# as.mask: Pixel Image Approximation of a Window In spatstat.geom: Geometrical Functionality of the 'spatstat' Family

## Description

Obtain a discrete (pixel image) approximation of a given window

## Usage

 `1` ``` as.mask(w, eps=NULL, dimyx=NULL, xy=NULL) ```

## Arguments

 `w` A window (object of class `"owin"`) or data acceptable to `as.owin`. `eps` (optional) width and height of pixels. `dimyx` (optional) pixel array dimensions `xy` (optional) data containing pixel coordinates

## Details

This function generates a rectangular grid of locations in the plane, tests whether each of these locations lies inside the window `w`, and stores the results as a binary pixel image or ‘mask’ (an object of class `"owin"`, see `owin.object`).

The most common use of this function is to approximate the shape of another window `w` by a binary pixel image. In this case, we will usually want to have a very fine grid of pixels.

This function can also be used to generate a coarsely-spaced grid of locations inside a window, for purposes such as subsampling and prediction.

The grid spacing and location are controlled by the arguments `eps`, `dimyx` and `xy`, which are mutually incompatible.

If `eps` is given, then it determines the grid spacing. If `eps` is a single number, then the grid spacing will be approximately `eps` in both the x and y directions. If `eps` is a vector of length 2, then the grid spacing will be approximately `eps[1]` in the x direction and `eps[2]` in the y direction.

If `dimyx` is given, then the pixel grid will be an m * n rectangular grid where m, n are given by `dimyx[2]`, `dimyx[1]` respectively. Warning: `dimyx[1]` is the number of pixels in the y direction, and `dimyx[2]` is the number in the x direction.

If `xy` is given, then this should be some kind of data specifing the coordinates of a pixel grid. It may be

• a list or structure containing elements `x` and `y` which are numeric vectors of equal length. These will be taken as x and `y` coordinates of the margins of the grid. The pixel coordinates will be generated from these two vectors.

• a pixel image (object of class `"im"`).

• a window (object of class `"owin"`) which is of type `"mask"` so that it contains pixel coordinates.

If `xy` is given, `w` may be omitted.

If neither `eps` nor `dimyx` nor `xy` is given, the pixel raster dimensions are obtained from `spatstat.options("npixel")`.

There is no inverse of this function. However, the function `as.polygonal` will compute a polygonal approximation of a binary mask.

## Value

A window (object of class `"owin"`) of type `"mask"` representing a binary pixel image.

## Author(s)

and \rolf

`owin.object`, `as.rectangle`, `as.polygonal`, `spatstat.options`
 ```1 2 3 4 5 6 7 8 9``` ``` w <- owin(c(0,10),c(0,10), poly=list(x=c(1,2,3,2,1), y=c(2,3,4,6,7))) m <- as.mask(w) if(interactive()) { plot(w) plot(m) } x <- 1:9 y <- seq(0.25, 9.75, by=0.5) m <- as.mask(w, xy=list(x=x, y=y)) ```