How do I convert ppi into dpi for Android images?

26
votes

I have started making graphics for my Android app using Adobe Photoshop. But I am unable to proceed, as the resolution in Photoshop is set in pixels per inch where as the official Google documentation says Android will require images set in dpi. I have searched the web for the conversion between the two but never ended up with any proper formula.

I know that the Android documentation describes the relation as px = dp*dpi/160. But my problem is that if I know dpi where do I get the value of dp to be used in this calculation? Or is there any assumption about the value of dp? I am confused.

5
androiddpi
The ppi set in photoshop only affects how large image prints on paper, and has no affect on how image displays in android... – Steven Byle
Hi @StevenByle, while that's true in general, it can't mean the same thing in Android and Android's phone context, can it? How much does print definition mean to Android or Android phones that all search results you can find on the Internet e.g. "HTC Sensation screen resolution" shows PPI instead of DPI? My experience in the media industry shows that people actually uses dpi for print design and Photoshop talks as well. Many use both ppi and dpi interchangeably – ericn
@fuzzybee While ppi and dpi and not equivalent (since some screen technologies use several pixels to generate a dot, etc), Android can use either. However, Android does not use this metadata for images. For instance, create a 100x100 image at 100dpi and then at 1000dpi in photoshop. While there is no difference on how they look on your screen (always 100x100), if you print them they will be diff physical sizes. Android knows the dpi of the image depending on the drawable density folder you put it in (not the setting in photoshop), drawable-hdpi = 240dpi, drawable-xhdpi = 320dpi, etc. – Steven Byle
@fuzzybee So when you look at screen specs, you may see them in ppi, which is fine, as most screen technologies translate to 1 pixel = 1 dot. Android references dots with dpi when drawing the screen in software, since 1 pixel may not be 1 dot on all displays, it's a way of normalizing the unit. Check out my blog on Understanding Density Independence in Android for more info. – Steven Byle

5 Answers

37
votes

Dp are Density independant pixels and are used to generalise the number of pixels a screen has. These are generalised figures taken from http://developer.android.com/guide/practices/screens_support.html

  • xlarge screens are at least 960dp x 720dp
  • large screens are at least 640dp x 480dp
  • normal screens are at least 470dp x 320dp
  • small screens are at least 426dp x 320dp

Generalised Dpi values for screens:

  • ldpi Resources for low-density (ldpi) screens (~120dpi)
  • mdpi Resources for medium-density (mdpi) screens (~160dpi). (This is the baseline density.)
  • hdpi Resources for high-density (hdpi) screens (~240dpi).
  • xhdpi Resources for extra high-density (xhdpi) screens (~320dpi).

Therefore generalised size of your resources (assuming they are full screen):

  • ldpi
    • Vertical = 426 * 120 / 160 = 319.5px
    • Horizontal = 320 * 120 / 160 = 240px
  • mdpi
    • Vertical = 470 * 160 / 160 = 470px
    • Horizontal = 320 * 160 / 160 = 320px
  • hdpi
    • Vertical = 640 * 240 / 160 = 960px
    • Horizontal = 480 * 240 / 160 = 720px

Edit - adding xhdpi as they are becoming more popular

  • xhdpi
    • Vertical = 960 * 320 / 160 = 1920px
    • Horizontal = 720 * 320 / 160 = 1440px

These values should be suitable for most xhdpi screens such as TVs and the Nexus 4, including the Nexus 10 (assuming they don't create a new category for this as it is 25k x 16k, don't know as I haven't got hands on one yet).

/Edit

If you use these sizes your images will look great on any screen. Be sure to define sizes in code in dp however, Android will handle the conversion described above on its own.

5
votes

I don't agree with answer by Michael Allen because the resulting resolutions for ldpi, mdpi, hdpi and xdpi doesn't satisfy the 3:4:6:8 scaling ratios for alternative bitmaps mentioned in the google docs here under 'Alternative Drawables'

http://developer.android.com/guide/practices/screens_support.html#testing

Therefore I would suggest that you take the baseline example that has a minimum size of

470 x 320 dp now using formula from same documentation we calculate full screen resolution for baseline screen size

px = dp * (dpi/160); for baseline px = dp * (160/160) = dp * 1 so px = dp. This means the full screen size for our baseline config. in pixels would be

470 X 320 px (mdpi)

now to follow the 3:4:6:8 scaling ratios for alternative drawables sizes for ldpi, hdpi and xhdpi we need to derive the unit values from mdpi. i.e.

470/4 = 117.5

320/4 = 80

divide by 4 because scaling ratio for mdpi is 4, scaling ratios for ldpi, hdpi and xhdpi are 3,6 and 8 respectively. now just multiply the unit results 117.5 and 80 with these scaling factors

ldpi

117.5 * 3 = 352.5

80 * 3 = 240

mdpi

117.5 * 4 = 470

80 * 4 = 320

hdpi

117.5 * 6 = 705

80 * 6 = 480

xhdpi

117.5 * 8 = 940

80 * 8 = 640

These sizes are now in perfect 3:4:6:8 scaling ratios.

1
votes

Yes, Dp (Density independant pixels) is all Photoshop will care about. It should not consider what the density of the pixels will be. That image of a super high number of pixels may be put on a super small high definition UI button for all it cares about. Or you may have few pixels for a retro style A0 poster of Tetris. This is because they can get scaled. Few pixels are scaled up for the large area, and many pixels scaled down for the small area on a screen.

So pick your target link to sizes screen size and use that in photoshop. (also check out who uses what... pie chart of sizes used)

For the intuition this site helped me link to blog page:

At run time, the platform transparently handles any scaling of the dp units needed, based on the actual density of the screen in use.

(the platform does the scaling- the density ratio handling is done by Android; not you) and this is extremely relevant to the UI:

Using dp units to define your application’s UI is highly recommended, as a way of ensuring proper display of your UI on different screens.

Because you want the UI elements to appear the same on all devices so it will do scaling based on the number of pixels it computes in that formula. And if the dp is scaled in a way so that it becomes too spare or dense, well that won't look good.

1
votes

As the resolution in Photoshop is set in pixels per inch where as the official Google documentation says Android will require images set in dpi.

0
votes

The same on all devices so it will do scaling based on the number of pixels it computes in that formula.