# discretise

##### Safely Convert Point Pattern Window to Binary Mask

Given a point pattern, discretise its window by converting it to a binary pixel mask, adjusting the mask so that it still contains all the points.

##### Usage

`discretise(X, eps = NULL, dimyx = NULL, xy = NULL)`

##### Arguments

- X
A point pattern (object of class

`"ppp"`

) to be converted.- eps
(optional) width and height of each pixel

- dimyx
(optional) pixel array dimensions

- xy
(optional) pixel coordinates

##### Details

This function modifies the point pattern `X`

by converting its
observation window `Window(X)`

to a binary pixel image (a window
of type `"mask"`

). It ensures that no points of `X`

are
deleted by the discretisation.

The window is first discretised using `as.mask`

.
It can happen that points of `X`

that were inside the original
window may fall outside the new mask.
The `discretise`

function corrects this by augmenting the mask
(so that the mask includes any pixel that contains a point of the pattern).

The arguments `eps`

, `dimyx`

and `xy`

control the fineness of the pixel array. They are passed to
`as.mask`

.

If `eps`

, `dimyx`

and `xy`

are all absent or
`NULL`

, and if the window of `X`

is of type `"mask"`

to start with, then `discretise(X)`

returns `X`

unchanged.

See `as.mask`

for further details
about the arguments `eps`

, `dimyx`

,
and `xy`

, and the process of converting
a window to one of type `mask`

.

##### Value

A point pattern (object of class `"ppp"`

),
identical to `X`

, except that
its observation window has been converted to one
of type `mask`

.

##### Error checking

Before doing anything, `discretise`

checks that
all the points of the pattern are actually
inside the original window. This is guaranteed to
be the case if the pattern was constructed using `ppp`

or `as.ppp`

. However anomalies are possible if the
point pattern was created or manipulated inappropriately.
These will cause an error.

##### See Also

##### Examples

```
# NOT RUN {
data(demopat)
X <- demopat
plot(X, main="original pattern")
Y <- discretise(X, dimyx=50)
plot(Y, main="discretise(X)")
stopifnot(npoints(X) == npoints(Y))
# what happens if we just convert the window to a mask?
W <- Window(X)
M <- as.mask(W, dimyx=50)
plot(M, main="window of X converted to mask")
plot(X, add=TRUE, pch=16)
plot(X[M], add=TRUE, pch=1, cex=1.5)
XM <- X[M]
cat(paste(npoints(X) - npoints(XM), "points of X lie outside M\n"))
# }
```

*Documentation reproduced from package spatstat, version 1.57-1, License: GPL (>= 2)*