This program displays a standard shape, or optotype, (e.g. Snell E)) in various orientations and sampling a given range of sizes. An interface is provided to the human to enter their perception, which is recorded along with chosen values to be post-processed into a visual acuity measure.
On a typical computer screen (about 125 dpi), aiming for at least 10 pixels per arc minute to reach a decent precision, means almost 7 m between eye and screen. To accommodate such a setup, the input interface is not tied to the display, but rather is an embedded HTTP server, so it can be used e.g. with a smartphone.