Teeth as an extracted blob contour, not a clipped band

The band filled the mouth because a band is the wrong reduction: the bright
region is a blob, and reading it as "everything above a line" throws the shape
away.

Extracting a contour reintroduces the vertex-correspondence problem that made
me avoid it, but for a blob there is a way out. Radial sampling from the
centroid along N fixed directions makes vertex k always mean "the extent in
direction k": correspondence holds by construction, the count is fixed, and
temporal smoothing cannot reorder anything. It also yields a star-shaped
reduction, which suits flat colour.

Tongue rejection, which the band had no way to express:
- pixels red relative to their own brightness are dropped (teeth are neutral)
- component choice is biased toward the top of the cavity, since area alone
  picks the tongue when the mouth is wide
- separate inner and outer controls: cavity erode pulls the sampled region off
  the lip edge, blob grow/erode resizes the found blob

Also: a knob wired in app.js but missing from index.html threw during wiring
and left a blank page with nothing useful in the console - which is exactly
what happened to teethDwell in the previous commit. el() now names the missing
id, and window.onerror surfaces it in the status line.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -65,25 +65,33 @@ is hand-drawn head plates, which this tool does not yet do.
 ## Teeth
 
 MediaPipe has no landmarks inside the lips: the inner ring bounds the cavity and
-everything within it is just pixels. So teeth come from the image — but tracing
-the bright blob would produce a new contour every frame with no vertex
-correspondence, which is precisely the boil the design exists to avoid.
+everything within it is just pixels. So teeth come from the image.
 
-So the extraction yields a **scalar, not a shape**. The teeth polygon is the inner
-lip ring clipped to a horizontal line, and only that line's height is measured
-(Otsu threshold within the cavity, scanned from the top). The silhouette is
-therefore always the mouth's own shape — stable by construction — and the only
-per-frame variable is one number, which smooths trivially. It is also how the
-shape gets drawn by hand: a band bounded by the lip.
+The hazard is vertex correspondence — a traced contour reorders between frames
+and boils. The way out for a blob specifically is **radial sampling**: march
+outward from the blob's centroid along N fixed directions and take the last pixel
+inside. Vertex *k* is then always "the extent in direction *k*", so correspondence
+holds by construction, the vertex count is fixed, and temporal smoothing is well
+defined with no reordering possible. It also produces a star-shaped reduction,
+which is what flat blocks of colour want.
 
-Presence uses hysteresis plus a minimum dwell, the same treatment plate selection
-gets, because a teeth block that blinks on and off for single frames is worse
-than one that is simply absent.
+The **teeth measurement** panel shows exactly what is sampled: region dimmed,
+kept pixels green, extracted contour amber. Tune against that, not the numbers.
 
-**Tongue** would work the same way — a shape filling the lower cavity, gated on a
-redness rather than a brightness statistic. Not implemented, because it is not
-visible in the test footage: the cavity reads as dark with a bright upper-teeth
-band and nothing else.
+| Knob | What it does |
+| --- | --- |
+| teeth contrast | Gate on the separation between the cavity's dark and bright class means. Otsu always returns *some* threshold, so this is what stops it inventing teeth in a dark mouth. |
+| cavity erode | Pulls the sampled region in from the lip edge — MediaPipe's inner lip landmarks sit slightly outside the real opening, and lips are bright. |
+| blob grow/erode | Resizes the found blob. An open pass always runs first to despeckle. |
+| tongue reject | Drops pixels red relative to their own brightness. Teeth are near-neutral; tongue is not. |
+| prefer upper | Biases component choice toward the top of the cavity. Area alone picks the tongue when the mouth is wide. |
+| teeth vertices | Radial sample count. |
+| teeth avg ±f | Temporal average over the contour. |
+| teeth dwell | Frames a presence change must persist before it takes effect. |
+
+**Tongue** as its own part would work the same way, gated on redness instead of
+brightness and biased low rather than high. Not implemented: it is not visible in
+the test footage, which reads as a dark cavity with a bright upper-teeth band.
 
 ## The plate is reference, not art