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What is the paper-like display resolution for e-book reading? Meaning, the resolution where the text display is 100% indistinguishable to average human with 20/20 vision from a regular paper book.
If the answers differ between different screen types, I'm more interested in LCD but eInk answers are fine as well.
There is no display resolution "where the text display is 100% indistinguishable to average human with 20/20 vision from a regular paper book." In fact, display resolution is not the most important factor in producing text as legible as a paper book. Notice I used legible, not indistinguishable.
Start by reading Wikipedia on font rasterization. Compare that to offset printing, the process that is used to produce most printed books. In offset printing, the quality of the paper affects how much the ink spreads as it is absorbed into the paper. There's no way to duplicate that effect in a digital display.
The perceived quality of text display on digital devices is affected by the font rendering technology (i.e. Freetype, Microsoft renderer, Adobe's, etc.), the text rendering technology (usually either an HTML/CSS engine like WebKit or a PDF renderer), the particular font, and the type of display (LCD, eInk, etc.). If all the other factors are executed correctly, the display resolution is only important at smaller type sizes. However, higher display resolution are almost always better because they give the various rendering technologies more resources to use.
When ink is printed on paper, the human eye can't really differentiate dots smaller than about one-six-hundredth of an inch. Possibly those of us with the sharpest eyes might distinguish 800 dots in an inch. Thus, before the era of desktop publishing, low-end photo-typesetting equipment was typically 1000 or 1200 dpi.
Why not just 800 dpi? Well, first there was the margin of error; and second the text being set might need to be enlarged — for instance a headline might need to be larger to fit a particular width — and enlarging any specific resolution lowers the dpi.
(High-end typesetting equipment ran as high as 3000 or 3500 dpi; this enabled gradients or blends to be smoother.
You can't really compare a reflective medium like ink on paper with display media, but display resolution higher than 200 pixels per inch or so is indistinguishable under most conditions. As with print, color gradients or blends will appear smoother with even higher resolutions.
As LCD resolution goes higher than 200 ppi, you probably won't notice any difference in display or normal-sized text. However, text below 6 pts in size does seem sharper.
I haven't done any studies on type color at display resolutions between 200 and 350 ppi, but the higher available resolutions may well benefit bold and italic words within normal text.
So, to summarize: basically 200 ppi display resolution is equivalent to print on better-than-newsprint-quality paper, but variant weights and styles within a single font at the same size may benefit from "retina" displays above 300 ppi.
