readPNG {png}

Read a bitmap image stored in the PNG format

Description

Reads an image from a PNG file/content into a raster array.

Usage

readPNG(source, native = FALSE, info = FALSE)

Arguments

source

Either name of the file to read from or a raw vector representing the PNG file content.

native

determines the image representation - if FALSE (the default) then the result is an array, if TRUE then the result is a native raster representation.

info

logical, if TRUE additional "info" attribute is attached to the result containing information from optional tags in the file (such as bit depth, resolution, gamma, text etc.). If the PNG file contains R metadata, it will also contain a "metadata" attribute with the unserialized R object.

Value

If native is FALSE then an array of the dimensions height x width x channels. If there is only one channel the result is a matrix. The values are reals between 0 and 1. If native is TRUE then an object of the class nativeRaster is returned instead. The latter cannot be easily computed on but is the most efficient way to draw using rasterImage.

Most common files decompress into RGB (3 channels), RGBA (4 channels), Grayscale (1 channel) or GA (2 channels). Note that G and GA images cannot be directly used in rasterImage unless native is set to TRUE because rasterImage requires RGB or RGBA format (nativeRaster is always 8-bit RGBA).

As of png 0.1-2 files with 16-bit channels are converted in full resolution to the array format, but the nativeRaster format only supports 8-bit and therefore a truncation is performed (eight least significant bits are dropped) with a warning if native is TRUE.

See Also

rasterImage, writePNG

Examples

# read a sample file (R logo)
img <- readPNG(system.file("img", "Rlogo.png", package="png"))

# read it also in native format
img.n <- readPNG(system.file("img", "Rlogo.png", package="png"), TRUE)

# if your R supports it, we'll plot it
if (exists("rasterImage")) { # can plot only in R 2.11.0 and higher
  plot(1:2, type='n')

  if (names(dev.cur()) == "windows") {
    # windows device doesn't support semi-transparency so we'll need
    # to flatten the image
    transparent <- img[,,4] == 0
    img <- as.raster(img[,,1:3])
    img[transparent] <- NA

    # interpolate must be FALSE on Windows, otherwise R will
    # try to interpolate transparency and fail
    rasterImage(img, 1.2, 1.27, 1.8, 1.73, interpolate=FALSE)

  } else {
    # any reasonable device will be fine using alpha
    rasterImage(img, 1.2, 1.27, 1.8, 1.73)
    rasterImage(img.n, 1.5, 1.5, 1.9, 1.8)

  }
}