@var{tri} =} delaunay (@dots{}, @var{options})
Compute the Delaunay triangulation for a 2-D or 3-D set of points.

For 2-D sets, the return value @var{tri} is a set of triangles which
satisfies the Delaunay circum-circle criterion, i.e., only a single data
point from [@var{x}, @var{y}] is within the circum-circle of the defining
triangle.  The set of triangles @var{tri} is a matrix of size [n, 3].  Each
row defines a triangle and the three columns are the three vertices of the
triangle.  The value of @code{@var{tri}(i,j)} is an index into @var{x} and
@var{y} for the location of the j-th vertex of the i-th triangle.

For 3-D sets, the return value @var{tetr} is a set of tetrahedrons which
satisfies the Delaunay circum-circle criterion, i.e., only a single data
point from [@var{x}, @var{y}, @var{z}] is within the circum-circle of the
defining tetrahedron.  The set of tetrahedrons is a matrix of size [n, 4].
Each row defines a tetrahedron and the four columns are the four vertices of
the tetrahedron.  The value of @code{@var{tetr}(i,j)} is an index into
@var{x}, @var{y}, @var{z} for the location of the j-th vertex of the i-th
tetrahedron.

The input @var{x} may also be a matrix with two or three columns where the
first column contains x-data, the second y-data, and the optional third
column contains z-data.

The optional last argument, which must be a string or cell array of strings,
contains options passed to the underlying qhull command.
See the documentation for the Qhull library for details
@url{http://www.qhull.org/html/qh-quick.htm#options}.
The default options are @code{@{"Qt", "Qbb", "Qc", "Qz"@}}.

If @var{options} is not present or @code{[]} then the default arguments are
used.  Otherwise, @var{options} replaces the default argument list.
To append user options to the defaults it is necessary to repeat the
default arguments in @var{options}.  Use a null string to pass no arguments.

@example
@group
x = rand (1, 10);
y = rand (1, 10);
tri = delaunay (x, y);
triplot (tri, x, y);
hold on;
plot (x, y, "r*");
axis ([0,1,0,1]);
@end group
@end example
@seealso{@ref{XREFdelaunayn,,delaunayn}, @ref{XREFconvhull,,convhull}, @ref{XREFvoronoi,,voronoi}, @ref{XREFtriplot,,triplot}, @ref{XREFtrimesh,,trimesh}, @ref{XREFtetramesh,,tetramesh}, @ref{XREFtrisurf,,trisurf}}
@end deftypefn


For 3-D inputs @code{delaunay} returns a set of tetrahedra that satisfy the
Delaunay circum-circle criteria.  Similarly, @code{delaunayn} returns the
N-dimensional simplex satisfying the Delaunay circum-circle criteria.
The N-dimensional extension of a triangulation is called a tessellation.

@c delaunayn scripts/geometry/delaunayn.m
@anchor{XREFdelaunayn}
@deftypefn  {Function File} {@var{T} =} delaunayn (@var{pts})
@deftypefnx {Function File} {@var{T} =} delaunayn (@var{pts}, @var{options})
Compute the Delaunay triangulation for an N-dimensional set of points.
The Delaunay triangulation is a tessellation of the convex hull of a set
of points such that no N-sphere defined by the N-triangles contains
any other points from the set.

The input matrix @var{pts} of size [n, dim] contains n points in a space of
dimension dim.  The return matrix @var{T} has size [m, dim+1].  Each row
of @var{T} contains a set of indices back into the original set of points
@var{pts} which describes a simplex of dimension dim.  For example, a 2-D
simplex is a triangle and 3-D simplex is a tetrahedron.

An optional second argument, which must be a string or cell array of strings,
contains options passed to the underlying qhull command.
See the documentation for the Qhull library for details
@url{http://www.qhull.org/html/qh-quick.htm#options}.
The default options depend on the dimension of the input:

@itemize
@item 2-D and 3-D: @var{options} = @code{@{"Qt", "Qbb", "Qc", "Qz"@}}

@item 4-D and higher: @var{options} = @code{@{"Qt", "Qbb", "Qc", "Qx"@}}
@end itemize

If @var{options} is not present or @code{[]} then the default arguments are
used.  Otherwise, @var{options} replaces the default argument list.
To append user options to the defaults it is necessary to repeat the
default arguments in @var{options}.  Use a null string to pass no arguments.

