Added post about Android display density.

know-how/hacking/android/_posts/2015-12-02-display-density.md

@@ -0,0 +1,117 @@
+---
+title: Display density on Android
+language: en
+layout: default
+created: 2015-12-02 17:19:42 +0200
+updated: 2015-12-02 17:19:42 +0200
+toc: false
+tags:
+  - know-how
+  - hacking
+  - android
+  - display
+  - resolution
+  - lcd
+---
+Methods
+=======
+
+Android supports changing the display resolution in 2 different ways.
+
+wm density
+----------
+
+This method works without root and changes the density of the *window manager*.
+You can call this command only from the adb shell.
+
+The syntax is:
+
+    wm density [ppi]
+
+E.g. the Nexus 9 has a real display density of 288ppi, so you would run:
+
+    wm density 288
+
+To reset the value to the factory setting (for the N9 that is 320), just run:
+
+    wm density reset
+
+
+build.prop
+----------
+
+The other way to change the display density is to edit the `build.prop`. This
+only works with root access.
+
+To edit the file, first make the `system` partition writable by remounting it.
+
+Then find the file `/system/build.prop` and open it in a text editor. I use
+my favourite file manager [FX File Explorer](https://play.google.com/store/apps/details?id=nextapp.fx)
+with its integrated editor for that.
+
+Find the following line (Example from Nexus 9 with currently 320ppi):
+
+    ro.sf.lcd_density=320
+
+And change the number to your desired value. BE SURE TO NOT CHANGE ANYTHING ELSE.
+Save the file and reboot your device.
+
+
+Advantages
+==========
+
+Setting the correct density value makes the display appear sharper and it may
+even make your device run better.
+
+Android knows different *densities* and if an app supplies graphics in different
+qualities, it loads them according to the detected density. This means: The
+higher the density, the larger graphics Android has to load and keep in
+memory. So reducing the density (to the real value) can make Android load
+a bit less detailed graphics and thus save RAM.
+
+Android knows these quality classes:
+
+| Class |   PPI   |
+|------:|:-------:|
+|  ldpi | 100-140 |
+|  mdpi | 141-199 |
+|  hdpi | 200-319 |
+| xhdpi | 320-340 |
+| xxhdpi|  ~480   |
+|xxxhdpi|  ~640   |
+
+As you can see, the 320ppi of the Nexus 9 makes Android use *xhdpi* resources.
+If you change the density to 318, you won't notice anything, but Android will
+suddenly use the *hdpi* resources and save memory when running apps.
+
+
+Problems
+========
+
+If you don't have root access and try to change the density using the `wm density`
+command only, it might seem to work on the first glance, but you will notice
+strange effects.
+
+If you're using SwiftKey, you'll notice it right away:
+
+![]({{ site.url }}/assets/android-dpichanged-swiftkey-broken.png)
+
+
+Other problems are more subtle (Nexus 9 at 288ppi):
+
+![]({{ site.url }}/assets/android-dpichanged.png)
+
+Compare this to the same screen at the original 320ppi:
+
+![]({{ site.url }}/assets/android-originaldpi.png)
+
+(If you're still searching: Notice the huge "chevron" icon after the
+developer's name. And also the **huge** background image.)
+
+<p><div class="noteimportant" markdown="1">
+These problems only occur if you use only one method to change the density. If
+you use **both** methods and set them to the same value, everything will look
+just fine.
+</div></p>
+
+More info at the [xda forum](http://forum.xda-developers.com/nexus-9/general/guide-little-trick-improvimg-nexus-9-t3224931).