fade-tiles.sh

#!/bin/bash

# The minimum age (in days) of tiles to modify
MIN_AGE=7
# Number of simultaneous image modifications to run, default is the number of cores
MAX_PROCS="$(grep ^processor /proc/cpuinfo | wc -l)"
# Percent of saturation to apply to new image, should be an integer from 0-100
DESATURATION=90

if [ -z "$1" ]; then
    cat <<EOF
Usage: $0 path/to/tiles/ path/to/lighting/ <etc..>
EOF
elif [[ "$1" == "--helper-mode" ]]; then
    IMAGEFILE="$2"
    MTIME="$(stat -c %y "${IMAGEFILE}")"
    mogrify -modulate "100,${DESATURATION}" "${IMAGEFILE}"
    touch -d "${MTIME}" "${IMAGEFILE}"
else
    find "$@" -mtime "+${MIN_AGE}" -type f -print0 | \
        xargs -r -0 -n 1 -P "${MAX_PROCS}" -- "$0" --helper-mode 
fi

updated with -type f