zpx.sh

function int {

	if [[ $1 = *.* ]]; then

		i=${1%.*}
		d=${1#*.}; if (( ${d:0:1} >= 5 )); then i=$[i+1]; fi

	     echo $i
	else echo $1; fi

}

function asp {

	w=$(file $1); h=$w; w=${w#*,}; w=${w%*x*}
	                    h=${h#*x}; h=${h%%,*}

	int $(echo $w $h | awk '{ print ($1/$2)*100 }')

}

function zpx {

	z=1000

	if (( $1 > $2 )); then px=($(echo $z $(int $(echo $1 $2 $z | awk '{ print $3/($1/$2) }'))   ))
	else                   px=($(echo    $(int $(echo $2 $1 $z | awk '{ print $3/($1/$2) }')) $z)); fi

	sips -z ${px[1]} ${px[0]} $png -s dpiWidth 72 $png -s dpiHeight 72 $png -s format jpeg $png --out "$1 $2"

}

dir=~/Downloads; cd $dir; sips -s format png *.jpg --out $dir; rm *.jpg

for png in $(ls *.png); do

	case $(asp $png) in 6[8-9] |      7[0-9] |  8[0-3]) zpx 12 16;;
	                    9[0-9] |     10[0-9] |     110) zpx  1  1;;
	               1[2-3][0-9] |     14[0-7]          ) zpx 16 12;;
	                   13[5-9] | 1[4-5][0-9] | 16[0-5]) zpx 24 16;;
	               2[4-8][0-9] |     29[0-3]          ) zpx 32 12;;
	                                                 *) osascript -e "tell application \"Terminal\" to display dialog \"$png\"";; esac

done; rm *.png