Next: Interpolation on Scattered Data, Previous: Voronoi Diagrams, Up: Geometry [Contents][Index]

The convex hull of a set of points is the minimum convex envelope
containing all of the points. Octave has the functions `convhull`

and `convhulln`

to calculate the convex hull of 2-dimensional and
N-dimensional sets of points.

- :
`H`=**convhull***(*`x`,`y`) - :
`H`=**convhull***(*`x`,`y`,`z`) - :
`H`=**convhull***(*`x`) - :
`H`=**convhull***(…,*`options`) - :
*[*`H`,`V`] =**convhull***(…)* Compute the convex hull of a 2-D or 3-D set of points.

The hull

`H`is a linear index vector into the original set of points that specifies which points form the enclosing hull. For 2-D inputs only, the output is ordered in a counterclockwise manner around the hull.The input

`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.An optional final 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 http://www.qhull.org/html/qh-quick.htm#options. The default option is

`{"Qt"}`

.If

`options`is not present or`[]`

then the default arguments are used. Otherwise,`options`replaces the default argument list. To append user options to the defaults it is necessary to repeat the default arguments in`options`. Use a null string to pass no arguments.If the second output

`V`is requested the volume of the enclosing convex hull is calculated.

- :
`h`=**convhulln***(*`pts`) - :
`h`=**convhulln***(*`pts`,`options`) - :
*[*`h`,`v`] =**convhulln***(…)* Compute the convex hull of the set of points

`pts`.`pts`is a matrix of size [n, dim] containing n points in a space of dimension dim.The hull

`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 http://www.qhull.org/html/qh-quick.htm#options. The default options depend on the dimension of the input:

- 2D, 3D, 4D:
`options`=`{"Qt"}`

- 5D and higher:
`options`=`{"Qt", "Qx"}`

If

`options`is not present or`[]`

then the default arguments are used. Otherwise,`options`replaces the default argument list. To append user options to the defaults it is necessary to repeat the default arguments in`options`. Use a null string to pass no arguments.If the second output

`v`is requested the volume of the enclosing convex hull is calculated.- 2D, 3D, 4D:

An example of the use of `convhull`

is

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]);

The output of the above can be seen in Figure 30.5.

Next: Interpolation on Scattered Data, Previous: Voronoi Diagrams, Up: Geometry [Contents][Index]