Fix the hip/thigh collapse and knee pinch: rigid thigh/shin with a short knee blend

tools/blender_assets.py

@@ -676,9 +676,17 @@ def _auto_weights(obj, layout, rigid=None):
                 if abs(p.x) > 0.23:
                     allowed.remove("Spine")
         elif p.z < 0.80:
-            allowed = ["Hips", f"Hip{side}", f"Knee{side}"]
-            if p.z < 0.66:
-                allowed.remove("Hips")
+            # Legs: keep the shin rigid to the knee and the thigh rigid to the
+            # hip, with a short blend only at the knee joint, so deep flexion
+            # (seated) no longer collapses the thigh or pinches the knee.
+            if p.z < 0.28:
+                allowed = [f"Knee{side}"]
+            elif p.z < 0.34:
+                allowed = [f"Hip{side}", f"Knee{side}"]
+            elif p.z < 0.62:
+                allowed = [f"Hip{side}"]
+            else:
+                allowed = ["Hips", f"Hip{side}"]
         elif p.z > 1.40:
             allowed = ["Spine", "Neck", "Head"]
         else: