Compass Sprite Image and Scale

Question

I've spent the past 4 hours trying to find a way to create a sprite image with Compass and sass that also automatically scales each individual image for use with the background-size property.

Nothing I've found works, can't believe it's that difficult.

Does any one have a working example?

Edit: This is what I have so far

@mixin resize-sprite($map, $sprite, $percent) {
  $spritePath:    sprite-path($map);
  $spriteWidth:   image-width($spritePath);
  $spriteHeight:  image-height($spritePath);
  $width: image-width(sprite-file($map, $sprite));
  $height: image-height(sprite-file($map, $sprite));

  @include background-size(ceil($spriteWidth * ($percent/100)) ceil($spriteHeight * ($percent/100)));
  width: ceil($width*($percent/100));
  height: ceil($height*($percent/100));
  background-position: 0 floor(nth(sprite-position($map, $sprite), 2)  * ($percent/100) );
}

@mixin resize-sprite-set($map, $percent, $only...) {
  $name: sprite_map_name($map);

  @each $sprite in sprite_names($map) {
    @if length($only) == 0 or index($only, $sprite) != false {
      .#{$name}-#{$sprite} {
        @include resize-sprite($map, $sprite, $percent);
      }
    }
  }
}

The mixin returns no errors.

$my-icons-spacing: 10px; // give some space to avoid little pixel size issues on resize

@import "my-icons/*.png";

$my-icons-sprite-dimensions: true;

@include all-my-icons-sprites;

// the fun part

.small-icons { // overriding all sprites
  @include resize-sprite-set($my-icons-sprites, 40); // 40% sized
}

.some-part-of-my-site {
  @include resize-sprite-set($my-icons-sprites, 40, logo, ok); // will create overrides only for sprites "logo" and "ok"
}

I get the following error message from the above implementation when I try to compile. Via Prepros App.

remove ../images/my-icons-s9e77ab1ef1.png
   create ../images/my-icons-s9e77ab1ef1.png
    error style.scss (Line 62 of _mixins.scss: Undefined mixin 'resize-sprite-set'.)
identical ../css/style.css 

Answer 1

I've also done some research on this. This gist is what I came up with: https://gist.github.com/apauly/7917906

Update:

The solution depends on three key-parts:

1. scale width
2. scale height
3. get background-position

0. Grab the dimensions for both, the complete sprite and the single icon:

$icon-file: sprite-file($map, $icon);
$icon-width: image-width($icon-file);
$icon-height: image-height($icon-file);

$sprite-file: sprite-path($map);
$sprite-width: image-width($sprite-file);
$sprite-height: image-height($sprite-file);

1. Consider a div displaying a sprite as its background. Set background-size: 100%; to make sure, that the background sprite covers the full width of the div. If one would use width: 100%;, the result would be something like this:

+----------------+
|--|             | 
|----------------| 
|--------|       | <--- This is the sprite image we want to display 
|------|         |
+----------------+

So we need to enlarge our background to get something like this: (the div should have overflow:hidden though)

+----------------+
|---------|             | 
|-----------------------| 
|----------------|      | <---
|-------------|         |
+----------------+

To achieve that, just divide the width of the complete sprite by the width of the single icon:

width:percentage($sprite-width / $icon-width);

2. This one is basically inspired by a blog post form tkenny: http://inspectelement.com/tutorials/a-responsive-css-background-image-technique/

The resulting sass code is this:

display: block;
height: 0;
padding-bottom: percentage($icon-height / $icon-width);
background-size: 100%;

3. The rest is just some basic math to calculate the top spacing of the icon inside of the sprite as a relative value:

Get the space from the top in pixels (a negative value):

$space-top:floor(nth(sprite-position($map, $icon), 2));

Sass will need a px-value

@if $space-top == 0 {
  $space-top: 0px
}

Set the background position with percentages:

background-position:0 percentage(
  -1 * $space-top / ( $sprite-height - $icon-height )
);