[bl]

I quite agree: I don't think megapixels will get us very far, if we could even formulate a consensus definition. I much prefer a perception/discrimination measure, as you said, and as it seems neuroscientists in the visual field have settled on.

Clouding the issue even further is the fact that different areas of central vision processing handle different features. Some areas are tuned to react to points, some to bars, some to grids, some to movement, some to rotation, etc. Perhaps it would be simpler to make the comparison in the other direction: "How intricate do I have to make this visual scene to fully exercise the perceptual abilities of grating-sensitive neurons in the Lateral Geniculate Nucleus of the thalamus." That way, we could put some sort of upper bound on the useful specifications of a pixelated display. We would then have to iterate over all the known feature sensitivities (bars, grids, rotations, etc.).

Display engineers surely must know that below such-and-such pixels per inch, a screen can present any reasonable perceptible pattern. So outside of casual interest, ristretto and I think that eye "megapixels" is relatively meaningless.