Write a bitmap image in PNG format
Create a PNG image from an array or matrix.
writePNG(image, target = raw(), dpi = NULL, asp = NULL, text = NULL, metadata = NULL)
- 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 specifying the plane) and must have either one (grayscale), two (grayscale + alpha), three (RGB) or four (RGB + alpha) planes. (For alternative image specifications see deatils)
- 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.
- optional, if set, must be a numeric vector of length 1 or 2 specifying the resolution of the image in DPI (dots per inch) for x and y (in that order) - it is recycled to length 2.
- optional, if set, must be a numeric scalar specifying the
aspect ratio (
x / y).
aspare mututally exclusive, speciyfing both is an error.
- optional, named character vector of entries that will be
saved in the text chunk of the PNG. Names are used as keys. Note
"R.metadata"key is reserved for internal use - see below
- optional, an R object that will be serialized
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
which is an integer matrix with each entry representing one pixel in
binary encoded RGBA format (as used internally by R). It can be
native = TRUE.
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
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.
NULLif the target is a file or a raw vector containing the compressed PNG image if the target was a raw vector.
writePNG only produces 8-bit, deflate-compressed,
non-quantized, non-interlaced images. Note in particular that
readPNG can read 16-bit channels but storing them
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).
# 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) 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")