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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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22
implementations/main.cpp
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@ -1189,8 +1189,9 @@ public:
std::println(std::cerr, "baseline did not calibrate (floor stayed 0)");
return false;
}
std::println("idle floor = {}, finger threshold = {}", baseline_.Floor(),
baseline_.Threshold());
std::println("idle floor = {}, finger threshold = {}, settled threshold = {}",
baseline_.Floor(), baseline_.Threshold(),
baseline_.SettledThreshold());
return true;
}
@ -1373,6 +1374,10 @@ public:
std::uint32_t fid = 0;
int presses = 0;
int frames = 0;
// Frames that showed a finger arriving but were not settled enough to
// spend a verdict on. A press made only of these is a "try again", not
// a rejection.
int skippedUnsettled = 0;
};
// `accept` is the set of fids that count as a match for THIS request. The
@ -1404,7 +1409,7 @@ public:
en::TouchTracker tracker;
harvested_ = 0; // no fold has happened in this session yet
bool inPress = false, pressMatched = false, pressRejected = false;
int pressFrames = 0, rescans = 0;
int pressFrames = 0, rescans = 0, skipped = 0;
std::uint32_t pressFid = 0;
auto t0 = std::chrono::steady_clock::now();
auto msSince = [&](auto t) {
@ -1424,8 +1429,11 @@ public:
ta::BuildCapturePayload(cap);
auto c = SendCommand(app_, ta::Cmd::CaptureImage, cap);
bool finger = baseline_.IsFinger(c.metric);
// A press begins only once contact is settled; see TouchTracker.
bool settled = baseline_.IsSettled(c.metric);
fingerPresent_.store(finger);
out.frames++;
if (finger && !settled && !tracker.FingerDown()) skipped++;
std::string note;
// No recapture on the rising edge. It was tried, on the theory that
@ -1435,7 +1443,7 @@ public:
// still flagged FINGER from the first capture) and an empty image
// was reported to the matcher. A guaranteed miss on exactly the
// case it was meant to fix.
for (ta::Event ev : tracker.Observe(finger, en::Mode::Authenticate)) {
for (ta::Event ev : tracker.Observe(finger, settled, en::Mode::Authenticate)) {
if (ev == ta::Event::FingerTouched) {
inPress = true; pressMatched = false; pressRejected = false;
pressFrames = 0; rescans = 0; pressFid = 0;
@ -1521,9 +1529,11 @@ public:
else std::this_thread::sleep_for(std::chrono::milliseconds(g_frameGapMs));
}
fingerPresent_.store(false);
std::println(" verify loop: {} frames in {} ms ({} ms/frame incl. {} ms gap)",
std::println(" verify loop: {} frames in {} ms ({} ms/frame incl. {} ms gap){}",
out.frames, msSince(t0),
out.frames ? msSince(t0) / out.frames : 0, g_frameGapMs);
out.frames ? msSince(t0) / out.frames : 0, g_frameGapMs,
skipped ? std::format(", {} unsettled frame(s) skipped", skipped) : "");
out.skippedUnsettled = skipped;
if (cancel) {
SendCommand(app_, ta::Cmd::Cancel, {});
out.cancelled = true;