Drive SOMA fingers from the MCP/PIP/DIP joints, not the metacarpals

soma_body.py

@@ -34,6 +34,14 @@ import torch.nn as nn
 
 # SMPL-X joint index -> SOMA joint index.
 # SMPL-X hand order is index / middle / pinky / ring / thumb.
+#
+# Fingers need care: SOMA's four fingers carry one joint more than SMPL-X's.
+# `<Finger>1` sits ~3 cm from the wrist at the base of the metacarpal (the four
+# `*1` joints span only 4 cm across the palm), while `<Finger>2` sits ~9 cm out
+# on the knuckle line (6.2 cm span) — i.e. SOMA `2/3/4` are the MCP / PIP / DIP
+# joints that SMPL-X calls `1/2/3`. The metacarpals are left un-rotated, which
+# is what SMPL-X assumes anyway. The thumb chain has no extra joint, so
+# `Thumb1/2/3` map straight across.
 SMPLX_TO_SOMA = [
     1,   # 0  pelvis        -> Hips
     68,  # 1  left_hip      -> LeftLeg
@@ -60,16 +68,16 @@ SMPLX_TO_SOMA = [
     9,   # 22 jaw           -> Jaw
     10,  # 23 left_eye      -> LeftEye
     11,  # 24 right_eye     -> RightEye
-    20, 21, 22,  # left index
-    25, 26, 27,  # left middle
-    35, 36, 37,  # left pinky
-    30, 31, 32,  # left ring
-    16, 17, 18,  # left thumb
-    48, 49, 50,  # right index
-    53, 54, 55,  # right middle
-    63, 64, 65,  # right pinky
-    58, 59, 60,  # right ring
-    44, 45, 46,  # right thumb
+    21, 22, 23,  # left index   -> LeftHandIndex2/3/4
+    26, 27, 28,  # left middle  -> LeftHandMiddle2/3/4
+    36, 37, 38,  # left pinky   -> LeftHandPinky2/3/4
+    31, 32, 33,  # left ring    -> LeftHandRing2/3/4
+    16, 17, 18,  # left thumb   -> LeftHandThumb1/2/3
+    49, 50, 51,  # right index  -> RightHandIndex2/3/4
+    54, 55, 56,  # right middle -> RightHandMiddle2/3/4
+    64, 65, 66,  # right pinky  -> RightHandPinky2/3/4
+    59, 60, 61,  # right ring   -> RightHandRing2/3/4
+    44, 45, 46,  # right thumb  -> RightHandThumb1/2/3
 ]
 assert len(SMPLX_TO_SOMA) == 55 and len(set(SMPLX_TO_SOMA)) == 55