This function creates an object of class "im" representing
a ‘pixel image’ or two-dimensional array of values. The pixel grid is rectangular and occupies a rectangular window
in the spatial coordinate system.
The pixel values are scalars: they 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 no value is defined
at that location, and effectively that pixel is ‘outside’ the window.
Although the pixel values must be scalar,
photographic colour images (i.e., with red, green, and blue brightness
channels) can be represented as character-valued images in spatstat,
using R's standard encoding of colours as character strings.
The matrix mat contains the ‘greyscale’ values
for a rectangular grid of pixels.
Note carefully that the entry mat[i,j]
gives the pixel value at the location (xcol[j],yrow[i]).
That is, the row index of the matrix mat corresponds
to increasing y coordinate, while the column index of mat
corresponds to increasing x coordinate.
Thus yrow has one entry for each row of mat
and xcol has one entry for each column of mat.
Under the usual convention in R, a correct
display of the image would be obtained by transposing the matrix, e.g.
image.default(xcol, yrow, t(mat)), if you wanted to do it by hand.
The entries of mat may be numeric (real or integer), complex,
logical, character, or factor values.
If mat is not a matrix, it will be converted into
a matrix with nrow(mat) = length(yrow) and
ncol(mat) = length(xcol).
To make a factor-valued image, note that
R has a quirky way of handling matrices with
factor-valued entries. The command matrix cannot be used
directly, because it destroys factor information.
To make a factor-valued image, do one of the following:
-
Create a
factor containing the pixel values,
say mat <- factor(.....),
and then assign matrix dimensions to it by dim(mat) <- c(nr, nc)
where nr, nc are the numbers of rows and columns. The
resulting object mat is both a factor and a vector.
-
Supply
mat as a one-dimensional factor
and specify the arguments xcol and yrow
to determine the dimensions of the image.
-
Use the functions
cut.im or eval.im to make factor-valued
images from other images).
For a description of the methods available for pixel image objects,
see im.object.
To convert other kinds of data to a pixel image (for example,
functions or windows), use as.im.