R: Apply Function to Image Broken Down by Factor

by.im {spatstat.geom} | R Documentation

## Apply Function to Image Broken Down by Factor

### Description

Splits a pixel image into sub-images and applies a function to each sub-image.

### Usage

    ## S3 method for class 'im'
    by(data, INDICES, FUN, ...)

### Arguments

`data` | A pixel image (object of class `"im"`).
`INDICES` | Grouping variable. Either a tessellation (object of class `"tess"`) or a factor-valued pixel image.
`FUN` | Function to be applied to each sub-image of `data`.
`...` | Extra arguments passed to `FUN`.

### Details

This is a method for the generic function `by` for pixel images (class `"im"`).

The pixel image `data` is first divided into sub-images according to `INDICES`. Then the function `FUN` is applied to each subset. The results of each computation are returned in a list.

The grouping variable `INDICES` may be either

- a tessellation (object of class `"tess"`). Each tile of the tessellation delineates a subset of the spatial domain.

- a pixel image (object of class `"im"`) with factor values. The levels of the factor determine subsets of the spatial domain.

### Value

A list containing the results of each evaluation of `FUN`.

### Author(s)

Adrian Baddeley Adrian.Baddeley@curtin.edu.au

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

### See Also

`split.im`, `tess`, `im`

### Examples

      W <- square(1)
      X <- as.im(function(x,y){sqrt(x^2+y^2)}, W)
      Y <- dirichlet(runifrect(12, W))
      # mean pixel value in each subset
      unlist(by(X, Y, mean))
      # trimmed mean
      unlist(by(X, Y, mean, trim=0.05))

[Package spatstat.geom version 2.2-0 Index]