Retina Sprites: Ratios

What is this for?

These days, some devices like the iPhone 4, or the new Macbook Pro have screens with higher pixel density than usual. For example iPhone4’s Retina Display doubles the pixel density we use to see on handheld devices.

This change improves the sharpness of vector graphics, but not images... Why? These devices scale vector graphics like text without losing quality, but in order to make images bigger, images are automatically pixel-doubled like in the following example.

_images/nonretina.png

How can we solve this problem with high DPI devices? Basically, provide two different version of each image.

_images/retina.png

And... how can we detect wich image should we use? CSS Media Queries. Modern browsers (anything after IE 8.0) supports them, and they allow us to specify different styles based on the device-pixel-ratio of the browser.

Can glue help? Yes, using --ratios you can choose different ratios you want to build of each sprite. Glue will create one sprite for each ratio and will add all the neccesary CSS magic to make the browser use the high DPI image if the browser needs it. You can also use --retina, it’s a shortcut for --ration=2,1.

How –retina and –ratios work?

As glue cannot do magic scaling up the source images, it assumes that these images are the biggests you want to serve. (i.e. For iPhone 4 Retina these images should be 2x the final size you want), then glue will create one sprite for each ratio you set in the command line or only 2x if you use --retina:

$ glue icons sprites --retina

This command will generate the following files:

sprites
├── icons.css
├── icons.png
└── icons@2x.png
_images/sprites.png

icons.png

_images/sprites@2x.png

icons@2x.png

And this will be the content of icons.css:

.sprite-sprites-loopback,
.sprite-sprites-weather,
.sprite-sprites-magnify,
.sprite-sprites-chat{
    background-image:url(sprites.png);
    background-repeat:no-repeat
}

.sprite-sprites-loopback{ background-position:-1px -1px;width:32px;height:21px; }
.sprite-sprites-weather{ background-position:-1px -24px;width:24px;height:26px; }
.sprite-sprites-magnify{ background-position:-35px -1px;width:24px;height:24px; }
.sprite-sprites-chat{ background-position:-35px -27px;width:24px;height:22px; }

@media only screen and (-webkit-min-device-pixel-ratio: 2.0),
       only screen and (min--moz-device-pixel-ratio: 2.0),
       only screen and (-o-min-device-pixel-ratio: 200/100),
       only screen and (min-device-pixel-ratio: 2.0) {
        .sprite-sprites-loopback,
        .sprite-sprites-weather,
        .sprite-sprites-magnify,
        .sprite-sprites-chat{
            background-image:url(sprites@2x.png);
            -webkit-background-size: 60px 51px;
            -moz-background-size: 60px 51px;
            background-size: 60px 51px;
        }
}

What about if I need some other ratios?

The option --retina is only a shortcut for --ratios=2,1. You can manually use --ratios=A,B,C... to create different ones. For example you can use --ratios=2,1.5,1 to make glue build three diferent sprites:

sprites
├── icons.css
├── icons.png
├── icons@1.5.png
└── icons@2x.png

Wich ratios should I target?

Is up to you, but using 2 and 1.5 should be enough for most of the devices.

Here you have a list of suggested ratios for some famous devices, (full list):

Device Screen size dpi Suggested ratio
iPad 2048 × 1536 264ppi 2
iPhone 5/5S/5C 1136 × 640 326ppi 2
iPhone 4 960 × 640 326ppi 2
iPhone 4S 960 × 640 326ppi 2
iPad (3rd gen) 2048 × 1536 264ppi 2
MacBook Retina 2880 x 1800 220ppi 2
Xperia S 720 × 1280 342ppi 2
One X 720 × 1280 312ppi 2
EVO LTE 720 × 1280 312ppi 2
Galaxy Note 800 × 1280 285ppi 2
Galaxy SIII 720 × 1280 306ppi 2
Galaxy S4 1080 × 1920 441ppi 3
Galaxy Nexus 720 × 1280 316ppi 2
Nexus 4 768 × 1280 320ppi 2
Nexus 5 1920 x 1080 445ppi 3
Kindle Fire HDX 8.9 2560 x 1600 339ppi 1.5
Kindle Fire HD 8.9 1920 x 1200 254ppi 1.5
HTC Desire 480 × 800 252ppi 1.5
Nexus One 480 × 800 252ppi 1.5
Sensation 960 × 540 256ppi 1.5
Evo 3D 960 × 540 256ppi 1.5
Sensation XE 960 × 540 256ppi 1.5
LG Optimus 2X 480 × 800 233ppi 1.5
Defy+ 854 × 480 265ppi 1.5
Milestone 480 × 854 265ppi 1.5
Nexus S SAMOLED 480 × 800 235ppi 1.5
Nexus S LCD 480 × 800 235ppi 1.5
Galaxy S Plus 480 x 800 233ppi 1.5
Galaxy SII 480 × 800 219ppi 1.5
Galaxy Tab 600 × 1024 171ppi 1.5
iPad mini 1024 × 768 163ppi 1
iPhone 480 × 320 163ppi 1
iPhone 3G 480 × 320 163ppi 1
iPhone 3GS 480 × 320 163ppi 1
iPad (1st gen) 1024 × 768 132ppi 1
iPad 2 1024 × 768 132ppi 1
Kidle Fire 1024 × 600 169ppi 1
Galaxy Y (S5360) 240 × 320 133ppi 0.75

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