Startle

Something is about to appear on your screen. You don't know when.

Startle waits until it sees your face, measures your calm for a moment, and then - at a random time - hits you with a scare. The front camera watches what your face does next and turns it into one number from 0 to 100.

WHAT IT MEASURES

Startle does not guess how you feel. It measures movement and time, from 478 face points tracked on your own phone:

- REACTION TIME - milliseconds from the frame that shows the scare to the first real change in your face.

- RECOIL - how fast your head moves, counted in head-widths per second, so it makes no difference how far you sit from the phone.

- EYE WIDENING - how much wider your eyelids open compared with your calm face.

- MOUTH OPENING - how far your lips part compared with your calm face.

That is why Startle spends a couple of seconds calibrating before the scare. Without a calm face to compare against, there is nothing to measure.

WHY NOT "FEAR: 87%"

Models that claim to read fear from a face are wrong roughly a third of the time, and fear is the class they get wrong most often. A percentage like that would simply be made up. Milliseconds and pixels can be checked, compared and argued over - and that is what makes a score worth beating.

PASS THE PHONE

Every run can go to a leaderboard with a name on it, so the game is simple: hand the phone to the next person and see who holds still. Reaction times include a fixed camera delay that differs between phone models, which is exactly why the leaderboard stays on your device and compares people who used the same one.

BRING YOUR OWN NIGHTMARES

Five scares are built in. Load your own images and Startle will draw from those instead - they stay in the app's private storage on this phone and disappear when you uninstall it.

EVERYTHING STAYS ON YOUR PHONE

No account, no server, no internet permission at all. Face tracking runs on the device. Recording your reaction is optional and off by default; when you switch it on, the clip stays inside the app, only the most recent one is kept, and it leaves the phone only if you tap share yourself.

BEFORE YOU START

There will be a sudden image and a loud sound. Do not use Startle if you have photosensitive epilepsy, a heart condition, or if loud surprises are a bad idea for you. Not for children. There is no strobe and no flashing anywhere in the app - the scare works by being sudden, not by flickering.

You need a front camera and reasonable light. Prop the phone against something if you can: held in your hand, your own shake counts as head movement.