How One Developer Designed a Projected UI to Protect Privacy in a Public Room

A developer building a Windows desktop app for a Spanish festival group faced an unusual design challenge: the software would be projected on a wall in front of the very people whose data it displayed. The app, called Rueda de Actos, is used once a year to fairly assign participation slots across five events for 96 members of a Moros y Cristianos festival group in Alcoy, Spain. Because sensitive information like unpaid dues and penalties could be visible to the entire room, the developer chose to show blocked table cells without explaining why they were blocked. Three different eligibility rules all render as the same visual pattern — a subtle diagonal stripe — so the audience sees that a rule was applied without learning the specific reason. This deliberate ambiguity replaced the usual tooltip approach, prioritising the dignity and privacy of members over the interface's typical goal of maximum transparency.