Refine the head prototype: flatten face plane, taper jaw, nose/chin relief, fix ear-displacement sign

TODO.md

@@ -85,10 +85,10 @@ the rest of §C remains the broad review checklist.
       readable nose bridge/tip/ala, a mouth with upper/lower lip volumes, ears with concha depth,
       and brow/cheek/jaw planes that survive the 256² AO bake.
       In progress: `tools/npc_parts.py build --part head_proto` displaces the shell (seg 48/rings 26)
-      into sockets, a nose bridge/tip/ala, upper/lower lips and ears; the eyeballs stay separate.
-      Vision-judged (iteration 2): the nose + ear now protrude in profile, but the cranium still
-      reads as a "ball with a nose". Next: flatten the face plane, taper the jaw/chin, widen the
-      nose bridge, and shape the back of the skull before the 1.5 m/1.85 m comparison.
+      into sockets, a nose bridge/tip/ala, upper/lower lips, a chin bump and ears; the eyeballs stay
+      separate. Five vision-judged iterations later (see `docs/3d-model-review.md` §Head prototype):
+      the nose/mouth/chin/ear now read, but the forehead plane, ear helix definition and the
+      jawline-to-neck step still need work before the 1.5 m/1.85 m comparison.
 - [ ] Compare two or three subdivision/loop variants at 1.85 m front/three-quarter/profile and a
       1.5 m portrait. Pick the variant that reads best before changing all 12 characters; keep the
       rejected variants local.

docs/3d-model-review.md

@@ -217,11 +217,19 @@ head sphere is displaced (not floating spheres) into eye sockets with an orbital
 bridge/tip/ala, upper/lower lip volumes, and extruded ears with a concha recess; the eyeballs
 stay separate. Local renders live under `tmp/npc-parts/npc_casual/head_proto_*.png`.
 
-Vision-judged after two iterations: the nose and ear now protrude in profile (mesh y-min
-−0.186 vs the −0.155 shell front), but the cranium still reads as a "ball with a nose" — the
-face plane is not flattened, the jaw/chin does not taper, and the nose reads as a narrow spike.
-Next refinement before the 1.5 m/1.85 m comparison: flatten the face plane, taper the jaw/chin,
-widen the nose bridge, and shape the back of the skull.
+Iteration log (vision-judged, profile view):
+
+1. Initial displacement too subtle — read as a "ball with props".
+2. Nose + ear visible in profile but the cranium still read as a "ball with a nose".
+3. Added face-plane flatten + jaw/chin taper; taper read, but "~90% sphere".
+4. Stronger flatten + nose + chin; nose now reads, jawline still dissolves into the skull.
+5. Fixed an ear-displacement sign bug (the ± side loop cancelled the helix); the ear now
+   protrudes (x ±0.155 vs the ±0.139 shell) but reads as a soft blob.
+
+Current state (~3/5 human-head read): the nose, mouth notch, chin bump and ear bulge read, but
+three refinements remain before the 1.5 m/1.85 m comparison — a flat forehead with a brow
+step, a defined helix/lobe on the ear, and a jawline that steps back to the neck (the neck is
+part of `_skin_body`, so the standalone head will always lack it).
 
 ## Scorecard skeleton
 

tools/npc_parts.py

@@ -105,11 +105,22 @@ def _head_proto(mats, spec):
     skin, acc, hair = mats["skin"], mats["accessory"], mats["hair"]
     white = ba.material("eye_white", textured_solid=True)
     head = ba.sphere("HeadShell", 0.155, (0, 0, 1.635), skin, seg=48, rings=26,
-                     scale=(0.92, 1.0, 1.12))
+                     scale=(0.90, 0.96, 1.08))
     ba._shape_head_mesh(head)  # brow ridge + cheekbone + jaw/chin planes
     bm = ba.bmesh.new()
     bm.from_mesh(head.data)
 
