It's wrong but it's wrong in a way that causes minimal trouble and there's no better option. And if you add viewing distance explicitly, PPI+distance isn't meaningfully worse than PPD+distance, and people will understand PPI+distance better.
3840x2160@15.3" for example would be a nice even 3.0x display scale, at 287.96 PPI, and 128 PPD at 30° hfov to match the line pair resolving capability of the human eye [0] rather than the light dot resolving of 60 PPD, although of course still far from the 10x improvement over it via hyperacuity that you linked to earlier.
If 960xwhatever is okay at 12 inches, then 1366x768 wouldn't even be the baseline resolution for 15 inch laptops, it would be the baseline resolution for 17 inch laptops. That just sounds silly to me.
Assuming the laptop screen is just 20% closer goes a long way here to figuring out a good resolution. And it gives 720p to 12/13 inch laptops at 1x.
Won't deny, since again, PPD depends on your field of view.
Yes, if you shop for "resolution and diagonal size", you may as well shop for PPI directly. This just doesn't generalize to displays overall (see my other comment with a TV example), as it's not actually the right variable. Wrong method, "right" result.
> The threshold for sharp edges is much finer, and the things we put on computer displays have a lot of sharp edges.
And the cell density is even finer. It was merely an example using a known reference value that lots of people would find excellent; I didn't mean to argue that it's the be-all end-all of vision. It's just 20/20.