R: Randomly Shift a Line Segment Pattern

rshift.psp {spatstat.core} | R Documentation

## Randomly Shift a Line Segment Pattern

### Description

Randomly shifts the segments in a line segment pattern.

### Usage

       ## S3 method for class 'psp'
    rshift(X, ..., group=NULL, which=NULL)

### Arguments

`X` | Line segment pattern to be subjected to a random shift. An object of class `"psp"`.
`...` | Arguments controlling the randomisation and the handling of edge effects. See `rshift.ppp`.
`group` | Optional. Factor specifying a grouping of the line segments of `X`, or `NULL` indicating that all line segments belong to the same group. Each group will be shifted together, and separately from other groups.
`which` | Optional. Identifies which groups of the pattern will be shifted, while other groups are not shifted. A vector of levels of `group`.

### Details

This operation randomly shifts the locations of the line segments in a line segment pattern.

The function `rshift` is generic. This function `rshift.psp` is the method for line segment patterns.

The line segments of `X` are first divided into groups, then the line segments within a group are shifted by a common random displacement vector. Different groups of line segments are shifted independently. If the argument `group` is present, then this determines the grouping. Otherwise, all line segments belong to a single group.

The argument `group` should be a factor, of length equal to the number of line segments in `X`. Alternatively `group` may be `NULL`, which specifies that all line segments of `X` belong to a single group.

By default, every group of line segments will be shifted. The argument `which` indicates that only some of the groups should be shifted, while other groups should be left unchanged. `which` must be a vector of levels of `group` indicating which groups are to be shifted.

The displacement vector, i.e. the vector by which the data line segments are shifted, is generated at random. The default behaviour is to generate a displacement vector at random with equal probability for all possible displacements. This means that the x and y coordinates of the displacement vector are independent random variables, uniformly distributed over the range of possible coordinates.

Alternatively, the displacement vector can be generated by another random mechanism, controlled by the arguments `radius`, `width` and `height`.

rectangular:

  if `width` and `height` are given, then the displacement vector is uniformly distributed in a rectangle of these dimensions, centred at the origin. The maximum possible displacement in the x direction is `width/2`. The maximum possible displacement in the y direction is `height/2`. The x and y displacements are independent. (If `width` and `height` are actually equal to the dimensions of the observation window, then this is equivalent to the default.)

radial:

  if `radius` is given, then the displacement vector is generated by choosing a random line segment inside a disc of the given radius, centred at the origin, with uniform probability density over the disc. Thus the argument `radius` determines the maximum possible displacement distance. The argument `radius` is incompatible with the arguments `width` and `height`.

The argument `edge` controls what happens when a shifted line segment lies partially or completely outside the window of `X`. Currently the only option is `"erode"` which specifies that the segments will be clipped to a smaller window.

The optional argument `clip` specifies a smaller window to which the pattern should be restricted.

### Value

A line segment pattern (object of class `"psp"`).

### Author(s)

Adrian Baddeley Adrian.Baddeley@curtin.edu.au

and Rolf Turner r.turner@auckland.ac.nz

### See Also

`rshift`, `rshift.ppp`

### Examples

      X <- psp(runif(20), runif(20), runif(20), runif(20),  window=owin())
      Y <- rshift(X, radius=0.1)

[Package spatstat.core version 2.0-0 Index]