@seealso{@ref{XREFdelaunay,,delaunay}, @ref{XREFconvhulln,,convhulln}, @ref{XREFvoronoin,,voronoin}, @ref{XREFtrimesh,,trimesh}, @ref{XREFtetramesh,,tetramesh}}
@end deftypefn


An example of a Delaunay triangulation of a set of points is

@example
@group
rand ("state", 2);
x = rand (10, 1);
y = rand (10, 1);
T = delaunay (x, y);
X = [ x(T(:,1)); x(T(:,2)); x(T(:,3)); x(T(:,1)) ];
Y = [ y(T(:,1)); y(T(:,2)); y(T(:,3)); y(T(:,1)) ];
axis ([0, 1, 0, 1]);
plot (X, Y, "b", x, y, "r*");
@end group
@end example

@ifnotinfo
@noindent
The result of which can be seen in @ref{fig:delaunay}.

@float Figure,fig:delaunay
@center @image{delaunay,4in}
@caption{Delaunay triangulation of a random set of points}
@end float
@end ifnotinfo

@menu
* Plotting the Triangulation::
* Identifying Points in Triangulation::
@end menu

@node Plotting the Triangulation
@subsection Plotting the Triangulation

Octave has the functions @code{triplot}, @code{trimesh}, and @code{trisurf}
to plot the Delaunay triangulation of a 2-dimensional set of points.
@code{tetramesh} will plot the triangulation of a 3-dimensional set of points.

@c triplot scripts/plot/draw/triplot.m
@anchor{XREFtriplot}
@deftypefn  {Function File} {} triplot (@var{tri}, @var{x}, @var{y})
@deftypefnx {Function File} {} triplot (@var{tri}, @var{x}, @var{y}, @var{linespec})
@deftypefnx {Function File} {@var{h} =} triplot (@dots{})
Plot a 2-D triangular mesh.

@var{tri} is typically the output of a Delaunay triangulation over the
grid of @var{x}, @var{y}.  Every row of @var{tri} represents one triangle
and contains three indices into [@var{x}, @var{y}] which are the
vertices of the triangles in the x-y plane.

The linestyle to use for the plot can be defined with the argument
@var{linespec} of the same format as the @code{plot} command.

The optional return value @var{h} is a graphics handle to the created
patch object.
@seealso{@ref{XREFplot,,plot}, @ref{XREFtrimesh,,trimesh}, @ref{XREFtrisurf,,trisurf}, @ref{XREFdelaunay,,delaunay}}
@end deftypefn


@c trimesh scripts/plot/draw/trimesh.m
@anchor{XREFtrimesh}
@deftypefn  {Function File} {} trimesh (@var{tri}, @var{x}, @var{y}, @var{z}, @var{c})
@deftypefnx {Function File} {} trimesh (@var{tri}, @var{x}, @var{y}, @var{z})
@deftypefnx {Function File} {} trimesh (@var{tri}, @var{x}, @var{y})
@deftypefnx {Function File} {} trimesh (@dots{}, @var{prop}, @var{val}, @dots{})
@deftypefnx {Function File} {@var{h} =} trimesh (@dots{})
Plot a 3-D triangular wireframe mesh.

In contrast to @code{mesh}, which plots a mesh using rectangles,
@code{trimesh} plots the mesh using triangles.

@var{tri} is typically the output of a Delaunay triangulation over the
grid of @var{x}, @var{y}.  Every row of @var{tri} represents one triangle
and contains three indices into [@var{x}, @var{y}] which are the
vertices of the triangles in the x-y plane.  @var{z} determines the
height above the plane of each vertex.  If no @var{z} input is given then
the triangles are plotted as a 2-D figure.

The color of the trimesh is computed by linearly scaling the @var{z} values
to fit the range of the current colormap.  Use @code{caxis} and/or
change the colormap to control the appearance.

Optionally, the color of the mesh can be specified independently of @var{z}
by supplying a color matrix, @var{c}.  If @var{z} has N elements, then
@var{c} should be an Nx1 vector for colormap data or an Nx3 matrix for
RGB data.

Any property/value pairs are passed directly to the underlying patch object.

The optional return value @var{h} is a graphics handle to the created patch
object.
@seealso{@ref{XREFmesh,,mesh}, @ref{XREFtetramesh,,tetramesh}, @ref{XREFtriplot,,triplot}, @ref{XREFtrisurf,,trisurf}, @ref{XREFdelaunay,,delaunay}, @ref{XREFpatch,,patch}, @ref{XREFhidden,,hidden}}
@end deftypefn


@c trisurf scripts/plot/draw/trisurf.m
@anchor{XREFtrisurf}
@deftypefn  {Function File} {} trisurf (@var{tri}, @var{x}, @var{y}, @var{z}, @var{c})
@deftypefnx {Function File} {} trisurf (@var{tri}, @var{x}, @var{y}, @var{z})
@deftypefnx {Function File} {} trisurf (@dots{}, @var{prop}, @var{val}, @dots{})
@deftypefnx {Function File} {@var{h} =} trisurf (@dots{})
Plot a 3-D triangular surface.

