Stylus + gulp.spritesmith で複数 dpi に対応した Mixin

_mixin.styl

// import spritesmith file
@import _parts

// mixin
sprite-bg( $i_sprites, $i_ratio = 2 )

  $x = $i_sprites[2] / $i_ratio
  $y = $i_sprites[3] / $i_ratio
  $width = $i_sprites[4] / $i_ratio
  $height = $i_sprites[5] / $i_ratio
  $total_width = $i_sprites[6] / $i_ratio
  $total_height = $i_sprites[7] / $i_ratio

  width $width
  height $height
  background-image url( $i_sprites[8] )
  background-repeat no-repeat
  background-size $total_width $total_height
  background-position $x $y