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Hold, do not tap -- and stop spending the verdict on a frame that cannot carry it

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Jorijn worked out the technique and it changes what every number in this project
means: "press and LIFT (quick tap) is wrong, holding the sensor until it gives
the result is a 100% success rate."

The logs agree, on a properly controlled comparison. Same template, same
session, learning off for all four blocks, only the technique differing: tapped
4/15 and 3/15, held 15/15 and 15/15.

The frame data says why. Over every frame this project has a verdict for, split
at 2.5x the idle floor:

  full contact, interrupt settled     78/175 = 45% match
  full contact, interrupt asserted    28/142 = 20%
  partial,      interrupt settled      1/9   = 11%
  partial,      interrupt asserted     0/53  =  0%

A tap is caught while the finger is still arriving or already leaving. Such a
frame is not a hard verdict waiting to happen, it is a wasted one: with the
rescan budget at 0 every frame is terminal, so its rejection ends the press. 62
partial frames produced exactly one match between them.

So the tracker becomes a Schmitt trigger. A press now STARTS on settled contact
and ENDS on the finger leaving, which means a frame taken mid-landing produces
no event at all rather than a false rejection. A press that never settles simply
yields no verdict and the loop waits for the next one, which is an honest try
again. Enrolment is untouched: it passes one threshold for both and keeps its
own sample-quality gate inside the trustlet.

fptrial.sh now says hold, and defaults to fifteen presses. Instructing a tap for
its whole life is what quietly made every rate this project has quoted a worst
case, and a tap is not a case the product has -- nobody taps a phone sensor and
walks away, they rest a finger until it unlocks.
This commit is contained in:
Jorijn van der Graaf 2026-09-05 00:17:13 +02:00
commit 284282350b
3 changed files with 80 additions and 11 deletions
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42
interfaces/Fingerprintd-Engine.cppm
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@ -69,9 +69,33 @@ export namespace fingerprintd::engine {
}
}
// A SECOND, higher bar: contact good enough to spend the press's one
// terminal verdict on. Detection at 1.5x answers "is a finger there";
// this answers "is it all the way down and still".
//
// Measured over every frame this project has a verdict for (2026-09-05,
// n=379), split at 2.5x the floor:
//
// full contact, interrupt settled 78/175 = 45% match
// full contact, interrupt asserted 28/142 = 20%
// partial, interrupt settled 1/9 = 11%
// partial, interrupt asserted 0/53 = 0%
//
// A partial frame is not a hard verdict waiting to happen, it is a
// WASTED one: with max_authentication_rescan_times at 0 every frame is
// terminal, so a partial frame's rejection ends the press. 62 partial
// frames produced exactly one match. Skipping them costs essentially
// nothing and saves 61 killed presses.
static constexpr std::int32_t SettledNum = 5;
static constexpr std::int32_t SettledDen = 2;
bool Ready() const { return seen_ >= want_ && floor_ > 0; }
std::int32_t Floor() const { return floor_; }
std::int32_t Threshold() const { return floor_ * MultiplierNum / MultiplierDen; }
std::int32_t SettledThreshold() const { return floor_ * SettledNum / SettledDen; }
bool IsSettled(std::int32_t metric) const {
return Ready() && metric >= SettledThreshold();
}
// Nothing is a finger until the floor is known. An uncalibrated
// Baseline reports false for everything rather than inventing a
@ -115,8 +139,24 @@ export namespace fingerprintd::engine {
class TouchTracker {
public:
// Returns the events to report for this frame, in order.
std::vector<Event> Observe(bool finger, Mode mode) {
//
// TWO thresholds, deliberately: a press STARTS on settled contact and
// ENDS on the finger leaving. A Schmitt trigger, and the reason is
// measured -- a frame taken while the finger is still arriving matches
// 0 times in 53, and at a rescan budget of 0 that rejection is terminal
// and ends the press. Starting the press on the settled frame instead
// spends the verdict on an image that can actually carry it. A press
// that never settles produces no event at all, which is an honest
// "try again" rather than a false rejection.
//
// `settled` defaults to `present` so enrolment, which has its own
// sample-quality gate inside the trustlet, is unchanged.
std::vector<Event> Observe(bool present, Mode mode) {
return Observe(present, present, mode);
}
std::vector<Event> Observe(bool present, bool settled, Mode mode) {
std::vector<Event> out;
bool finger = prev_ ? present : settled; // enter high, leave low
bool rising = finger && !prev_;
bool falling = !finger && prev_;
if (rising)