Title: | Read and write JPEG images |
---|---|
Description: | This package provides an easy and simple way to read, write and display bitmap images stored in the JPEG format. It can read and write both files and in-memory raw vectors. |
Authors: | Simon Urbanek <[email protected]> |
Maintainer: | Simon Urbanek <[email protected]> |
License: | GPL-2 | GPL-3 |
Version: | 0.1-10 |
Built: | 2024-11-01 11:15:01 UTC |
Source: | https://github.com/s-u/jpeg |
Reads an image from a JPEG file/content into a raster array.
readJPEG(source, native = FALSE)
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.
# 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) }
# 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) }
Create a JPEG image from an array or matrix.
writeJPEG(image, target = raw(), quality = 0.7, bg = "white", color.space)
writeJPEG(image, target = raw(), quality = 0.7, bg = "white", color.space)
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 ( |
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 |
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).
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.
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)) 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)
# 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)