http://tech.natemurray.com/2007/12/convert-white-to-transparent.html

cd images/im/masking
# doesnt work
convert -fuzz .2% -transparent white tool.png ugly_fuzz.png
composite  -compose Dst_Over -tile pattern:checkerboard ugly_fuzz.png ugly_fuzz_check.png

# doesnt work
convert -fuzz 10% -transparent white tool.png removed_too_much.png
composite  -compose Dst_Over -tile pattern:checkerboard removed_too_much.png removed_too_much_check.png

# works okay
convert tool.png -bordercolor white -border 1x1 -matte -fill none -fuzz 7% -draw 'matte 1,1 floodfill' -shave 1x1 white_floodfill.png
composite  -compose Dst_Over -tile pattern:checkerboard white_floodfill.png white_floodfill_check.png

# start real
convert tool.png \( +clone -fx 'p{0,0}' \)  -compose Difference  -composite   -modulate 100,0  +matte  difference.png

# remove the black, replace with transparency
convert difference.png -bordercolor white -border 1x1 -matte -fill none -fuzz 7% -draw 'matte 1,1 floodfill' -shave 1x1 removed_black.png
composite  -compose Dst_Over -tile pattern:checkerboard removed_black.png removed_black_check.png

# create the matte 
convert removed_black.png -channel matte -separate  +matte matte.png

# negate the colors
convert matte.png -negate -blur 0x1 matte-negated.png

# you are going for: white interior, black exterior
composite -compose CopyOpacity matte-negated.png tool.png finished.png
composite  -compose Dst_Over -tile pattern:checkerboard finished.png finished_check.png