Apple devices have been running HiDPI screens smoothly for years, while Windows and Android still struggle at times to make it work. Here's a look at what "Retina" means and why it's still important.

(1) For retinal projection systems, see virtual retinal display. (2) The LCD screen display technology from Apple, introduced with the iPhone 4 in 2010. Retina Displays have high pixel densities, and ...