Installation

You can install from GitHub with:

# install.package('remotes')
remotes::install_github('coolbutuseless/spng')

What’s in the box

• depng(raw_vec, fmt, flags) - convert a vector of raw values containing a PNG image into a vector of raw bytes representing individual pixel values.

• get_info(raw_vec) - interrogate a vector of raw values containing a PNG image to determine image information i.e. width, height, bit_depth, color_type, compression_method, filter_method, interlace_method.

Example: Decompress a PNG in memory

library(spng)

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# A PNG file everyone should have!
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
png_file <- system.file("img", "Rlogo.png", package="png")
img <- magick::image_read(png_file)
img

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Read in the raw bytes
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
png_data <- readBin(png_file, 'raw', n = file.size(png_file))
png_data[1:100]
  [1] 89 50 4e 47 0d 0a 1a 0a 00 00 00 0d 49 48 44 52 00 00 00 64 00 00 00 4c 08
 [26] 06 00 00 00 9b 1d 12 0f 00 00 00 06 62 4b 47 44 00 ff 00 ff 00 ff a0 bd a7
 [51] 93 00 00 00 09 70 48 59 73 00 00 2e 23 00 00 2e 23 01 78 a5 3f 76 00 00 00
 [76] 07 74 49 4d 45 07 d5 02 10 10 08 0e 97 b9 27 bc 00 00 20 00 49 44 41 54 78

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Get info about the PNG 
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
(png_info <- spng::png_info(png_data))
$width
[1] 100

$height
[1] 76

$bit_depth
[1] 8

$color_type
[1] 6

$compression_method
[1] 0

$filter_method
[1] 0

$interlace_method
[1] 0

$color_desc
[1] "RGB + Alpha"

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Unpack the raw PNG bytes into RGB 8-bits-per-color packed format. 
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
img_data <- spng::depng(png_data, fmt = spng_format$SPNG_FMT_RGB8)
img_data[1:200]
  [1] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 [26] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 [51] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 [76] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[101] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 c6 c8 c5 98 9b
[126] 96 8f 93 8c 87 8b 84 83 87 80 81 85 7e 80 84 7d 7e 82 7a 81 85 7e 7e 83 7b
[151] 7e 83 7b 7e 83 7b 78 7d 75 80 85 7d 7e 82 7b 80 84 7d 81 85 7e 88 8c 85 9a
[176] 9d 97 be bf bc 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Pick the green channel and plot as greyscale
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
N <- length(img_data)
mat <- matrix(img_data[seq(2, N, 3)], nrow = png_info$width, ncol = png_info$height)

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Image had an alpha channel, let's replace that with white background
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mat[mat == 0] <- as.raw(255)

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# PNG bytes are in row-major order. R is in column major order
# so need to transpose to view correctly
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
plot(as.raster(t(mat)))

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Construct an array by plucking the relevant bytes.
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
arr <- array(
  c(
    img_data[seq(1, N, 3)], # R
    img_data[seq(2, N, 3)], # G
    img_data[seq(3, N, 3)]  # B
  ),
  dim = c(png_info$width, png_info$height, 3)
)

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# This array had an alpha channel. Let's replace the background with white.
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
arr[arr == 0] <- as.raw(255)

#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# PNG bytes are in row-major order. R is in column major order
# so need to transpose to view correctly
#~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
arr <- aperm(arr, c(2, 1, 3))
plot(as.raster(arr))

Acknowledgements

• R Core for developing and maintaining the language.
• CRAN maintainers, for patiently shepherding packages onto CRAN and maintaining the repository