I am starting to see resolution doesn't matter. According to the specs, Valve Index has a very low average pixel density of about 13-15ppi. Criminal. Compare that to the high end iPhone display's pixel density of 128ppi. So if we could have ocular displays around 100ppi @720p, that would look a lot better than that of Valve Index displays at its seemingly impressive 1440x1600.
It's hard to discuss a monitor being 100ppi@720p because those 2 variables are related via a third variable size. A 720p screen would be 100dpi at about 14" and indeed that is a common size for crappy laptops. A tiny screen with that PPI strapped to your face would look like shit. For instance a 2" square screen would be 200px x 200px.
A 20/20 or 1.0 visual acuity equals 1MOA(1/60deg).
A 20/20 equivalent 360 degrees panorama image is has a size of 21600x10800px minimum. For a VR headset with 120x120 degrees H/VFOV(178deg diagonally), you need a 7200x7200px panel. 2 x 7200px x 7200px x 24bpp x 144Hz = 360Gbps, or worth 4x 100GE LAG’d, or one PCIe 4.0 x23 link, or one DDR5 channel.
Our retina only needs ~8 megapixels - the same number of pixels as 4K/UHD (3840x2160 is 8,294,400 pixels).
High pixel density is only required at the middle of where our gaze is - the surrounding can be highly compressed because our eyes are not very sensitive. No pixels are required where our blind spot is, and our colour and light level sensitivities varies from fovea outwards, so there are other compression possibilities too.
If the display can "move" with your eyes (high pixel density only for fovea e.g. contact lens) then the display also only needs a limited number of pixels.
Valve index has a PPI of 598. What I assume you are talking about is PPD, or Pixel per Degree. Iphone doesn't have a fixed PPD since the distance to your eyes is not fixed.
An ocular 720p display at 100ppd would be be about an inch diagonally at 10cm from you eye balls. That's basically unuseable
