Extend (or crop) the canvas background of an image to a multiple of a modulus (such as 32)

extend-crop-background.sh

#!/bin/bash

if [ $# -lt 1 ]; then
   echo "extend-crop-background <imagefile> [modulus [extent]]"
   echo "Extend or crop image to a multiple of modulus ±'extent'"
   echo
   echo "modulus: default 32"
   echo "extent: default=modulus"
   echo
   echo "Examples:"
   echo "extend-crop-background in.jpg 32"
   echo "extend-crop-background in.jpg 32 -32"
   echo "extend-crop-background in.jpg 100 -32"
   exit
fi

magick "$1" \
  -background none \
  -gravity center \
  -set option:xmod "${2:-32}" \
  -set option:zz "$3" \
  -set wx '%[fx:mod(w,xmod)==0?w:w-mod(w,xmod)+zz+xmod]' \
  -set hx '%[fx:mod(h,xmod)==0?h:h-mod(h,xmod)+zz+xmod]' \
  -extent '%[wx]x%[hx]' \
  -set filename:mod '%[xmod]' \
  -set filename:size '%wx%h' \
  -set filename:name '%t' \
  '%[filename:name]_mod%[filename:mod]-%[filename:size].png'

# to be used in a small script for composing small images as sprites in a bigger png
#
# #!/bin/bash
#
# if [ $# -lt 1 ] ; then
#   echo "Make a sprite sheet from images"
#   echo "Usage: make-sprite-sheet.sh *.png"
#   exit
# fi
#
# now=$(date +"%Y%m%d-%H%M%S")
# for i in "$@";do extend-crop-background.sh "$i";done
# montage -mode concatenate -background transparent *.png sprite-sheet-${now}.png