writeJPEG: Write a bitmap image in JPEG format

Description

Create a JPEG image from an array or matrix.

Usage

writeJPEG(image, target = raw(), quality = 0.7, bg = "white", color.space)

Value

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.

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 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)
target: 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.
quality: 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.
bg: 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: 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).

Details

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).

Examples

Run this code

# 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))
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))
stopifnot(max(abs(img - img5)) < 0.3)