# chull

##### Compute Convex Hull of a Set of Points

Computes the subset of points which lie on the convex hull of the set of points specified.

- Keywords
- graphs

##### Usage

`chull(x, y = NULL)`

##### Arguments

- x, y
- coordinate vectors of points. This can be specified as two
vectors
`x`

and`y`

, a 2-column matrix`x`

, a list`x`

with two components, etc, see`xy.coords`

.

##### Details

`xy.coords`

is used to interpret the specification of the
points. Infinite, missing and `NaN`

values are not allowed.

The algorithm is that given by Eddy (1977).

##### Value

- An integer vector giving the indices of the unique points lying on the convex hull, in clockwise order. (The first will be returned for duplicate points.)

##### References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
*The New S Language*.
Wadsworth & Brooks/Cole.

Eddy, W. F. (1977) A new convex hull algorithm for planar sets.
*ACM Transactions on Mathematical Software*, **3**, 398--403.

Eddy, W. F. (1977) Algorithm 523. CONVEX, A new convex hull
algorithm for planar sets[Z]. *ACM Transactions on
Mathematical Software*, **3**, 411--412.

##### See Also

##### Examples

`library(grDevices)`

```
X <- matrix(stats::rnorm(2000), ncol = 2)
chull(X)
# Example usage from graphics package
plot(X, cex = 0.5)
hpts <- chull(X)
hpts <- c(hpts, hpts[1])
lines(X[hpts, ])
```

*Documentation reproduced from package grDevices, version 3.3, License: Part of R 3.3*

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