Package ‘png’
November 29, 2022
Version 0.1-8
Title Read and write PNG images
Author Simon Urbanek <[email protected]>
Maintainer Simon Urbanek <[email protected]>
Depends R (>= 2.9.0)
Description This package provides an easy and simple way to read, write and display bitmap im-
ages stored in the PNG format. It can read and write both files and in-memory raw vectors.
License GPL-2 | GPL-3
SystemRequirements libpng
URL http://www.rforge.net/png/
NeedsCompilation yes
Repository CRAN
Date/Publication 2022-11-29 11:12:53 UTC
R topics documented:
readPNG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
writePNG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Index 6
readPNG 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)
1
2 readPNG
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
writePNG 3
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)
}
}
writePNG Write a bitmap image in PNG format
Description
Create a PNG image from an array or matrix.
Usage
writePNG(image, target = raw(), dpi = NULL, asp = NULL,
text = NULL, metadata = NULL)
Arguments
image image represented by a real matrix or array with values in the range of 0 to
1. Values outside this range will be clipped. The object must be either two-
dimensional (grayscale matrix) or three dimensional array (third dimension spec-
ifying the plane) and must have either one (grayscale), two (grayscale + alpha),
three (RGB) or four (RGB + alpha) planes. (For alternative image specifications
see details)
target Either name of the file to write, a binary connection or a raw vector (raw() - the
default - is good enough) indicating that the output should be a raw vector.
dpi optional, if set, must be a numeric vector of length 1 or 2 specifying the resolu-
tion of the image in DPI (dots per inch) for x and y (in that order) - it is recycled
to length 2.
asp optional, if set, must be a numeric scalar specifying the aspect ratio (x / y). dpi
and asp are mutually exclusive, speciyfing both is an error.
text optional, named character vector of entries that will be saved in the text chunk of
the PNG. Names are used as keys. Note that the "R.metadata" key is reserved
for internal use - see below
metadata optional, an R object that will be serialized into the "R.metadata" text key
4 writePNG
Details
writePNG takes an image as input and compresses it into PNG format. The image input is usually
a matrix (for grayscale images - dimensions are width, height) or an array (for color and alpha
images - dimensions are width, height, planes) of reals. The planes are interpreted in the sequence
red, green, blue, alpha.
Alternative representation of an image is of nativeRaster class which is an integer matrix with
each entry representing one pixel in binary encoded RGBA format (as used internally by R). It can
be obtained from readPNG using native = TRUE.
Finally, writePNG also supports raw array containing the RGBA image as bytes. The dimensions of
the raw array have to be planes, width, height (because the storage is interleaved). Currently only 4
planes (RGBA) are supported and the processing is equivalent to that of a native raster.
The result is either stored in a file (if target is a file name), in a raw vector (if target is a raw
vector) or sent to a binary connection.
If either dpi or asp is set, the sPHy chunk is generated based on that information. Note that not all
image viewers interpret this setting, and even fewer support non-square pixels.
Value
Either NULL if the target is a file or a raw vector containing the compressed PNG image if the target
was a raw vector.
Note
Currently writePNG only produces 8-bit, deflate-compressed, non-quantized, non-interlaced im-
ages. Note in particular that readPNG can read 16-bit channels but storing them back using writePNG
will strip the 8 LSB (irrelevant for display purposes but possibly relevant for use of PNG in signal-
processing if the input is truly 16-bit wide).
Author(s)
Simon Urbanek
See Also
readPNG
Examples
# read a sample file (R logo)
img <- readPNG(system.file("img","Rlogo.png",package="png"))
# write the image into a raw vector
r <- writePNG(img)
# read it back again
img2 <- readPNG(r)
# it better be the same
identical(img, img2)
# try to write a native raster
img3 <- readPNG(system.file("img","Rlogo.png",package="png"), TRUE)
r2 <- writePNG(img3)
writePNG 5
img4 <- readPNG(r2, TRUE)
identical(img3, img4)
## text and metadata
r <- writePNG(img, text=c(source=R.version.string),
metadata=sessionInfo())
img5 <- readPNG(r, info=TRUE)
attr(img5, "info")
attr(img5, "metadata")