In contrast to @code{surf}, which plots a surface mesh using rectangles,
@code{trisurf} plots the mesh using triangles.

@var{tri} is typically the output of a Delaunay triangulation over the
grid of @var{x}, @var{y}.  Every row of @var{tri} represents one triangle
and contains three indices into [@var{x}, @var{y}] which are the
vertices of the triangles in the x-y plane.  @var{z} determines the
height above the plane of each vertex.

The color of the trimesh is computed by linearly scaling the @var{z} values
to fit the range of the current colormap.  Use @code{caxis} and/or
change the colormap to control the appearance.

Optionally, the color of the mesh can be specified independently of @var{z}
by supplying a color matrix, @var{c}.  If @var{z} has N elements, then
@var{c} should be an Nx1 vector for colormap data or an Nx3 matrix for
RGB data.

Any property/value pairs are passed directly to the underlying patch object.

The optional return value @var{h} is a graphics handle to the created patch
object.
@seealso{@ref{XREFsurf,,surf}, @ref{XREFtriplot,,triplot}, @ref{XREFtrimesh,,trimesh}, @ref{XREFdelaunay,,delaunay}, @ref{XREFpatch,,patch}, @ref{XREFshading,,shading}}
@end deftypefn


@c tetramesh scripts/plot/draw/tetramesh.m
@anchor{XREFtetramesh}
@deftypefn  {Function File} {} tetramesh (@var{T}, @var{X})
@deftypefnx {Function File} {} tetramesh (@var{T}, @var{X}, @var{C})
@deftypefnx {Function File} {} tetramesh (@dots{}, @var{property}, @var{val}, @dots{})
@deftypefnx {Function File} {@var{h} =} tetramesh (@dots{})
Display the tetrahedrons defined in the m-by-4 matrix @var{T} as 3-D patches.

@var{T} is typically the output of a Delaunay triangulation
of a 3-D set of points.  Every row of @var{T} contains four indices into
the n-by-3 matrix @var{X} of the vertices of a tetrahedron.  Every row in
@var{X} represents one point in 3-D space.

The vector @var{C} specifies the color of each tetrahedron as an index
into the current colormap.  The default value is 1:m where m is the number
of tetrahedrons; the indices are scaled to map to the full range of the
colormap.  If there are more tetrahedrons than colors in the colormap then
the values in @var{C} are cyclically repeated.

Calling @code{tetramesh (@dots{}, "property", "value", @dots{})} passes all
property/value pairs directly to the patch function as additional arguments.

The optional return value @var{h} is a vector of patch handles where each
handle represents one tetrahedron in the order given by @var{T}.
A typical use case for @var{h} is to turn the respective patch
@qcode{"visible"} property @qcode{"on"} or @qcode{"off"}.

Type @code{demo tetramesh} to see examples on using @code{tetramesh}.
@seealso{@ref{XREFtrimesh,,trimesh}, @ref{XREFdelaunay,,delaunay}, @ref{XREFdelaunayn,,delaunayn}, @ref{XREFpatch,,patch}}
@end deftypefn


The difference between @code{triplot}, and @code{trimesh} or @code{triplot},
is that the former only plots the 2-dimensional triangulation itself, whereas
the second two plot the value of a function @code{f (@var{x}, @var{y})}.  An
example of the use of the @code{triplot} function is

@example
@group
rand ("state", 2)
x = rand (20, 1);
y = rand (20, 1);
tri = delaunay (x, y);
triplot (tri, x, y);
@end group
@end example

@noindent
which plots the Delaunay triangulation of a set of random points in
2-dimensions.
@ifnotinfo
The output of the above can be seen in @ref{fig:triplot}.

@float Figure,fig:triplot
@center @image{triplot,4in}
@caption{Delaunay triangulation of a random set of points}
@end float
@end ifnotinfo

@node Identifying Points in Triangulation
@subsection 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 @code{tsearch} and @code{dsearch} perform this
function in a triangulation, and @code{tsearchn} and @code{dsearchn} in
an N-dimensional tessellation.

