gosplit is tool for split image.
go install github.com/desktopgame/gosplit/cmd/gosplit
gosplit -r $rowCount -c $columnCount -i $input_file
gosplit -r 3 -c 13 -i /Users/koya/Documents/Gimp/AlphaMapFill_14x28.png
command
gosplit -r 3 -c 13 -i ./image/AlphaMapFill_14x28.png