R: Convert Point Pattern to Pixel Image

pixellate.ppp {spatstat} | R Documentation

## Convert Point Pattern to Pixel Image

### Description

Converts a point pattern to a pixel image. The value in each pixel is the number of points falling in that pixel, and is typically either 0 or 1.

### Usage

    ## S3 method for class 'ppp'
    pixellate(x, W=NULL, ..., weights = NULL,
                            padzero=FALSE, fractional=FALSE, preserve=FALSE,
                            DivideByPixelArea=FALSE, savemap=FALSE)

    ## S3 method for class 'ppp'
    as.im(X, ...)

### Arguments

`x,X` | Point pattern (object of class `"ppp"`).
`...` | Arguments passed to `as.mask` to determine the pixel resolution
`W` | Optional window mask (object of class `"owin"`) determining the pixel raster.
`weights` | Optional vector of weights associated with the points.
`padzero` | Logical value indicating whether to set pixel values to zero outside the window.
`fractional,preserve` | Logical values determining the type of discretisation. See Details.
`DivideByPixelArea` | Logical value, indicating whether the resulting pixel values should be divided by the pixel area.
`savemap` | Logical value, indicating whether to save information about the discretised coordinates of the points of `x`.

### Details

The functions `pixellate.ppp` and `as.im.ppp` convert a spatial point pattern `x` into a pixel image, by counting the number of points (or the total weight of points) falling in each pixel.

Calling `as.im.ppp` is equivalent to calling `pixellate.ppp` with its default arguments. Note that `pixellate.ppp` is more general than `as.im.ppp` (it has additional arguments for greater flexibility).

The functions `as.im.ppp` and `pixellate.ppp` are methods for the generic functions `as.im` and `pixellate` respectively, for the class of point patterns.

The pixel raster (in which points are counted) is determined by the argument `W` if it is present (for `pixellate.ppp` only). In this case `W` should be a binary mask (a window object of class `"owin"` with type `"mask"`). Otherwise the pixel raster is determined by extracting the window containing `x` and converting it to a binary pixel mask using `as.mask`. The arguments `...` are passed to `as.mask` to control the pixel resolution.

If `weights` is `NULL`, then for each pixel in the mask, the algorithm counts how many points in `x` fall in the pixel. This count is usually either 0 (for a pixel with no data points in it) or 1 (for a pixel containing one data point) but may be greater than 1. The result is an image with these counts as its pixel values.

If `weights` is given, it should be a numeric vector of the same length as the number of points in `x`. For each pixel, the algorithm finds the total weight associated with points in `x` that fall in the given pixel. The result is an image with these total weights as its pixel values.

By default (if `zeropad=FALSE`) the resulting pixel image has the same spatial domain as the window of the point pattern `x`. If `zeropad=TRUE` then the resulting pixel image has a rectangular domain; pixels outside the original window are assigned the value zero.

The discretisation procedure is controlled by the arguments `fractional` and `preserve`.

- The argument `fractional` specifies how data points are mapped to pixels. If `fractional=FALSE` (the default), each data point is allocated to the nearest pixel centre. If `fractional=TRUE`, each data point is allocated with fractional weight to four pixel centres (the corners of a rectangle containing the data point).

- The argument `preserve` specifies what to do with pixels lying near the boundary of the window, if the window is not a rectangle. If `preserve=FALSE` (the default), any contributions that are attributed to pixel centres lying outside the window are reset to zero. If `preserve=TRUE`, any such contributions are shifted to the nearest pixel lying inside the window, so that the total mass is preserved.

If `savemap=TRUE` then the result has an attribute `"map"` which is a 2-column matrix containing the row and column indices of the discretised positions of the points of `x` in the pixel grid.

### Value

A pixel image (object of class `"im"`).

### Author(s)

Adrian Baddeley Adrian.Baddeley@curtin.edu.au, Rolf Turner r.turner@auckland.ac.nz and Ege Rubak rubak@math.aau.dk.

### See Also

`pixellate`, `im`, `as.im`, `density.ppp`, `Smooth.ppp`.

### Examples

      plot(pixellate(humberside))
      plot(pixellate(humberside, fractional=TRUE))

[Package spatstat version 1.63-3 Index]