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image: Display a Color Image

image

R Documentation

Display a Color Image

Description

Creates a grid of colored or gray-scale rectangles with colors corresponding to the values in z. This can be used to display three-dimensional or spatial data aka images. This is a generic function.

NOTE: the grid is drawn as a set of rectangles by default; see the useRaster argument to draw the grid as a raster image.

The function hcl.colors provides a broad range of sequential color palettes that are suitable for displaying ordered data, with n giving the number of colors desired.

Usage

image(x, ...)

## Default S3 method:
image(x, y, z, zlim, xlim, ylim,
      col = hcl.colors(12, "YlOrRd", rev = TRUE),
      add = FALSE, xaxs = "i", yaxs = "i", xlab, ylab,
      breaks, oldstyle = FALSE, useRaster, ...)

Arguments

`x, y`	locations of grid lines at which the values in `z` are measured. These must be finite, non-missing and in (strictly) ascending order. By default, equally spaced values from 0 to 1 are used. If `x` is a `list`, its components `x$x` and `x$y` are used for `x` and `y`, respectively. If the list has component `z` this is used for `z`.
`z`	a numeric or logical matrix containing the values to be plotted (`NA`s are allowed). Note that `x` can be used instead of `z` for convenience.
`zlim`	the minimum and maximum `z` values for which colors should be plotted, defaulting to the range of the finite values of `z`. Each of the given colors will be used to color an equispaced interval of this range. The midpoints of the intervals cover the range, so that values just outside the range will be plotted.
`xlim, ylim`	ranges for the plotted `x` and `y` values, defaulting to the ranges of `x` and `y`.
`col`	a list of colors such as that generated by `hcl.colors`, `gray.colors` or similar functions.
`add`	logical; if `TRUE`, add to current plot (and disregard the following four arguments). This is rarely useful because `image` ‘paints’ over existing graphics.
`xaxs, yaxs`	style of x and y axis. The default `"i"` is appropriate for images. See `par`.
`xlab, ylab`	each a character string giving the labels for the x and y axis. Default to the ‘call names’ of `x` or `y`, or to `""` if these were unspecified.
`breaks`	a set of finite numeric breakpoints for the colours: must have one more breakpoint than colour and be in increasing order. Unsorted vectors will be sorted, with a warning.
`oldstyle`	logical. If true the midpoints of the colour intervals are equally spaced, and `zlim[1]` and `zlim[2]` were taken to be midpoints. The default is to have colour intervals of equal lengths between the limits.
`useRaster`	logical; if `TRUE` a bitmap raster is used to plot the image instead of polygons. The grid must be regular in that case, otherwise an error is raised. For the behaviour when this is not specified, see ‘Details’.
`...`	graphical parameters for `plot` may also be passed as arguments to this function, as can the plot aspect ratio `asp` and `axes` (see `plot.window`).

Details

The length of x should be equal to the nrow(z)+1 or nrow(z). In the first case x specifies the boundaries between the cells: in the second case x specifies the midpoints of the cells. Similar reasoning applies to y. It probably only makes sense to specify the midpoints of an equally-spaced grid. If you specify just one row or column and a length-one x or y, the whole user area in the corresponding direction is filled. For logarithmic x or y axes the boundaries between cells must be specified.

Rectangles corresponding to missing values are not plotted (and so are transparent and (unless add = TRUE) the default background painted in par("bg") will show through and if that is transparent, the canvas colour will be seen).

If breaks is specified then zlim is unused and the algorithm used follows cut, so intervals are closed on the right and open on the left except for the lowest interval which is closed at both ends.

The axes (where plotted) make use of the classes of xlim and ylim (and hence by default the classes of x and y): this will mean that for example dates are labelled as such.

Notice that image interprets the z matrix as a table of f(x[i], y[j]) values, so that the x axis corresponds to row number and the y axis to column number, with column 1 at the bottom, i.e. a 90 degree counter-clockwise rotation of the conventional printed layout of a matrix.

Images for large z on a regular grid are rendered more efficiently with useRaster = TRUE and can prevent rare anti-aliasing artifacts, but may not be supported by all graphics devices. Some devices (such as postscript and X11(type = "Xlib")) which do not support semi-transparent colours may emit missing values as white rather than transparent, and there may be limitations on the size of a raster image. (Problems with the rendering of raster images have been reported by users of windows() devices under Remote Desktop, at least under its default settings.)

The graphics files in PDF and PostScript can be much smaller under this option.

If useRaster is not specified, raster images are used when the getOption("preferRaster") is true, the grid is regular and either dev.capabilities("rasterImage")$rasterImage is "yes" or it is "non-missing" and there are no missing values.

Note

Originally based on a function by Thomas Lumley.

Examples

require("grDevices") # for colours
x <- y <- seq(-4*pi, 4*pi, length.out = 27)
r <- sqrt(outer(x^2, y^2, "+"))
image(z = z <- cos(r^2)*exp(-r/6), col = gray.colors(33))
image(z, axes = FALSE, main = "Math can be beautiful ...",
      xlab = expression(cos(r^2) * e^{-r/6}))
contour(z, add = TRUE, drawlabels = FALSE)

# Volcano data visualized as matrix. Need to transpose and flip
# matrix horizontally.
image(t(volcano)[ncol(volcano):1,])

# A prettier display of the volcano
x <- 10*(1:nrow(volcano))
y <- 10*(1:ncol(volcano))
image(x, y, volcano, col = hcl.colors(100, "terrain"), axes = FALSE)
contour(x, y, volcano, levels = seq(90, 200, by = 5),
        add = TRUE, col = "brown")
axis(1, at = seq(100, 800, by = 100))
axis(2, at = seq(100, 600, by = 100))
box()
title(main = "Maunga Whau Volcano", font.main = 4)