Declaring Tablet Layouts for Android 3.2 - mild confusion

Question

According to this guide's chapter px = dp * (dpi / 160) and to that layout specifications change from buckets(physical size in inches) to "dp" units so that:

• 320dp: a typical phone screen (240x320 ldpi, 320x480 mdpi, 480x800 hdpi, etc).
• 480dp: a tweener tablet like the Streak (480x800 mdpi).
• 600dp: a 7” tablet (600x1024 mdpi).
• 720dp: a 10” tablet (720x1280 mdpi, 800x1280 mdpi, etc).

Since "dp" values depend on "px" and "dpi" values so that: dp = px / (dpi / 160) - isn't it possible that a phone device with high resolution will take a layout from w600dp folder?

For instance: Samsung galaxy s3 dp = 1280 / (303 / 160) = 670; Samsung galaxy s2 dp = 800 / (217 / 160) = 592. So, galaxy s3 is going to take "tablet-oriented" template, right. Is it okay, especially if a tablet version contains more elements and overall interface ergonomics gonna suffer from the tablet layout in case of s3 ? Why don't just stick to display buckets ?

You're probably going to recommend using "sw" key but I just want make sure that those "dp" units actually relate to sizes so that tablet-oriented template won't be shown on phone devices.

So, would you mind sharing your practical knowledge on applying the new approach ?

Thanks

Answer 1

A dp or dip is a (screen) Density Independant Pixel. The confusing part about that is that is mentions pixel . It is actually a unit similar to a centimeter or inch.

Each device defines a dpi bucket for it's screen, for example mdpi which is 160dpi. That means that you find 160 pixel across one inch on the screen. It's not 100% accurate since physical screens are usually not exactly 160 dpi (or whatever value that is defined for the bucket). But it's a value that comes close.

That means that 160dp on a perfect 160dpi screen are actually 160 pixel. On a device with a perfect xhdpi (=320dpi) screen those 160dp would be 320 pixel. ( px = 160dp * (320dpi / 160) )

To work with dp don't think in pixel. Think in inch, milimeter or what unit you prefer. The recommended 48dp rythm for UI elements for example explains that

On average, 48dp translate to a physical size of about 9mm (with some variability)

To approximate that: 50dp = one finger wide.

That hopefully explains why a typical 320dp phone screen is always smaller than a 720dp tablet screen although the smaller screen can have more pixel than the larger. The dpi / pixel thing is already factored into dp.

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How to do different layout for different screensizes? (ie phone vs table)

Use the screen size buckets ( small , ..., xlarge ) for the layout. They represent physical small to large screens. You don't need sw600dp etc unless you need to adjust very special cases.

The different dpi buckets should only concern you for images. Eg high resolution images for high resolution screens of any size in the drawable-hdpi folder.

Answer 2

S3的密度为160。