spatstat.geom (version 3.3-3)

eval.im: Evaluate Expression Involving Pixel Images

Description

Evaluates any expression involving one or more pixel images, and returns a pixel image.

Usage

eval.im(expr, envir, harmonize=TRUE, warn=TRUE)

Value

An image object of class "im".

Arguments

expr

An expression.

envir

Optional. The environment in which to evaluate the expression, or a named list containing pixel images to be used in the expression.

harmonize

Logical. Whether to resolve inconsistencies between the pixel grids.

warn

Logical. Whether to issue a warning if the pixel grids were inconsistent.

Author

Adrian Baddeley and Rolf Turner

Details

This function is a wrapper to make it easier to perform pixel-by-pixel calculations in an image.

Pixel images in spatstat are represented by objects of class "im" (see im.object). These are essentially matrices of pixel values, with extra attributes recording the pixel dimensions, etc.

Suppose X is a pixel image. Then eval.im(X+3) will add 3 to the value of every pixel in X, and return the resulting pixel image.

Suppose X and Y are two pixel images with compatible dimensions: they have the same number of pixels, the same physical size of pixels, and the same bounding box. Then eval.im(X + Y) will add the corresponding pixel values in X and Y, and return the resulting pixel image.

In general, expr can be any expression in the R language involving (a) the names of pixel images, (b) scalar constants, and (c) functions which are vectorised. See the Examples.

First eval.im determines which of the variable names in the expression expr refer to pixel images. Each such name is replaced by a matrix containing the pixel values. The expression is then evaluated. The result should be a matrix; it is taken as the matrix of pixel values.

The expression expr must be vectorised. There must be at least one pixel image in the expression.

All images must have compatible dimensions. If harmonize=FALSE, images that are incompatible will cause an error. If harmonize=TRUE, images that have incompatible dimensions will be resampled so that they are compatible; if warn=TRUE, a warning will be issued.

See Also

im.apply for operations similar to apply, such as taking the sum of a list of images.

as.im, compatible.im, harmonise.im, im.object

Examples

  # test images
  X <- as.im(function(x,y) { x^2 - y^2 }, unit.square())
  Y <- as.im(function(x,y) { 3 * x + y }, unit.square())

  eval.im(X + 3)
  eval.im(X - Y)
  eval.im(abs(X - Y))
  Z <- eval.im(sin(X * pi) + Y)

  ## Use of 'envir': bei.extra is a list with components 'elev' and 'grad'
  W <- eval.im(atan(grad) * 180/pi, bei.extra)