Batch append images in groups of two with Imagemagick - imagemagick

Question

I have a directory of images and need to merge those images horizontally in groups of two, then save the output of each to a new image file:
image-1.jpeg
image-2.jpeg
image-3.jpeg
image-4.jpeg
image-5.jpeg
image-6.jpeg
Using Imagemagick via command line, is there a way to loop through every other image in a directory and run magick convert image-1.jpeg image-2.jpeg +append image-combined-*.jpg?
So the result would be combined pairs of images:
image-1.jpeg image-2.jpeg -> image-combined-1.jpg
image-3.jpeg image-4.jpeg -> image-combined-2.jpg
image-5.jpeg image-6.jpeg -> image-combined-3.jpg

Answer 1

Get them all appended succinctly and in parallel with GNU Parallel and actually use all those lovely CPU cores you paid Intel for!
parallel -N2 convert {1} {2} +append combined-{#}.jpeg ::: *jpeg
where:
-N2 says to take two files at a time
{1} and {2} are the first two parameters
{#} is the sequential job number, and
::: demarcates the start of the parameters
If your CPU has 8 cores, GNU Parallel will run 8 converts at once, unless you specify say 4 jobs at a time by adding -j4.
If you are learning and just finding your way with GNU Parallel add:
--dry-run so you can see what it would do without actually doing anything
-k to keep the outputs in order
So, I mean:
parallel --dry-run -k -N2 convert {1} {2} +append combined-{#}.jpeg ::: *jpeg
Sample Output
convert image-1.jpeg image-2.jpeg +append combined-1.jpeg
convert image-3.jpeg image-4.jpeg +append combined-2.jpeg
convert image-5.jpeg image-6.jpeg +append combined-3.jpeg
On macOS, you can simply install GNU Parallel with:
brew install parallel
If you have thousands, or hundreds of thousands of files, you may run into an error Argument list too long - although this is pretty rare on macOS because the limit is 262,144 characters:
sysctl -a kern.argmax
kern.argmax: 262144
If that happens, you can use this syntax to pipe the filenames in GNU Parallel instead:
find /somewhere -iname "*.jpeg" -print0 | parallel -0 -N2 convert {1} {2} +append combined-{#}.jpeg

Answer 2

If the images are all the same size and orientation, and if your system has the memory to read in all the images in the directory, it can be done as simply as this...
magick *.jpeg -set option:doublewide %[fx:w*2] \
+append +repage -crop %[doublewide]x%[h] +repage image-combined-%02d.jpg

Answer 3

This can be scripted easily using ImageMagick. I could show you how in Unix. But if you have more than 9 images, then you may have to rename with leading zeros, since alphabetically image-10 will come before image-2. You do not mention your IM version or platform and scripting will differ depending upon OS.
Here is a Unix solution. I have images rose-01.jpg ... rose-06.jpg in folder test on my desktop (Mac OSX). Each image has a label under it with its filename so we can keep track of the files.
cd
cd desktop/test
arr=(`ls *.jpg`)
num=${#arr[*]}
for ((i=0; i<num; i=i+2)); do
j=$((i+1))
k=$((i+2))
magick ${arr[$i]} ${arr[$j]} +append newimage_${j}_${k}.jpg
done
Note that arrays start with index 0. So I use j=i+1 and k=i+2 for the images that correspond to 1,2 3,4 5,6 in the filenames from ls in the array.
The result is (newimage_1_2.jpg, newimage_3_4.jpg, newimage_5_6.jpg)

Answer 4

An alternate solution is to montage all the images together two-by-two as an array of 2x3 and then equally crop them into 3 sections vertically. So in ImageMagick, this also works since these images are all the same size.
cd
cd desktop/test
arr=(`ls *.jpg`)
num=${#arr[*]}
num2=`magick xc: -format "%[fx:ceil($num/2)]" info:`
magick montage ${arr[*]} -tile 2x -geometry +0+0 miff:- | magick - -crop 1x3# +repage newimage.jpg
The results are: newimage-0.jpg, newimage-1.jpg, newimage-2.jpg

Answer 5

Ole Tang wrote:
Fails on filenames like My summer photo.jpg
So here is the solution using ImageMagick as modified from my original post.
Images:
rose 1.png
rose 2.png
rose 3.png
rose 4.png
rose 5.png
rose 6.png
OLDIFS=IFS
IFS=$'\n'
arr=(`ls *.png`)
for ((i=0;i<6;i++)); do
echo "${arr[$i]}"
done
IFS=OLDIFS
num=${#arr[*]}
for ((i=0; i<num; i=i+2)); do
j=$((i+1))
k=$((i+2))
magick "${arr[$i]}" "${arr[$j]}" +append newimage_${j}_${k}.jpg
done
This produces:
newimage_1_2.jpg
newimage_3_4.jpg
newimage_5_6.jpg