To identify whether a particular point represented by a vector @var{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 @code{@var{N} + 1} vectors @var{t}(@var{i},:), then the Barycentric
coordinates defining the point @var{p} are given by

@example
@var{p} = sum (@var{beta}(1:@var{N}+1) * @var{t}(1:@var{N}+1),:)
@end example

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

@example
sum (@var{beta}(1:@var{N}+1)) == 1
@end example

@noindent
is imposed, and we can therefore write the above as

@example
@group
@var{p} - @var{t}(end, :) = @var{beta}(1:end-1) * (@var{t}(1:end-1, :)
      - ones (@var{N}, 1) * @var{t}(end, :)
@end group
@end example

@noindent
Solving for @var{beta} we can then write

@example
@group
@var{beta}(1:end-1) = (@var{p} - @var{t}(end, :)) / (@var{t}(1:end-1, :)
      - ones (@var{N}, 1) * @var{t}(end, :))
@var{beta}(end) = sum (@var{beta}(1:end-1))
@end group
@end example

@noindent
which gives the formula for the conversion of the Cartesian coordinates
of the point @var{p} to the Barycentric coordinates @var{beta}.  An
important property of the Barycentric coordinates is that for all points
in the N-simplex

@example
0 <= @var{beta}(@var{i}) <= 1
@end example

@noindent
Therefore, the test in @code{tsearch} and @code{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
@var{beta}.  This is exactly the implementation used in
@code{tsearchn}.  @code{tsearch} is optimized for 2-dimensions and the
Barycentric coordinates are not explicitly formed.

@c tsearch libinterp/corefcn/tsearch.cc
@anchor{XREFtsearch}
@deftypefn {Built-in Function} {@var{idx} =} tsearch (@var{x}, @var{y}, @var{t}, @var{xi}, @var{yi})
Search for the enclosing Delaunay convex hull.  For @code{@var{t} =
delaunay (@var{x}, @var{y})}, finds the index in @var{t} containing the
points @code{(@var{xi}, @var{yi})}.  For points outside the convex hull,
@var{idx} is NaN.
@seealso{@ref{XREFdelaunay,,delaunay}, @ref{XREFdelaunayn,,delaunayn}}
@end deftypefn


@c tsearchn scripts/geometry/tsearchn.m
@anchor{XREFtsearchn}
@deftypefn {Function File} {[@var{idx}, @var{p}] =} tsearchn (@var{x}, @var{t}, @var{xi})
Search for the enclosing Delaunay convex hull.  For @code{@var{t} =
delaunayn (@var{x})}, finds the index in @var{t} containing the
points @var{xi}.  For points outside the convex hull, @var{idx} is NaN.
If requested @code{tsearchn} also returns the Barycentric coordinates @var{p}
of the enclosing triangles.
@seealso{@ref{XREFdelaunay,,delaunay}, @ref{XREFdelaunayn,,delaunayn}}
@end deftypefn


An example of the use of @code{tsearch} can be seen with the simple
triangulation

@example
@group
@var{x} = [-1; -1; 1; 1];
@var{y} = [-1; 1; -1; 1];
@var{tri} = [1, 2, 3; 2, 3, 1];
@end group
@end example

@noindent
consisting of two triangles defined by @var{tri}.  We can then identify
which triangle a point falls in like

@example
@group
tsearch (@var{x}, @var{y}, @var{tri}, -0.5, -0.5)
@result{} 1
tsearch (@var{x}, @var{y}, @var{tri}, 0.5, 0.5)
@result{} 2
@end group
@end example

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

@example
@group
tsearch (@var{x}, @var{y}, @var{tri}, 2, 2)
@result{} NaN
@end group
@end example

The @code{dsearch} and @code{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.

@c dsearch scripts/geometry/dsearch.m
@anchor{XREFdsearch}
@deftypefn  {Function File} {@var{idx} =} dsearch (@var{x}, @var{y}, @var{tri}, @var{xi}, @var{yi})
@deftypefnx {Function File} {@var{idx} =} dsearch (@var{x}, @var{y}, @var{tri}, @var{xi}, @var{yi}, @var{s})
Return the index @var{idx} or the closest point in @code{@var{x}, @var{y}}
to the elements @code{[@var{xi}(:), @var{yi}(:)]}.  The variable @var{s} is
accepted for compatibility but is ignored.
@seealso{@ref{XREFdsearchn,,dsearchn}, @ref{XREFtsearch,,tsearch}}
@end deftypefn


