alphafy.sh

#!/bin/sh

# Applies a greyscale image as the alpha channel to another image.
# Black will be transparent, white will be opaque.
# Useful for tilesets, ie: find . -name "*.png" -exec alphafy.sh {} \;

tile=$1
fill_tile=/mnt/white.png

echo "Tile: $tile"

mv "$tile" "$tile.bak" && \
    composite -compose CopyOpacity "$tile.bak" "$fill_tile" "$tile"