Extract a subset or subregion of a pixel image.
# S3 method for im
[(x, i, j, …, drop=TRUE, tight=FALSE,
raster=NULL, rescue=is.owin(i))A two-dimensional pixel image.
An object of class "im".
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 linear network (object of
class "linnet") or a point pattern (an object
of class "ppp"), or any type of index that applies to a
matrix, or something that can be converted to a point pattern
by as.ppp (using the window of x).
An integer or logical vector serving as the column index if
matrix indexing is being used. Ignored if i is a spatial object.
Ignored.
Logical value, specifying whether to return a vector containing
the selected pixel values (drop=TRUE, the default)
or to return a pixel image containing
these values in their original spatial positions
(drop=FALSE).
The exception is that if i is a point pattern,
then drop specifies whether to delete NA values.
See Details.
Logical value. If tight=TRUE, and if the result of the
subset operation is an image, the image will be trimmed
to the smallest possible rectangle.
Optional. An object of class "owin" or "im"
determining a pixel grid.
Logical value indicating whether rectangular blocks of data should always be returned as pixel images.
Either a pixel image or a vector of pixel values. See Details.
If you have a 2-column matrix containing the \(x,y\) coordinates
of point locations, then to prevent this being interpreted as an
array index, you should convert it to a data.frame
or to a point pattern.
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].
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 arguments i,j
according to the following rules (which are checked in this order):
i is a spatial object such as a window,
a pixel image with logical values,
a linear network, or a point pattern; or
i,j are indices for the matrix as.matrix(x); or
i can be converted to a point pattern
by as.ppp(i, W=Window(x)),
and i is not a matrix.
If i is a spatial window (an object of class "owin"),
the pixels inside this window are selected.
If drop=TRUE (the default) and either
is.rectangle(i)=FALSE or rescue=FALSE,
the pixel values are extracted;
the result is a vector, with one entry for each pixel of x
that lies inside the window i.
Pixel values may be NA, indicating that the selected pixel
lies outside the spatial domain of the image.
if drop=FALSE,
the result is another pixel image, obtained by setting
the pixel values to NA outside the window i.
The effect is that the pixel image x is clipped to the
window i.
if i is a rectangle and rescue=TRUE,
the result is a pixel image as described above.
To ensure that an image is produced in all circumstances,
set drop=FALSE.
To ensure that pixel values are extracted as a vector
in all circumstances, set drop=TRUE, rescue=FALSE.
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 linear network (object of class "linnet"),
the pixels which lie on this network are selected.
If drop=TRUE (the default),
the pixel values are extracted;
the result is a vector, with one entry for each pixel of x
that lies along the network i.
Pixel values may be NA, indicating that the selected pixel
lies outside the spatial domain of the image.
if drop=FALSE,
the result is a pixel image on a linear network (object of
class "linim"), obtained by setting the pixel values of
x to NA except for those which lie on the network i.
The effect is that the pixel image x is restricted to the
network i.
If i is a point pattern (an object of class
"ppp") or something that can be converted to a point pattern,
then the values of the pixel image at the points of
this pattern are extracted.
The result is a vector of pixel values.
This is a simple way to read the
pixel values at a given spatial location.
if drop=FALSE the length of the result
is equal to the number of points in the pattern. It may contain
NA values which indicate that the corresponding point
lies outside the spatial domain of the image.
if drop=TRUE (the default), NA values are
deleted. The result is a vector whose length may be shorter than
the number of points of the pattern.
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.
If i does not satisfy any of the conditions above, then
the algorithm attempts to interpret i and j
as indices for the matrix as.matrix(x).
Either i or j may be missing or blank.
The result is usually a vector or matrix of pixel values.
Exceptionally the result is a pixel image if i,j determines
a rectangular subset of the pixel grid, and if the user specifies
rescue=TRUE.
Finally, if none of the above conditions is met,
the object i may also be a data frame or list of x,y
coordinates which will be converted to a point pattern, taking the
observation window to be Window(x). Then the pixel values
at these points will be extracted as a vector.
# NOT RUN {
# 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
R <- affine(letterR, diag(c(1,1)/2), c(-2,-0.7))
plot(X[R, drop=FALSE])
plot(X[R, drop=FALSE, tight=TRUE])
# a point pattern
Y <- X[cells]
# 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]
# }
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