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When designing an Android application, dpi scale is used (from mdpi to xxxhdpi, as presented on material.io); but on https://material.io/devices/ there's a list of many Android-based devices for which physical px/in density is very different.

Why do 6 "constants" represent all variety of screen densities on different devices?

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In the devices list on the right you can sort the list by pixel ratio.

 - 4:    1 device  <- xxxhdpi
 - 3.5:  3 devices
 - 3:    9 devices <- xxhdpi
 - 2.6:  2 devices
 - 2:   28 devices <- xhdpi
 - 1.5: 10 devices <- hdpi
 - 1.3:  4 devices
 - 1:   10 devices <- mdpi

So they select the ones that have more number of devices, which have a difference with the previous of .5 or 1. We could argue that rather than 4 they could have chosen 3.5 for xxxhdpi, but I guess more devices will be release from now on with 4.

So for example:

  • Nexus 7 ('13) has a pixel ratio of 2 and a physical screen of 1200x1920px. So it's effective screen is 600x1920dp (as every two pixels will act as one).

  • Surface Book has a pixel ratio of 2 and a physical screen of 3000x2000px. So it's effective screen is 1500x1000dp (as every two pixels will act as one).

  • Perfectly put. Also, its not like Google is resistant to introducing new density inflection categories as more devices hop into a particular density bracket. For example, as more devices started popping up in the 1.3 x mdpi bracket, Google introduced the tvdpi to cater to the same (though this category generally caters to the Smart TV apps). – ikartik90 Jan 13 '17 at 7:55

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