Analysing spatial point patterns in R - CSIRO

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10 Pixel images in spatstat
An object of class "im" represents a pixel image. It specifies a rectangular grid of locations
(“pixels”) in two dimensional space, and a numerical value for each pixel. The pixel values
can be real numbers, integers, complex numbers, single characters or strings, logical values or
categorical values. A pixel’s value can also be NA, meaning that it is not defined at that location.
A pixel image represents a spatial function Z(u) in many different contexts. It may contain
experimental data (such as a map of terrain elevation) or computed values (such as a kernel
estimate of point process intensity) or it may be directly obtained from a camera (such as a
satellite image).
10.1 Creating a pixel image
10.1.1 Creating an image from raw data
To create a pixel image from raw data, use im:
> im(mat, xcol, yrow)
where mat is a matrix containing the pixel values. The pixel values could have been generated
by hand, or read from a file.
The correspondence between matrix indices mat[i,j] and cartesian coordinates is slightly
idiosyncratic: the rows ofmcorrespond to the y coordinate, and the columns to the x coordinate.
The argument xcol is a vector of equally-spaced x coordinate values corresponding to the
columns of mat, and yrow is a vector of equally-spaced y coordinate values corresponding to
the rows of mat. These vectors determine the spatial position of the pixel grid. The length of
xcol is ncol(mat) while the length of yrow is nrow(mat). If mat is not a matrix, it will be
converted into a matrix with nrow(mat) = length(yrow) and ncol(mat) = length(xcol).
> vec mat noisy plot(noisy)
noisy
−5 0 5
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