image scaling for srcset

picture.sh

#!/usr/bin/env bash

SOURCE="$2"
ORIGINAL_SCALE="$1"
EXTENSION=$(echo "$SOURCE" | ag -o "\.[a-z]+$")
# http://hints.macworld.com/article.php?story=20031124140353993
ORIGINAL_W=$(sips -g pixelWidth "$SOURCE" | ag -o "pixelWidth:\ [0-9]+" | sed 's:[^0-9]::g')
ORIGINAL_H=$(sips -g pixelHeight "$SOURCE" | ag -o "pixelHeight:\ [0-9]+" | sed 's:[^0-9]::g')
FILENAME=$(echo "$SOURCE" | sed "s:_*[0-9]*\.[a-zA-Z]*$::")

if [[ "$ORIGINAL_SCALE" -lt 2 ]]; then
  echo "---I'm not going to inflate 1x into 3x"
  exit 1
else
  # make 1x
  width1x=$(( ORIGINAL_W / ORIGINAL_SCALE ))
  height1x=$(( ORIGINAL_H / ORIGINAL_SCALE ))
  convert -resize "$width1x"x"$height1x" "$SOURCE" "$FILENAME"_"$width1x"x"$height1x""$EXTENSION"
  diff=$(( ORIGINAL_SCALE - 1))
  while [[ $diff -gt 1 ]]; do
    width=$(( width1x * diff ))
    height=$(( height1x * diff ))
    convert -resize "$width"x"$height" "$SOURCE" "$FILENAME"_"$width"x"$height""$EXTENSION"
    (( diff-- ))
  done
fi