@c dsearchn scripts/geometry/dsearchn.m
@anchor{XREFdsearchn}
@deftypefn  {Function File} {@var{idx} =} dsearchn (@var{x}, @var{tri}, @var{xi})
@deftypefnx {Function File} {@var{idx} =} dsearchn (@var{x}, @var{tri}, @var{xi}, @var{outval})
@deftypefnx {Function File} {@var{idx} =} dsearchn (@var{x}, @var{xi})
@deftypefnx {Function File} {[@var{idx}, @var{d}] =} dsearchn (@dots{})
Return the index @var{idx} or the closest point in @var{x} to the elements
@var{xi}.  If @var{outval} is supplied, then the values of @var{xi} that are
not contained within one of the simplices @var{tri} are set to
@var{outval}.  Generally, @var{tri} is returned from @code{delaunayn
(@var{x})}.
@seealso{@ref{XREFdsearch,,dsearch}, @ref{XREFtsearch,,tsearch}}
@end deftypefn


An example of the use of @code{dsearch}, using the above values of
@var{x}, @var{y} and @var{tri} is

@example
@group
dsearch (@var{x}, @var{y}, @var{tri}, -2, -2)
@result{} 1
@end group
@end example

If you wish the points that are outside the tessellation to be flagged,
then @code{dsearchn} can be used as

@example
@group
dsearchn ([@var{x}, @var{y}], @var{tri}, [-2, -2], NaN)
@result{} NaN
dsearchn ([@var{x}, @var{y}], @var{tri}, [-0.5, -0.5], NaN)
@result{} 1
@end group
@end example

@noindent
where the point outside the tessellation are then flagged with @code{NaN}.

@node Voronoi Diagrams
@section Voronoi Diagrams

A Voronoi diagram or Voronoi tessellation of a set of points @var{s} in
an N-dimensional space, is the tessellation of the N-dimensional space
such that all points in @code{@var{v}(@var{p})}, a partitions of the
tessellation where @var{p} is a member of @var{s}, are closer to @var{p}
than any other point in @var{s}.  The Voronoi diagram is related to the
Delaunay triangulation of a set of points, in that the vertexes of the
Voronoi tessellation are the centers of the circum-circles of the
simplices of the Delaunay tessellation.

@c voronoi scripts/geometry/voronoi.m
@anchor{XREFvoronoi}
@deftypefn  {Function File} {} voronoi (@var{x}, @var{y})
@deftypefnx {Function File} {} voronoi (@var{x}, @var{y}, @var{options})
@deftypefnx {Function File} {} voronoi (@dots{}, "linespec")
@deftypefnx {Function File} {} voronoi (@var{hax}, @dots{})
@deftypefnx {Function File} {@var{h} =} voronoi (@dots{})
@deftypefnx {Function File} {[@var{vx}, @var{vy}] =} voronoi (@dots{})
Plot the Voronoi diagram of points @code{(@var{x}, @var{y})}.
The Voronoi facets with points at infinity are not drawn.

If @qcode{"linespec"} is given it is used to set the color and line style
of the plot.  If an axis graphics handle @var{hax} is supplied then the
Voronoi diagram is drawn on the specified axis rather than in a new
figure.

The @var{options} argument, which must be a string or cell array of strings,
contains options passed to the underlying qhull command.
See the documentation for the Qhull library for details
@url{http://www.qhull.org/html/qh-quick.htm#options}.

If a single output argument is requested then the Voronoi diagram will be
plotted and a graphics handle @var{h} to the plot is returned.
[@var{vx}, @var{vy}] = voronoi (@dots{}) returns the Voronoi vertices
instead of plotting the diagram.

@example
@group
x = rand (10, 1);
y = rand (size (x));
h = convhull (x, y);
[vx, vy] = voronoi (x, y);
plot (vx, vy, "-b", x, y, "o", x(h), y(h), "-g");
legend ("", "points", "hull");
@end group
@end example

@seealso{@ref{XREFvoronoin,,voronoin}, @ref{XREFdelaunay,,delaunay}, @ref{XREFconvhull,,convhull}}
@end deftypefn


@c voronoin scripts/geometry/voronoin.m
@anchor{XREFvoronoin}
@deftypefn  {Function File} {[@var{C}, @var{F}] =} voronoin (@var{pts})
@deftypefnx {Function File} {[@var{C}, @var{F}] =} voronoin (@var{pts}, @var{options})
Compute N-dimensional Voronoi facets.  The input matrix @var{pts}
of size [n, dim] contains n points in a space of dimension dim.
@var{C} contains the points of the Voronoi facets.  The list @var{F}
contains, for each facet, the indices of the Voronoi points.

