Even since the introduction of the iPhone 4, people seem to care a lot about pixels per inch (ppi). It was originally thought that extremely high resolutions on very small displays was a waste of pixels, or even a major drawback to UI design (the theory was that elements become too small once the resolution gets too high and the screen can’t expand with the larger amount of pixels). However, with resolution independence – a technology that allows UI elements and windows to scale with increasing pixel size independently of the display size – found in iOS, this has become a non-issue. Instead, because of the iPhone 4 we now know the concept of a “retina display” where pixel density becomes high enough that text and graphics become amazingly sharp.
Yet the iPad, for all the great features of its display, lacks “retina” pixel density – around 330 ppi on the iPhone 4. The rumors so far indicate that the next iPad will double its resolution in each direction, becoming a QXGA display (limited to the 4:3 aspect ratio). This resolution, on a 9.7″ display, would yield a pixel density of around 260 ppi, high but not retina display high. 260 ppi to the casual observer might seem high enough, but the interesting thing is that pixel density appears to have exponential effects on our vision, by which I mean increasing pixel density does not linearly affect our perception of the image, 20% increase in pixel density does not lead to 20% less perception of each individual pixel. Case in point: the Droid with a 854×480 resolution on a 3.7″ display also has a ppi of 260 – yet is distinctly NOT considered “retina display” quality, despite being nearly 80% of the iPhone 4’s ppi.
So last night I was experimenting with an online DPI/PPI calculator that, among offering many examples of electronics of differing resolution and display size, also allows you to create your own numbers for calculation. I wanted to know what resolution is actually necessary for the iPad to have a “retina display”. There are guiding factors I had to remember: horizontal and vertical resolution have to increase by the same amount (Apple wants developers to be able to easily scale their apps or have the device do it for them), and the aspect ratio of 4:3 must be maintained. So doubling the horizontal and vertical resolution of the iPad lead to a 2048×1536 resolution with a ppi of 260, but a 3X change was far too high (no display exists with that resolution). So I tried 2.5, and I yielded a resolution of 2560×1920 – nice, familiar numbers for display buffs. And lo and behold, the resulting ppi is 329.9, exactly the same as the iPhone 4.
So there you go, one day down the line I can almost guarantee we’ll see this resolution on a 9.7″ iPad.