Fluid Ratio images

fluid-ratio.scss

// credit Rolf Timmermans
// http://voormedia.com/blog/2012/11/responsive-background-images-with-fixed-or-fluid-aspect-ratios

/* Calculate fluid ratio based on two dimensions (width/height) */
@mixin fluid-ratio($large-size, $small-size) {
  $width-large: nth($large-size, 1);
  $width-small: nth($small-size, 1);
  $height-large: nth($large-size, 2);
  $height-small: nth($small-size, 2);
  $slope: ($height-large - $height-small) / ($width-large - $width-small);
  $height: $height-small - $width-small * $slope;

  padding-top: $slope * 100%;
  height: $height;

  background-size: cover;
  -moz-background-size: cover;  /* Firefox 3.6 */
  background-position: center;  /* Internet Explorer 7/8 */
}

figure.fluidratio {
  /* This element will have fluid ratio from 4:1 at 800px to 2:1 at 300px. */
  @include fluid-ratio(800px 200px, 300px 150px);

  background-image: url(http://voormedia.com/examples/amsterdam.jpg);
}