Pixel densities

Devices of many different screen dimensions dominate today's mobile landscape. Some are small with 4.5-inch screens and others are quite large with 6-inch screens. Tablets, of course, are even larger at 7 inches, 8.9 inches, 10 inches, and so on (the recently released iPad Pro is 12.9 inches).

To further complicate matters, the resolutions of these screens are also very different. The physical dimension of a screen tells you very little about how many pixels it contains—a 4.5-inch screen might have a resolution of 320 x 480 pixels or it might be 640 x 960, or 1280 x 800. Slightly larger screens may have significantly more pixels – a 6-inch phone may have a resolution of 2560 x 1600.

Most developers are used to thinking about pixels as discrete units over which they have direct control. In other words, the developer has the idea that they can determine the color and brightness of each and every pixel on the screen individually. While it is indeed possible to control each pixel...