This approach could produce a simple rule-of-thumb, e.g. **in my case** something like 105 PPI at 1 metre distance, 52.5 PPI at 2 metres, 210 PPI at 50cm, etc., and double that PPI for higher contrast detail, Vernier acuity, and sharpening.

To expand a little bit further for an 'average' person (which I'm not

) with 1 arc minute acuity (20/20 vision), and for the theoretically best possible visual acuity 0.4 arc minute (due to the size limits of the human eye's cones):

Rule of thumb for the required PPI for viewing (large format) output at a given distance:

- 1 arc minute at 1 metre equals 87.32 PPI.
- 0.4 arc minutes at 1 metre equals 218.3 PPI.

Divide by the viewing distance in metres to find the required minimum PPI.

For the metrically challenged this would become:

- 1 arc minute at 1 foot equals 286.48 PPI.
- 0.4 arc minutes at 1 foot equals 716.2 PPI.

Divide by the viewing distance in feet to find the required minimum PPI.

This is before upsampling to the printer driver's native resolution (to avoid low quality printer driver interpolation, and to allow output sharpening at the printer's native output resolution).

I still recommend to use twice that minimum PPI requirement if one wants to exploit output sharpening to its maximum potential, and make sure that the most critical subjects (requiring Vernier acuity) with high contrast will be accommodated with very high quality.

Here is a nice demonstration of what Vernier acuity is capable of. The best I can achieve with that test is 0.02 pixels at half a metre viewing distance, which would translate for my display resolution to some 3183 PPI at 1 metre, i.e some 25x the display PPI, and some 14.6x the maximum spatial resolution limit of 0.4 arc minutes. Therefore, the above recommendation of using 2x the PPI that the rule of thumb suggests is not unrealistic.

Cheers,

Bart