readJPEG | R Documentation |
Reads an image from a JPEG file/content into a raster array.
readJPEG(source, native = FALSE)
source |
Either name of the file to read from or a raw vector representing the JPEG file content. |
native |
determines the image representation - if |
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) or
Grayscale (1 channel). Note that Grayscale 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).
JPEG doesn't support alpha channel, you may want to use PNG instead in such situations.
CMYK JPEG images saved by Adobe Photoshop may have inverted ink values due
to a bug in Photoshop. Unfortunately this includes some sample CMYK
images that are floating around, so beware of the source when
converting the result to other color spaces. readJPEG
will
preserve values exactly as they are encoded in the file.
rasterImage
, writeJPEG
# read a sample file (R logo) img <- readJPEG(system.file("img", "Rlogo.jpg", package="jpeg")) # read it also in native format img.n <- readJPEG(system.file("img", "Rlogo.jpg", package="jpeg"), 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') rasterImage(img, 1.2, 1.27, 1.8, 1.73) rasterImage(img.n, 1.5, 1.5, 1.9, 1.8) }
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