An optional second argument, which must be a string or cell array of strings,
contains options passed to the underlying qhull command.
See the documentation for the Qhull library for details
@url{http://www.qhull.org/html/qh-quick.htm#options}.

The default options depend on the dimension of the input:

@itemize
@item 2-D and 3-D: @var{options} = @code{@{"Qbb"@}}

@item 4-D and higher: @var{options} = @code{@{"Qbb", "Qx"@}}
@end itemize

If @var{options} is not present or @code{[]} then the default arguments are
used.  Otherwise, @var{options} replaces the default argument list.
To append user options to the defaults it is necessary to repeat the
default arguments in @var{options}.  Use a null string to pass no arguments.

@seealso{@ref{XREFvoronoi,,voronoi}, @ref{XREFconvhulln,,convhulln}, @ref{XREFdelaunayn,,delaunayn}}
@end deftypefn


An example of the use of @code{voronoi} is

@example
@group
rand ("state",9);
x = rand (10,1);
y = rand (10,1);
tri = delaunay (x, y);
[vx, vy] = voronoi (x, y, tri);
triplot (tri, x, y, "b");
hold on;
plot (vx, vy, "r");
@end group
@end example

@ifnotinfo
@noindent
The result of which can be seen in @ref{fig:voronoi}.  Note that the
circum-circle of one of the triangles has been added to this figure, to
make the relationship between the Delaunay tessellation and the Voronoi
diagram clearer.

@float Figure,fig:voronoi
@center @image{voronoi,4in}
@caption{Delaunay triangulation and Voronoi diagram of a random set of points}
@end float
@end ifnotinfo

Additional information about the size of the facets of a Voronoi
diagram, and which points of a set of points is in a polygon can be had
with the @code{polyarea} and @code{inpolygon} functions respectively.

@c polyarea scripts/general/polyarea.m
@anchor{XREFpolyarea}
@deftypefn  {Function File} {} polyarea (@var{x}, @var{y})
@deftypefnx {Function File} {} polyarea (@var{x}, @var{y}, @var{dim})

Determine area of a polygon by triangle method.  The variables
@var{x} and @var{y} define the vertex pairs, and must therefore have
the same shape.  They can be either vectors or arrays.  If they are
arrays then the columns of @var{x} and @var{y} are treated separately
and an area returned for each.

If the optional @var{dim} argument is given, then @code{polyarea}
works along this dimension of the arrays @var{x} and @var{y}.

@end deftypefn


An example of the use of @code{polyarea} might be

@example
@group
rand ("state", 2);
x = rand (10, 1);
y = rand (10, 1);
[c, f] = voronoin ([x, y]);
af = zeros (size (f));
for i = 1 : length (f)
  af(i) = polyarea (c (f @{i, :@}, 1), c (f @{i, :@}, 2));
endfor
@end group
@end example

Facets of the Voronoi diagram with a vertex at infinity have infinity
area.  A simplified version of @code{polyarea} for rectangles is
available with @code{rectint}

@c rectint scripts/geometry/rectint.m
@anchor{XREFrectint}
@deftypefn {Function File} {@var{area} =} rectint (@var{a}, @var{b})

Compute the area of intersection of rectangles in @var{a} and
rectangles in @var{b}.  Rectangles are defined as [x y width height]
where x and y are the minimum values of the two orthogonal
dimensions.

If @var{a} or @var{b} are matrices, then the output, @var{area}, is a
matrix where the i-th row corresponds to the i-th row of a and the j-th
column corresponds to the j-th row of b.

@seealso{@ref{XREFpolyarea,,polyarea}}
@end deftypefn


@c inpolygon scripts/geometry/inpolygon.m
@anchor{XREFinpolygon}
@deftypefn {Function File} {[@var{in}, @var{on}] =} inpolygon (@var{x}, @var{y}, @var{xv}, @var{yv})

For a polygon defined by vertex points @code{(@var{xv}, @var{yv})}, determine
if the points @code{(@var{x}, @var{y})} are inside or outside the polygon.
The variables @var{x}, @var{y}, must have the same dimension.  The optional
output @var{on} gives the points that are on the polygon.

