diff --git a/crates/track/src/face.rs b/crates/track/src/face.rs
--- a/crates/track/src/face.rs
+++ b/crates/track/src/face.rs
@@ -186,9 +186,16 @@ fn to_layer(c: [f64; 2], b: f64, roll_deg: f64, q: [f64; 2]) -> [f64; 2] {
 
 /// Layer pixels → face frame.
 fn to_face(c: [f64; 2], b: f64, roll_deg: f64, p: [f64; 2]) -> [f64; 2] {
+    face_mapper(c, b, roll_deg)(p)
+}
+
+/// [`to_face`] with the rotation worked out once, for mapping many points.
+fn face_mapper(c: [f64; 2], b: f64, roll_deg: f64) -> impl Fn([f64; 2]) -> [f64; 2] {
     let (sn, cs) = roll_deg.to_radians().sin_cos();
-    let d = [(p[0] - c[0]) / b, (p[1] - c[1]) / b];
-    [cs * d[0] + sn * d[1], -sn * d[0] + cs * d[1]]
+    move |p| {
+        let d = [(p[0] - c[0]) / b, (p[1] - c[1]) / b];
+        [cs * d[0] + sn * d[1], -sn * d[0] + cs * d[1]]
+    }
 }
 
 /// A synthetic face's landmarks in layer pixels.
@@ -578,6 +585,44 @@ impl Window {
     }
 }
 
+/// Per pixel of a `w × h` window: not skin and inside `poly` (even-odd rule, same comparisons as
+/// [`crate::mask::inside_polygon`] at `layer(i, j)`). A scanline crosses the polygon at the same
+/// edges for every pixel of a row, so the crossings are found once per row and each pixel's
+/// parity follows from how many lie right of it.
+fn non_skin_inside(poly: &[[f64; 2]], skin: &[bool], layer: impl Fn(usize, usize) -> [f64; 2], w: usize, h: usize) -> Vec<bool> {
+    let mut feat = vec![false; w * h];
+    let n = poly.len();
+    if n < 3 {
+        return feat;
+    }
+    let mut xs: Vec<f64> = Vec::with_capacity(n);
+    for j in 0..h {
+        let y = layer(0, j)[1];
+        xs.clear();
+        let mut prev = poly[n - 1];
+        for &cur in poly {
+            let (a, b) = (cur, prev);
+            if (a[1] > y) != (b[1] > y) {
+                xs.push((b[0] - a[0]) * (y - a[1]) / (b[1] - a[1]) + a[0]);
+            }
+            prev = cur;
+        }
+        if xs.is_empty() {
+            continue;
+        }
+        xs.sort_by(f64::total_cmp);
+        let mut left = 0;
+        for i in 0..w {
+            let x = layer(i, j)[0];
+            while left < xs.len() && xs[left] <= x {
+                left += 1;
+            }
+            feat[j * w + i] = (xs.len() - left) % 2 == 1 && !skin[j * w + i];
+        }
+    }
+    feat
+}
+
 fn median(v: &mut [f32]) -> f32 {
     v.sort_by(f32::total_cmp);
     v[v.len() / 2]
@@ -857,18 +902,12 @@ impl FaceTracker {
         let b = major;
         // Non-skin components inside the face outline (shrunk a little: anti-aliased edges).
         let inner: Vec<[f64; 2]> = outline.iter().map(|p| [center[0] + 0.9 * (p[0] - center[0]), center[1] + 0.9 * (p[1] - center[1])]).collect();
-        let mut feat = vec![false; w * h];
-        for j in 0..h {
-            for i in 0..w {
-                if !skin[j * w + i] && crate::mask::inside_polygon(&inner, win.layer(frame, i, j)) {
-                    feat[j * w + i] = true;
-                }
-            }
-        }
+        let feat = non_skin_inside(&inner, &skin, |i, j| win.layer(frame, i, j), w, h);
         let min_area = (0.0006 * b * b).max(2.0) as usize;
         let mut seen = vec![false; w * h];
         let mut blobs: Vec<Blob> = vec![];
         let skin_luma = self.skin.med[0] as f64;
+        let to_face = face_mapper(center, b, roll);
         for j0 in 0..h {
             for i0 in 0..w {
                 if !feat[j0 * w + i0] || seen[j0 * w + i0] {
@@ -878,7 +917,7 @@ impl FaceTracker {
                 seen[j0 * w + i0] = true;
                 let mut pts = vec![];
                 while let Some((i, j)) = st.pop() {
-                    let q = to_face(center, b, roll, win.layer(frame, i, j));
+                    let q = to_face(win.layer(frame, i, j));
                     pts.push((q, skin_luma - luma[j * w + i] as f64));
                     for dj in -1i64..=1 {
                         for di in -1i64..=1 {
@@ -1174,6 +1213,35 @@ mod tests {
         assert!(err < 0.06 * 50.0, "max error {err}");
     }
 
+    #[test]
+    fn row_crossings_match_the_point_in_polygon_test() {
+        let (w, h) = (97, 83);
+        let off = [13.0, 7.0];
+        let layer = |i: usize, j: usize| [(20 + i) as f64 + 0.5 - off[0] + 0.25, (11 + j) as f64 + 0.5 - off[1]];
+        let star: Vec<[f64; 2]> = (0..48)
+            .map(|k| {
+                let a = k as f64 / 48.0 * std::f64::consts::TAU;
+                let r = 30.0 + 9.0 * (5.0 * a).sin() + 3.0 * (k % 3) as f64;
+                [50.0 + a.cos() * r, 40.0 + a.sin() * r * 1.3]
+            })
+            .collect();
+        // Vertices on pixel-centre rows and columns exercise the tie cases.
+        let tied: Vec<[f64; 2]> = vec![[10.0, 5.0], [70.0, 5.0], [70.0, 60.5], [40.0, 30.5], [10.0, 60.5], [10.0, 5.0]];
+        for poly in [star, tied, vec![[1.0, 1.0], [5.0, 5.0]]] {
+            let skin: Vec<bool> = (0..w * h).map(|i| (i * 7 + i / w) % 5 == 0).collect();
+            let got = non_skin_inside(&poly, &skin, layer, w, h);
+            let mut inside = 0;
+            for j in 0..h {
+                for i in 0..w {
+                    let want = !skin[j * w + i] && crate::mask::inside_polygon(&poly, layer(i, j));
+                    assert_eq!(got[j * w + i], want, "({i}, {j})");
+                    inside += want as usize;
+                }
+            }
+            assert!(inside > 0 || poly.len() < 3);
+        }
+    }
+
     #[test]
     fn tracks_a_moving_rotating_synthetic_face() {
         let (w, h) = (320, 240);