writeJPEG.RdCreate a JPEG image from an array or matrix.
writeJPEG(image, target = raw(), quality = 0.7, bg = "white", color.space)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 to, or a binary
connection, or a raw vector (raw() - the default - is good
enough) indicating that the output should be a raw vector.
JPEG quality - a real number between 0 (lowest) and 1 (highest) controlling the quality of the output. Lower quality produces smaller, but more lossy files.
background color - used only if the input contains alpha channel since JPEG does not support storage of the alpha channel and thus the image needs to be flattened as if it was placed over the background of this color.
color space in which the image data is to be
interpreted. Defaults to the "color.space" attribute of the
image and NULL is interpreted as the default color space.
The color space specified here must match the image array
dimensions, no conversions are performed. Currently the only
supported non-default color space is "CMYK" for four-channel
images (which would be interpreted as "RGBA" if the color
space is not specified).
NULL if the target is either a file or connection, or a raw
vector containing the compressed JPEG image if the target was a raw
vector.
writeJPEG takes an image as input and compresses it into JPEG
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. For
convenience writeJPEG allows the source to include alpha
channel, but JPEG does NOT support alpha channel so it will be
blended against the specified background.
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 readJPEG using native = TRUE.
Finally, writeJPEG also supports raw array containing the RGBA
(or CMYK) 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 and CMYK) are supported and the processing of RGBA
is equivalent to that of a native raster.
The result is either stored in a file (if target is a file
name), send to a binary connection (if target is a connection)
or stored in a raw vector (if target is a raw vector).
Currently writeJPEG only produces 8-bit, non-progressive JPEG
format with no additional tags.
# read a sample file (R logo)
img <- readJPEG(system.file("img","Rlogo.jpg",package="jpeg"))
# write the image into a raw vector - using a low quality
r <- writeJPEG(img, raw(), quality=0.3)
# read it back again
img2 <- readJPEG(r)
# it will be slightly different since JPEG is a lossy format
# in particular at the low quality
max(abs(img - img2))
#> [1] 0.2941176
stopifnot(max(abs(img - img2)) < 0.4)
# try to write a native raster
img3 <- readJPEG(system.file("img","Rlogo.jpg",package="jpeg"), TRUE)
r2 <- writeJPEG(img3, raw())
img4 <- readJPEG(r2, TRUE)
# comparing nativeRaster values is not easy, so let's do write/read again
img5 <- readJPEG(writeJPEG(img4, raw()))
max(abs(img - img5))
#> [1] 0.1843137
stopifnot(max(abs(img - img5)) < 0.3)