Stacking PNGs in R

gistfile1.txt

# Stack two PNGs on top of each other,
# filling any leftover pixels with white
# 
# N.B. I'm assuming the alpha channel is always 1; 
# I *think* this will handle transparency with no changes, but haven't tested

library("png")

# these are plain R arrays with 3 dimensions (row, col, rgba) 
# -- not even wrapped in a class
a = readPNG("some_figure.png")
b = readPNG("other_figure.png")

adim = dim(a)
bdim = dim(b)

# channels should be RGBA, encoded as 0-1 not 0-255
# Might be unnecessary -- should check if readPNG guarantees this already
stopifnot(
	adim[3] == 4,
	bdim[3] == 4,
	all(a >= 0 & a <= 1),
	all(b >= 0 & b <= 1))

outdim=c(adim[1]+bdim[1], max(adim[2], bdim[2]), adim[3])
out = array(NA, dim=outdim)
out[1:adim[1], 1:adim[2], 1:adim[3]] = a
out[(adim[1]+1):outdim[1], 1:bdim[2], 1:bdim[3]] = b

if(adim[2] < bdim[2]){ # ==> need to fill pixels to the right of a
	out[1:adim[1], (adim[2]+1):outdim[2], 1:outdim[3]] = 1
	# Or `out[...] = c(1,1,1,0)` to fill transparent, etc
}

# if adim[2] == bdim[2] ==> no need to fill anything

if(adim[2] > bdim[2]){ # ==> need to fill pixels to the right of b
	out[(adim[1]+1):outdim[1], (bdim[2]+1):outdim[2], 1:outdim[3]] = 1	
}

writePNG(out, "joined_figure.png")