# chull: Compute Convex Hull of a Set of Points

### Description

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

### Usage

 `1` ```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.

`xy.coords`, `polygon`
 ``` 1 2 3 4 5 6 7 8 9 10``` ```X <- matrix(stats::rnorm(2000), ncol = 2) chull(X) ## Not run: # Example usage from graphics package plot(X, cex = 0.5) hpts <- chull(X) hpts <- c(hpts, hpts[1]) lines(X[hpts, ]) ## End(Not run) ```