What is the minimum PPF for mask detection?

25 Pixels Per Foot (PPF) is the minimum required for mask detection that's included with automated social distancing detection (camio.com/sd).

For example, the person in the foreground is detected as wearing a mask, whereas the person in the background has too few pixels on the face to detect the mask.

There must be at least 25 pixels from the top of the head to the bottom of the chin:

Example courtesy of SDF (https://www.sdef.com/SDF)