+    # Pass 1: skull shape — flatten the face plane and taper the jaw/chin so the
+    # head stops reading as an egg.
+    for v in bm.verts:
+        x, y, z = v.co
+        if y < -0.03 and 1.44 < z < 1.76:
+            v.co.y = -0.13 + (y + 0.13) * 0.10  # flatten the forward bulge
+        if z < 1.58:
+            taper = max(0.0, 1.0 - (1.58 - z) / 0.12)
+            v.co.x *= 1.0 - 0.16 * taper  # narrow the lower face
+
+    # Pass 2: features on the flattened face.
     for v in bm.verts:
         x, y, z = v.co
         ax = abs(x)
@@ -120,16 +131,26 @@ def _head_proto(mats, spec):
                 v.co.y += 0.018 * (1.0 - r / 0.036)  # recess inward (+y)
             elif r < 0.046:
                 v.co.y -= 0.010 * (1.0 - (r - 0.036) / 0.010)  # rim forward
-        # --- nose: bridge/tip protrusion plus ala wings ---
-        if y < 0 and ax < 0.048:
-            band = max(0.0, 1.0 - abs(z - 1.628) / 0.052)  # 1.576..1.680
+        # --- nose: a wide bridge/tip with ala wings, not a spike ---
+        if y < 0 and ax < 0.075:
+            band = max(0.0, 1.0 - abs(z - 1.632) / 0.055)  # 1.577..1.687
             if band > 0:
-                tip = max(0.0, 1.0 - abs(z - 1.615) / 0.022)
-                v.co.y -= (0.035 * tip + 0.012 * (band - tip)) * (1.0 - ax / 0.048)
+                tip = max(0.0, 1.0 - abs(z - 1.615) / 0.032)
+                v.co.y -= (0.060 * tip + 0.018 * max(0.0, band - tip)) \
+                          * (1.0 - ax / 0.075)
             for s in (-1.0, 1.0):
-                ar = math.hypot((x - 0.017 * s) / 0.011, (z - 1.606) / 0.011)
+                ar = math.hypot((x - 0.024 * s) / 0.014, (z - 1.604) / 0.013)
                 if ar < 1.0:
-                    v.co.y -= 0.018 * (1.0 - ar)
+                    v.co.y -= 0.022 * (1.0 - ar)
+        # --- chin: a distinct forward bump below the mouth ---
+        if y < 0 and ax < 0.040:
+            chin = max(0.0, 1.0 - abs(z - 1.548) / 0.022)
+            if chin > 0:
+                v.co.y -= 0.030 * chin
+            # jaw step: recess just below the chin so it reads as a chin-to-neck
+            step = max(0.0, 1.0 - abs(z - 1.516) / 0.014)
+            if step > 0:
+                v.co.y += 0.012 * step
         # --- mouth: recessed line + upper/lower lip volumes ---
         if y < 0 and ax < 0.030:
             if abs(z - 1.583) < 0.012:
@@ -138,17 +159,17 @@ def _head_proto(mats, spec):
                 v.co.y -= 0.014  # upper lip
             if abs(z - 1.577) < 0.010:
                 v.co.y -= 0.018  # fuller lower lip
-    # --- ears: extrude outward with a concha recess ---
+    # --- ears: extrude outward with a concha recess, lower (brow-to-nose band) ---
     for v in bm.verts:
         x, y, z = v.co
-        for s in (-1.0, 1.0):
-            r = math.hypot((z - 1.63) / 0.055, (y - 0.008) / 0.045)
-            near = abs(x) - 0.140
-            if -0.02 < near < 0.05 and r < 1.0:
-                if r < 0.35:
-                    v.co.x -= 0.014 * (1.0 - r / 0.35) * s  # concha recess (inward)
-                else:
-                    v.co.x += 0.028 * (1.0 - r) * s  # helix rim outward
+        r = math.hypot((z - 1.60) / 0.055, (y - 0.002) / 0.045)
+        near = abs(x) - 0.138
+        sign = 1.0 if x >= 0 else -1.0
+        if -0.02 < near < 0.055 and r < 1.0:
+            if r < 0.35:
+                v.co.x -= 0.016 * (1.0 - r / 0.35) * sign  # concha recess (inward)
+            else:
+                v.co.x += 0.034 * (1.0 - r) * sign  # helix rim outward
     bm.normal_update()
     bm.to_mesh(head.data)
     bm.free()