Create Retina Display Images in Mobile Web Themes and PhoneGap Apps

Looking at my iPhone 4 screen makes me wish all my screens had such high DPI.  Everything looks so crisp.  During the development of GoodDay I realized that the icons and images I’d used in my own app were not being rendered in high resolution.  I wanted to share how I addressed that and how you could address it with your own mobile web site/theme/app as well.   Basically the solution is to use divs and set specific sized background images for the high-DPI displays.

Lets look at the edit icon for example:

Since this is a PhoneGap app it’s rendered in Webkit.  Webkit treats all screens as if they were normal DPI. So 1px on an iPhone 4 is actually 2 retina display pixels.  On the page I used the following HTML:

<div id=”goalEditButton”></div> <!– To represent the edit icon. –>

Then in the CSS I do the following:

#goalEditButton

{

height: 16px;

width: 16px;

background: url(./themes/apple/img/edit.png) no-repeat; // This is actually a 32×32 image

-webkit-background-size: 16px 16px;

}

The 32×32 image is used so the pixel resolution is doubled when it’s forced down to 16×16.   Of course if you want to save some load time for lower DPI screens then you’ll need to have a CSS file specifically for each case and switch between them ala:

<link rel=”stylesheet” media=”only screen and (-webkit-min-device-pixel-ratio: 2)”
type=”text/css” href=”retina.css” />

If you want more information and even slicker techniques (leverage jquery to switch images automatically) then I recomend reading the following articles:

http://blog.iwalt.com/2010/06/targeting-the-iphone-4-retina-display-with-css3-media-queries.html

http://aralbalkan.com/3331


3 responses to “Create Retina Display Images in Mobile Web Themes and PhoneGap Apps”

  1. Good to know as I experiment with PG.

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