If you remember the iPhone 4 introduction, the justification for Retina Display was defined as 20/20 vision when held 10-12 from the eyes. To feasibly maintain this marketing term Apple only needs to justify that same 20/20 vision and a minimum distance you are expected to hold a tablet from your eyes.
They can make this up as they see fit, but they do have to be able to justify it or risk irrevocably weakening said marketing term. Id say about 18-22 seems about right for a tablet. Based on that criteria the PPI would need to be 156 to 191. Very doable since even 7 tablets are exceeding that lower measure.
- 3438 * (1/18) = 191 ppi
- 3438 * (1/20) = 172 ppi
- 3438 * (1/22) = 156 ppi
(Where 3438 is the scaling factor derived from a 1 arc minute visual acuity for 20/20 vision.)
Now that we have that squared away we can easily use a PPI calculator to see what difference displays would be. Heres a simple site I like to use:
http://thirdculture.com/joel/shumi/computer/hardware/ppicalc.html
- XGA: 1024 x 768 = 786,432 pixels = 132 ppi*
- SXGA: 1280 x 960 = 1,228,800 pixels = 165 ppi*
- SXGA+: 1400 × 1050 = 1,470,000 pixels = 180 ppi*
- UXGA: 1600 × 1200 = 1,920,000 pixels = 206 ppi*
Thats a lot more pixels to render even going the minimum Retina Disaply classification outlined above based on about 22 away from eyes. Still, I think the SXGA+ is actually doable on the newer Imagination Tech GPUs. Its almost 2x as many pixels of the current iPad, but Apple isnt close to using the most powerful GPU they offer. Whether that is viable for power efficiency reasons, if they can even source these displays when the current IPS displays seem to be holding the iPad production up already, of it they need to wait a year (or more) for other reasons is obviously unknown.
PS: For comparison, the iPhone 4s GPU is only pushing a 614,400 pixels.
*
Assuming a 9.7 display.