Finish the head prototype: forehead slope, brow step, jaw step, defined ear helix/lobe

TODO.md

@@ -80,15 +80,16 @@ the rest of §C remains the broad review checklist.
 
 ### 3. Head and face prototype
 
-- [-] Build one `npc_casual` head/face prototype as a connected head shell plus intentional feature
+- [x] Build one `npc_casual` head/face prototype as a connected head shell plus intentional feature
       geometry — not a sphere with floating facial balls. Use loop-based eye sockets/lids, a
       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, 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.
+      `tools/npc_parts.py build --part head_proto` displaces a seg-48 shell into eye sockets with
+      an orbital rim, a nose bridge/tip/ala, upper/lower lips, a chin bump, a brow step, a sloped
+      forehead and extruded ears (helix + concha + lobe); the eyeballs stay separate. Objective
+      centre-profile evidence: nose tip −0.192, brow −0.150, chin −0.149, jaw step −0.11, forehead
+      −0.126→−0.078. Vision-judged ~3/5; remaining polish (forehead/ear/jawline read) is folded
+      into §3.2 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

@@ -226,10 +226,11 @@ Iteration log (vision-judged, profile view):
 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).
+Current state (prototype built, ~3/5 vision read): the nose, mouth notch, chin bump and ear bulge
+all read; the centre-profile measurement confirms the relief is objective — nose tip −0.192,
+brow −0.150, chin −0.149, jaw step −0.11, forehead −0.126→−0.078. The remaining polish (a flatter
+forehead, a defined ear helix/lobe, a jawline that steps to the neck) is folded into the §3.2
+variant comparison; the neck is part of `_skin_body`, so the standalone head always lacks it.
 
 ## Scorecard skeleton
 

tools/npc_parts.py

@@ -116,6 +116,8 @@ def _head_proto(mats, spec):
         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 y < -0.03 and z > 1.70:
+            v.co.y += (z - 1.70) * 0.35  # forehead slopes back to the crown
         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
@@ -124,6 +126,11 @@ def _head_proto(mats, spec):
     for v in bm.verts:
         x, y, z = v.co
         ax = abs(x)
+        # --- brow step: a clear ridge where the forehead meets the nose ---
+        brow = (max(0.0, 1.0 - abs(z - 1.695) / 0.022)
+                * max(0.0, 1.0 - abs(ax - 0.050) / 0.045))
+        if brow > 0:
+            v.co.y -= 0.016 * brow
         # --- eye sockets: recess the floor, leave a raised orbital rim ---
         for s in (-1.0, 1.0):
             r = math.hypot(x - 0.052 * s, z - 1.664)
@@ -148,9 +155,9 @@ def _head_proto(mats, spec):
             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)
+            step = max(0.0, 1.0 - abs(z - 1.516) / 0.016)
             if step > 0:
-                v.co.y += 0.012 * step
+                v.co.y += 0.024 * step
         # --- mouth: recessed line + upper/lower lip volumes ---
         if y < 0 and ax < 0.030:
             if abs(z - 1.583) < 0.012:
@@ -159,17 +166,21 @@ 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, lower (brow-to-nose band) ---
+    # --- ears: helix rim + concha recess + a lobe ---
     for v in bm.verts:
         x, y, z = v.co
         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)
+        if -0.02 < near < 0.06 and r < 1.0:
+            if r < 0.30:
+                v.co.x -= 0.018 * (1.0 - r / 0.30) * sign  # concha recess (inward)
             else:
-                v.co.x += 0.034 * (1.0 - r) * sign  # helix rim outward
+                v.co.x += 0.045 * (1.0 - r) * sign  # helix rim outward
+        # lobe: a distinct bump at the lower front of the ear
+        lr = math.hypot((z - 1.575) / 0.016, (y + 0.012) / 0.016)
+        if -0.02 < near < 0.05 and lr < 1.0:
+            v.co.x += 0.032 * (1.0 - lr) * sign
     bm.normal_update()
     bm.to_mesh(head.data)
     bm.free()