grDevices (version 3.2.0)

as.raster: Create a Raster Object

Description

Functions to create a raster object (representing a bitmap image) and coerce other objects to a raster object.

Usage

is.raster(x)
as.raster(x, ...)
"as.raster"(x, max = 1, ...)
"as.raster"(x, max = 1, ...)
"as.raster"(x, max = 1, ...)
"as.raster"(x, max = 1, ...)
"as.raster"(x, max = 1, ...)

Arguments

x

Any R object.

max

number giving the maximum of the color values range.

...

further arguments passed to or from other methods.

Value

For as.raster(), a raster object.For is.raster(), a logical indicating whether x is a raster object.

Details

An object of class "raster" is a matrix of colour values as given by rgb representing a bitmap image.

It is not expected that the user will need to call these functions directly; functions to render bitmap images in graphics packages will make use of the as.raster() function to generate a raster object from their input.

The as.raster() function is generic so methods can be written to convert other R objects to a raster object.

The default implementation for numeric matrices interprets scalar values on black-to-white scale.

Raster objects can be subsetted like a matrix and it is possible to assign to a subset of a raster object.

There is a method for converting a raster object to a matrix (of colour strings).

Raster objects can be compared for equality or inequality (with each other or with a colour string).

There is a is.na method which returns a logical matrix of the same dimensions as the raster object. Note that NA values are interpreted as the fully transparent colour by some (but not all) graphics devices.

Examples

# A red gradient
as.raster(matrix(hcl(0, 80, seq(50, 80, 10)),
                 nrow = 4, ncol = 5))

# Vectors are 1-column matrices ...
#   character vectors are color names ...
as.raster(hcl(0, 80, seq(50, 80, 10)))
#   numeric vectors are greyscale ...
as.raster(1:5, max = 5)
#   locigal vectors are black and white ...
as.raster(1:10 %% 2 == 0)

# ... unless nrow/ncol are supplied ...
as.raster(1:10 %% 2 == 0, nrow = 1)

# Matrix can also be logical or numeric ...
as.raster(matrix(c(TRUE, FALSE), nrow = 3, ncol = 2))
as.raster(matrix(1:3/4, nrow = 3, ncol = 4))

# An array can be 3-plane numeric (R, G, B planes) ...
as.raster(array(c(0:1, rep(0.5, 4)), c(2, 1, 3)))

# ... or 4-plane numeric (R, G, B, A planes)
as.raster(array(c(0:1, rep(0.5, 6)), c(2, 1, 4)))

# subsetting
r <- as.raster(matrix(colors()[1:100], ncol = 10))
r[, 2]
r[2:4, 2:5]

# assigning to subset
r[2:4, 2:5] <- "white"

# comparison
r == "white"