@end deftypefn


An example of the use of @code{inpolygon} might be

@example
@group
randn ("state", 2);
x = randn (100, 1);
y = randn (100, 1);
vx = cos (pi * [-1 : 0.1: 1]);
vy = sin (pi * [-1 : 0.1 : 1]);
in = inpolygon (x, y, vx, vy);
plot (vx, vy, x(in), y(in), "r+", x(!in), y(!in), "bo");
axis ([-2, 2, -2, 2]);
@end group
@end example

@ifnotinfo
@noindent
The result of which can be seen in @ref{fig:inpolygon}.

@float Figure,fig:inpolygon
@center @image{inpolygon,4in}
@caption{Demonstration of the @code{inpolygon} function to determine the
points inside a polygon}
@end float
@end ifnotinfo

@node Convex Hull
@section Convex Hull

The convex hull of a set of points is the minimum convex envelope
containing all of the points.  Octave has the functions @code{convhull}
and @code{convhulln} to calculate the convex hull of 2-dimensional and
N-dimensional sets of points.

@c convhull scripts/geometry/convhull.m
@anchor{XREFconvhull}
@deftypefn  {Function File} {@var{H} =} convhull (@var{x}, @var{y})
@deftypefnx {Function File} {@var{H} =} convhull (@var{x}, @var{y}, @var{options})
Compute the convex hull of the set of points defined by the
arrays @var{x} and @var{y}.  The hull @var{H} is an index vector into
the set of points and specifies which points form the enclosing hull.

An optional third argument, which must be a string or cell array of strings,
contains options passed to the underlying qhull command.
See the documentation for the Qhull library for details
@url{http://www.qhull.org/html/qh-quick.htm#options}.
The default option is @code{@{"Qt"@}}.

If @var{options} is not present or @code{[]} then the default arguments are
used.  Otherwise, @var{options} replaces the default argument list.
To append user options to the defaults it is necessary to repeat the
default arguments in @var{options}.  Use a null string to pass no arguments.

@seealso{@ref{XREFconvhulln,,convhulln}, @ref{XREFdelaunay,,delaunay}, @ref{XREFvoronoi,,voronoi}}
@end deftypefn


@c convhulln libinterp/dldfcn/convhulln.cc
@anchor{XREFconvhulln}
@deftypefn  {Loadable Function} {@var{h} =} convhulln (@var{pts})
@deftypefnx {Loadable Function} {@var{h} =} convhulln (@var{pts}, @var{options})
@deftypefnx {Loadable Function} {[@var{h}, @var{v}] =} convhulln (@dots{})
Compute the convex hull of the set of points @var{pts} which is a matrix
of size [n, dim] containing n points in a space of dimension dim.
The hull @var{h} is an index vector into the set of points and specifies
which points form the enclosing hull.

An optional second argument, which must be a string or cell array of strings,
contains options passed to the underlying qhull command.
See the documentation for the Qhull library for details
@url{http://www.qhull.org/html/qh-quick.htm#options}.
The default options depend on the dimension of the input:

@itemize
@item 2D, 3D, 4D: @var{options} = @code{@{"Qt"@}}

@item 5D and higher: @var{options} = @code{@{"Qt", "Qx"@}}
@end itemize

If @var{options} is not present or @code{[]} then the default arguments are
used.  Otherwise, @var{options} replaces the default argument list.
To append user options to the defaults it is necessary to repeat the
default arguments in @var{options}.  Use a null string to pass no arguments.

If the second output @var{v} is requested the volume of the enclosing
convex hull is calculated.

@seealso{@ref{XREFconvhull,,convhull}, @ref{XREFdelaunayn,,delaunayn}, @ref{XREFvoronoin,,voronoin}}
@end deftypefn


An example of the use of @code{convhull} is

@example
@group
x = -3:0.05:3;
y = abs (sin (x));
k = convhull (x, y);
plot (x(k), y(k), "r-", x, y, "b+");
axis ([-3.05, 3.05, -0.05, 1.05]);
@end group
@end example

@ifnotinfo
@noindent
The output of the above can be seen in @ref{fig:convhull}.

@float Figure,fig:convhull
@center @image{convhull,4in}
@caption{The convex hull of a simple set of points}
@end float
@end ifnotinfo

@node Interpolation on Scattered Data
@section Interpolation on Scattered Data

An important use of the Delaunay tessellation is that it can be used to
interpolate from scattered data to an arbitrary set of points.  To do
this the N-simplex of the known set of points is calculated with
@code{delaunay} or @code{delaunayn}.  Then the simplices in to which the
desired points are found are identified.  Finally the vertices of the simplices
are used to interpolate to the desired points.  The functions that perform this
interpolation are @code{griddata}, @code{griddata3} and @code{griddatan}.

