Posts tagged "dpi"

Comparing CSS Media Queries vs. Application Scaling

If you haven’t heard of either of these features in Flex 4.5, here’s a quick summary:

  • CSS Media Queries – Conditionally apply styles at runtime based on the DPI and/or OS of the target device
  • Application Scaling – When the application specifies an applicationDPI value, Flex applies a scale factor to the root application. The result is an application designed for one DPI value scales to look good on another device with a different DPI value.

The Flex 4.5 reference covers the differences in the two approaches to adapting a mobile application based on DPI. The documentation also spells out the steps it takes to implement each approach.

Testing both approaches is also straightforward. Flash Builder 4.5 can simulate screen resolution and DPI when you run your application in ADL on your PC. However, it’s not the same as seeing the results on an actual device at the real physical size.

I’ve put together some quick examples to demonstrate each approach and I’ve taken a few photos of actual devices to see the final result. I have 3 devices, going left to right:

  • Motorola Droid Pro, 320×480, DPI Classification = 160
  • HTC EVO, 480×800, DPI Classification = 240
  • iPod Touch 4th Gen, 640×960, DPI Classification = 320

The picture below shows 3 different applications:

  1. Application scaling is turned ON (applicationDPI=160) in the main Application tag. What this means is that I’ve designed my application with 160 DPI devices in mind. Flex will scale the entire application by a scale factor based on the runtimeDPI classification. From left to right you can see 1x, 1.5x and 2x scaling.
  2. Application scaling is turned OFF (applicationDPI is not set). At each DPI, I’ve adjusted my slider to a fontSize value that is readable based on the runtimeDPI of the device.
  3. Application scaling is turned OFF (applicationDPI is not set) but I’ve set single explicit fontSize value that is used for all 3 DPI classifications. Do not do this. As you can see, if you set an explicit fontSize for all 3 DPI classifications, you’ll run into issues at one end or the other. In this case, fontSize 16 is far too small on the iPod Touch.


What I should have done in the last scenario is define CSS style rules with media queries that select a proper fontSize based on the DPI of the device. I’ll use a simple label as my next example. The code below has scaling turned off, but this time I’ve defined a stylename with different fontSize values per-DPI classification.

In my ViewNavigatorApplication, I’ve turned off scaling:

<s:ViewNavigatorApplication> <!-- scaling is OFF when applicationDPI is not set-->


And in my View, I’ve defined a stylename with bold and italic styles that apply at all DPI classifications. After that, I’ve defined overrides using CSS media queries. Notice that I only supplied a new fontSize value for 240 and 320. The first rule is applied at all DPIs. I’m able to omit a 160 DPI-specific rule because it gets it’s fontSize style from the first rule.

@namespace s "library://";
/* applies to all DPIs */
  fontSize: 18;
  fontWeight: bold;
  fontStyle: italic;
@media (application-dpi: 240)
  fontSize: 28;
@media (application-dpi: 320)
  fontSize: 36;
<s:Label text="Heading Style, fontSize={fontSize}" styleName="heading"/>


The photo below shows my app running on all 3 devices. The Label with the “heading” stylename shows the correct DPI-specific fontSize on all 3 devices.

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