There are many articles on the Web that explain what Dots Per Inch (DPI), Pixels Per Inch (PPI), and Density-independent Pixel (DP/DiP) are. Although it may be important to understand the intricate details of such technologies and terms, let's keep the scope of the book in the realms of what the basis of high density screens is and what we need to understand to create sound responsive designs.
Vectors like SVGs, Icon Fonts, or regular fonts are a visual representation of mathematical equations, thus they never lose quality, no matter their size.
In order for bitmap images to display well on high-density screens, we have to export a high-resolution version of the normal-quality image. This means that we need to create two files (or more) for every bitmap image we plan to use: one normal-quality image for non-high-density screens (standard LCD displays, old TFT monitors, some TVs, and so on) and one (or more) high...