@c griddata scripts/geometry/griddata.m
@anchor{XREFgriddata}
@deftypefn  {Function File} {@var{zi} =} griddata (@var{x}, @var{y}, @var{z}, @var{xi}, @var{yi})
@deftypefnx {Function File} {@var{zi} =} griddata (@var{x}, @var{y}, @var{z}, @var{xi}, @var{yi}, @var{method})
@deftypefnx {Function File} {[@var{xi}, @var{yi}, @var{zi}] =} griddata (@dots{})

Generate a regular mesh from irregular data using interpolation.
The function is defined by @code{@var{z} = f (@var{x}, @var{y})}.
Inputs @code{@var{x}, @var{y}, @var{z}} are vectors of the same length
or @code{@var{x}, @var{y}} are vectors and @code{@var{z}} is matrix.

The interpolation points are all @code{(@var{xi}, @var{yi})}.  If
@var{xi}, @var{yi} are vectors then they are made into a 2-D mesh.

The interpolation method can be @qcode{"nearest"}, @qcode{"cubic"} or
@qcode{"linear"}.  If method is omitted it defaults to @qcode{"linear"}.
@seealso{@ref{XREFgriddata3,,griddata3}, @ref{XREFgriddatan,,griddatan}, @ref{XREFdelaunay,,delaunay}}
@end deftypefn


@c griddata3 scripts/geometry/griddata3.m
@anchor{XREFgriddata3}
@deftypefn  {Function File} {@var{vi} =} griddata3 (@var{x}, @var{y}, @var{z}, @var{v}, @var{xi}, @var{yi}, @var{zi})
@deftypefnx {Function File} {@var{vi} =} griddata3 (@var{x}, @var{y}, @var{z}, @var{v}, @var{xi}, @var{yi}, @var{zi}, @var{method})
@deftypefnx {Function File} {@var{vi} =} griddata3 (@var{x}, @var{y}, @var{z}, @var{v}, @var{xi}, @var{yi}, @var{zi}, @var{method}, @var{options})

Generate a regular mesh from irregular data using interpolation.
The function is defined by @code{@var{v} = f (@var{x}, @var{y}, @var{z})}.
The interpolation points are specified by @var{xi}, @var{yi}, @var{zi}.

The interpolation method can be @qcode{"nearest"} or @qcode{"linear"}.
If method is omitted it defaults to @qcode{"linear"}.

The optional argument @var{options} is passed directly to Qhull when
computing the Delaunay triangulation used for interpolation.  See
@code{delaunayn} for information on the defaults and how to pass different
values.
@seealso{@ref{XREFgriddata,,griddata}, @ref{XREFgriddatan,,griddatan}, @ref{XREFdelaunayn,,delaunayn}}
@end deftypefn


@c griddatan scripts/geometry/griddatan.m
@anchor{XREFgriddatan}
@deftypefn  {Function File} {@var{yi} =} griddatan (@var{x}, @var{y}, @var{xi})
@deftypefnx {Function File} {@var{yi} =} griddatan (@var{x}, @var{y}, @var{xi}, @var{method})
@deftypefnx {Function File} {@var{yi} =} griddatan (@var{x}, @var{y}, @var{xi}, @var{method}, @var{options})

Generate a regular mesh from irregular data using interpolation.
The function is defined by @code{@var{y} = f (@var{x})}.
The interpolation points are all @var{xi}.

The interpolation method can be @qcode{"nearest"} or @qcode{"linear"}.
If method is omitted it defaults to @qcode{"linear"}.

The optional argument @var{options} is passed directly to Qhull when
computing the Delaunay triangulation used for interpolation.  See
@code{delaunayn} for information on the defaults and how to pass different
values.
@seealso{@ref{XREFgriddata,,griddata}, @ref{XREFgriddata3,,griddata3}, @ref{XREFdelaunayn,,delaunayn}}
@end deftypefn


An example of the use of the @code{griddata} function is

@example
@group
rand ("state", 1);
x = 2*rand (1000,1) - 1;
y = 2*rand (size (x)) - 1;
z = sin (2*(x.^2+y.^2));
[xx,yy] = meshgrid (linspace (-1,1,32));
griddata (x,y,z,xx,yy);
@end group
@end example

@noindent
that interpolates from a random scattering of points, to a uniform grid.
@ifnotinfo
The output of the above can be seen in @ref{fig:griddata}.

@float Figure,fig:griddata
@center @image{griddata,4in}
@caption{Interpolation from a scattered data to a regular grid}
@end float
@end ifnotinfo
