# Extract.im

##### Extract Subset of Image

Extract a subset or subregion of a pixel image.

##### Usage

```
## S3 method for class 'im':
[(x, i, drop=TRUE, \dots, raster=NULL)
```

##### Arguments

- x
- A two-dimensional pixel image.
An object of class
`"im"`

. - i
- Object defining the subregion or subset to be extracted.
Either a spatial window (an object of class
`"owin"`

), or a pixel image with logical values, or a point pattern (an object of class`"ppp"`

), or something that - ...
- Ignored.
- drop
- Logical value.
Locations in
`w`

that lie outside the spatial domain of the image`x`

return a pixel value of`NA`

if`drop=FALSE`

, and are omitted if`drop=TRUE`

. - raster
- Optional. An object of class
`"owin"`

or`"im"`

determining a pixel grid.

##### Details

This function extracts a subset of the pixel values in a
pixel image. (To reassign the pixel values, see `[<-.im`

).

The image `x`

must be an object of class
`"im"`

representing a pixel image defined inside a
rectangle in two-dimensional space (see `im.object`

).

The subset to be extracted is determined by the argument `i`

.
If `i`

is a spatial window (an object of class `"owin"`

),
the values of the image inside this window are extracted
(after first clipping the window to the spatial domain of the image
if necessary). If `i`

is a pixel image with logical values,
it is interpreted as a spatial window (with `TRUE`

values
inside the window and `FALSE`

outside).
If `i`

is a point pattern (an object of class
`"ppp"`

), then the values of the pixel image at the points of
this pattern are extracted.
At locations outside the spatial domain of the image,
the pixel value is undefined, and is taken to be `NA`

.
The logical argument `drop`

determines whether such `NA`

values will be returned or omitted.
It also influences the format of the return value.

If `i`

is a point pattern (or something that can be
converted to a point pattern by `as.ppp`

such as a
list of `x,y`

coordinates), then
`X[i, drop=FALSE]`

is a numeric vector
containing the pixel values at each of the points of the pattern.
Its length is equal to the number of points in the pattern `i`

.
It may contain `NA`

s corresponding to points which lie outside
the spatial domain of the image `x`

.
By contrast, `X[i]`

or `X[i, drop=TRUE]`

contains only those
pixel values which are not `NA`

. It may be shorter.

If `i`

is a spatial window
then `X[i, drop=FALSE]`

is another pixel image
of the same dimensions as `x`

obtained by
setting all pixels outside the window `i`

to have value
`NA`

. When the result is displayed by `plot.im`

the effect
is that the pixel image `x`

is clipped to the window `i`

.

If `i`

is a spatial window which is *not* a rectangle
(`i$type != "rectangle"`

) then `X[i, drop=TRUE]`

is a numeric vector containing the pixel values for all pixels
that lie inside the window `i`

.

If `i`

is a rectangle (a spatial window
with `i$type = "rectangle"`

) then `X[i, drop=TRUE]`

is a pixel image. The spatial domain of this image is the
intersection of `i`

with the spatial domain of `x`

.

If the optional argument `raster`

is given, then it should
be a binary image mask or a pixel image. Then
`x`

will first be converted to an image defined on the
pixel grid implied by `raster`

, before the subset operation
is carried out.
In particular, `x[i, raster=i, drop=FALSE]`

will return
an image defined on the same pixel array as the object `i`

.

##### Value

- Either a pixel image or a vector of pixel values. See Details.

##### Warning

If `W`

is a window or a pixel image,
then `x[W, drop=FALSE]`

will return
an image defined on the same pixel array as the original image
`x`

. If you want to obtain an image whose pixel
dimensions agree with those of `W`

,
use the `raster`

argument, `x[W, raster=W, drop=FALSE]`

.

##### See Also

##### Examples

```
# make up an image
X <- setcov(unit.square())
plot(X)
# a rectangular subset
W <- owin(c(0,0.5),c(0.2,0.8))
Y <- X[W]
plot(Y)
# a polygonal subset
data(letterR)
R <- affine(letterR, diag(c(1,1)/2), c(-2,-0.7))
Y <- X[R, drop=FALSE]
plot(Y)
# a point pattern
P <- rpoispp(20)
Y <- X[P]
# look up a specified location
X[list(x=0.1,y=0.2)]
# 10 x 10 pixel array
X <- as.im(function(x,y) { x + y }, owin(c(-1,1),c(-1,1)), dimyx=10)
# 100 x 100
W <- as.mask(disc(1, c(0,0)), dimyx=100)
# 10 x 10 raster
X[W,drop=FALSE]
# 100 x 100 raster
X[W, raster=W, drop=FALSE]
```

*Documentation reproduced from package spatstat, version 1.18-2, License: GPL (>= 2)*