4K resolution
As noted above, DCI 4K is 40962160, while 4K UHD is 38402160, producing a slight difference in aspect ratio rather than a significant difference in resolution. Traditional displays, such as LCD or OLED, are 3840 pixels across the screen, with each pixel being 1/3840th of the screen width. They do not overlap—if they did, they would suffer reduced detail. The diameter of each pixel is basically 1/3840th of the screen width or 1/2160th of the screen height - either gives the same size pixel. That 38402160 works out to 8.3 megapixels, the official resolution of 4K UHD (and therefore Blu-ray UHD discs).
Home cinema
By the mid-2010s, the Blu-ray Disc medium had become a common home media standard, and online video streaming sources such as Netflix and YouTube were offering a range of high definition content, including some 4K content (although various compression technologies are applied to make this streamed content feasible). The first 4K Blu-ray discs were released in 2016. By this point, 4K TVs and computer monitors were rapidly declining in price and increasing in prevalence, despite a lack of native 4K content. While many DSP systems existed, DTS-HD Master Audio remained the studio standard for lossless surround sound encoding on Blu-ray, with five or seven native discrete channels. High definition video projectors also continued to improve and decrease in price, relative to performance.