Pixel density is the specification that defines how detailed a display is. It is expressed in units of DPI (Dots Per Inch) or PPI (Pixels Per Inch). DPI is typically used for OS and applications, and PPI is typically used for LCD panel specs. For example 96dpi means that 96 dots are placed in a 1 inch (2.54 cm) length. The level of detail that you can see on the screen depends on the distance it is viewed from (e.g. viewing a monitor versus a smartphone), so whether or not a certain dpi is high resolution or not depends on the device. However in general the larger the dpi value, the higher the definition.
Retina Display is a brand name used by Apple for its series of IPS LCD and OLED displays that have a higher pixel density than traditional Apple displays. .Apple calls this mode HiDPI mode. In simpler words, it is one logical pixel = four physical pixels. Until recently most monitors typically had a pixel density* of 96 ppi. Various different company’s monitors were designed to match the display size (inch size) with the resolution, therefore most OS and applications were also made based on this 96 dpi standard. In normal display (same magnification) the image with a resolution of 1280 × 1024 is displayed with that number of pixels. In other words, 1280 horizontal rows and 1024 vertical rows of pixels are used to display it. At that time 640 horizontal rows (1920 – 1280 = 640) and 176 vertical rows (1200 – 1024 = 176) are not lit up so black areas are created at the top, bottom, left and right.
The "number of pixels", or put another way "the number of points that light up", in an LCD screen is decided, and this "number of pixels" is the "native resolution." For example, this means that a monitor with a native resolution of "1920 × 1200" lights up, or turns off, 1920 horizontal rows (dots) and 1200 vertical rows (dots) of pixels to display images.
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