A printed checkerboard with squares of 25 mm. Because the spacing of its corners is known, software can work out how a camera renders the world.
In PatternScan Pro it is needed once, for the calibration profile of a device, photographed from about twenty positions. It plays no part in digitizing itself, where the four reference tiles sit around the pattern piece instead.

Digitize sewing patterns without tracing.
Test PatternScan Pro free for 14 days with the full feature set. You need your smartphone, a printer and one pattern you always wanted digital.