from Matt Watt
4K is high-resolution with twice the vertical and horizontal pixels compared to Full HD and refers to resolutions with approximately 4 million horizontal pixels.
Supports 4096 x 2160 pixels / approx. A 17: 9 screen that surpasses the 3840 x 2160 pixel / 16: 9 (UHD 4K) screen often used on 4K computer screens. Note the difference in horizontal resolution.
At the same time, the 4K screen environment is still in transition, so a few things should be noted, the first of which is the refresh rate issue.
4K is a (not sanely standardized) description of a number of physical pixels. It’s an approximation of the number of pixels wide something is (as opposed to the previous convention of how many pixels tall as in 480p, 720p, 1080p, etc.). Something with somewhere around 4000 pixels wide will be called 4K. So you have a variety starting from 3840 pixels up to at least 5120 pixels on “ultrawide 4K” displays, broadcasts, or video.
Ever since Vista, Windows has been gradually advancing the HiDPI compatibility of its OS and development platforms (multi-monitor compatibility was introduced from Windows 8.1). However there are still many applications that are not compatible or fully compatible due to the following reasons. In these cases, an application created in the previously standard way will be forcibly enlarged / shrunk by the OS, causing it to become blurry. Furthermore some applications may display without blurring, but the display size will be reduced.
What is pixel density (DPI or PPI)
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.
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