spatstat (version 1.23-1)

im: Create a Pixel Image Object

Description

Creates an object of class "im" representing a two-dimensional pixel image.

Usage

im(mat, xcol=seq_len(ncol(mat)), yrow=seq_len(nrow(mat)),
   xrange=NULL, yrange=NULL,
   unitname=NULL)

Arguments

mat

matrix or vector containing the pixel values of the image.

xcol

vector of $x$ coordinates for the pixel gid

yrow

vector of $y$ coordinates for the pixel grid

xrange,yrange

Optional. Vectors of length 2 giving the $x$ and $y$ limits of the enclosing rectangle. (Ignored if xcol, yrow are present.)

unitname

Optional. Name of unit of length. Either a single character string, or a vector of two character strings giving the singular and plural forms, respectively.

Warnings

The internal representation of images is likely to change in future releases of spatstat. The safe way to extract pixel values from an image object is to use as.matrix.im or [.im.

Details

This function creates an object of class "im" representing a two-dimensional pixel image. See im.object for details of this class.

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 Rhas 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 afactorcontaining the pixel values, saymat <- factor(.....), and then assign matrix dimensions to it bydim(mat) <- c(nr, nc)wherenr, ncare the numbers of rows and columns. The resulting objectmatis both a factor and a vector.
• Supplymatas a one-dimensional factor and specify the argumentsxcolandyrowto determine the dimensions of the image.
• Use the functionscut.imoreval.imto 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.

See Also

im.object, as.im, as.matrix.im, [.im, eval.im

Examples

vec <- rnorm(1200)
   mat <- matrix(vec, nrow=30, ncol=40)
   whitenoise <- im(mat)
   whitenoise <- im(mat, xrange=c(0,1), yrange=c(0,1))
   whitenoise <- im(mat, xcol=seq(0,1,length=40), yrow=seq(0,1,length=30))
   whitenoise <- im(vec, xcol=seq(0,1,length=40), yrow=seq(0,1,length=30))
   plot(whitenoise)

   # Factor-valued images:
   f <- factor(letters[1:12])
   dim(f) <- c(3,4)
   Z <- im(f)

   # Factor image from other image:
   cutwhite <- cut(whitenoise, 3)
   plot(cutwhite)

   # Factor image from raw data
   cutmat <- cut(mat, 3)
   dim(cutmat) <- c(30,40)
   cutwhite <- im(cutmat)
   plot(cutwhite)