When 1080p screens came to phones, the general consensus was that the resolution race could be coming to an end. After all, who needs more than full HD resolution on a phone? Whether or not we need it, LG took the stage a few months ago and announced the LG G3 with a quad HD (QHD) screen clocking in at 2560x1440 pixels. The G3 is the first device in the US market with a QHD panel, but LG had to make some sacrifices to get there. So is it all marketing nonsense, or did LG win the resolution race?
More Retina than Retina
The conventional wisdom has long been that anything north of 300 pixels per inch would be sufficiently high resolution that the average human would be unable to make out the individual pixels at arm's length (the G3 is 534 PPI). This is absolutely true if you're talking about picking out pixels, but reality is a bit more muddled than that.
While 300 PPI makes it impossible to see pixels for virtually everyone, the images displayed on the screen might benefit from a higher resolution. For example, the eye can detect very small changes in the angle of a line that are well below the normal "retina resolution." Likewise, the alignment of two parallel lines can be seen with a startling degree of clarity--on the order of 4-5 times that of normal visual acuity. So, you might conceivably need 1500 PPI to account for all these cases.
A QHD screen might also perform better when it comes to rendering curves--antialiasing, basically. The mathematical relationship between discrete points (pixels) and continuous elements (lines) is murky at best, but when you toss human vision into the mix, it can be hard to come to any firm conclusions. So what does this mean? A straight line made up of pixels you can't see is just a line. However, a curve made up of pixels exactly the same size might not look continuous as the pixels will produce a very subtle aliased (jaggy) edge. It would be up to software to clean that up, and having more pixels to work means better results.
The way the eyes and brain process this visual data probably varies from person to person, but some analyses of the numbers point to roughly double the resolution requirements to prevent visible aliasing. So we're talking about 600 PPI, and the G3 gets close with 534 pixels per inch.
The bottom line is that there's SOME basis for thinking that a QHD screen could offer a better viewing experience. Although, it's definitely not going to be a marked improvement in quality like jumping past 300 PPI.