characters/skin_leg_repair.gd

@@ -41,7 +41,17 @@ class_name SkinLegRepair
 ##      against 0.97-0.99 at the knee). Leg vertices keep at most MAX_TORSO of
 ##      hips/spine influence, and the excess goes to the leg bone that already
 ##      dominates them, so the thigh follows its own bone.
-##   4. Drop any triangle still spanning the two legs below the knee. Those are
+##   4. Hang DRAPING CLOTH from the hips instead of the legs. The skirt is
+##      weighted to both thighs so it can follow them, but at a running or
+##      jumping stride the two thighs pull it in opposite directions and it
+##      collapses flat against the body — its flared volume vanishes and the
+##      whole lower body reads as squashed from the side. Cloth that is clear of
+##      both leg bone chains keeps at most MAX_DRAPE_LEG of leg influence, and
+##      the rest goes to the hips, so the skirt holds its shape and swings as a
+##      unit. This is the "model the skirt differently from the legs" case, and
+##      it is separate from the limb work above: LIMB vertices are snapped to
+##      one leg, DRAPE vertices are taken off the legs almost entirely.
+##   5. Drop any triangle still spanning the two legs below the knee. Those are
 ##      the midline band between the ankles, which has no correct pose either
 ##      way.
 ##
@@ -53,6 +63,9 @@ const TORSO_BONE_HINTS := ["hips", "spine", "pelvis"]
 ## The most torso influence a leg vertex may keep. Some is wanted — it is what
 ## rounds the hip off — but past this the thigh stops following its own bone.
 const MAX_TORSO := 0.15
+## The most LEG influence a draping cloth vertex may keep. A little lets the
+## skirt react to the legs; more than this and a split stride tears it flat.
+const MAX_DRAPE_LEG := 0.20
 ## Ignore influences below this — they are rounding, not real weighting.
 const EPSILON := 0.005
 
@@ -126,12 +139,13 @@ static func _repair_mesh(mi: MeshInstance3D, skeleton: Skeleton3D, knee: float,
 		limb_radius: float) -> Array:
 	var side := _side_map(mi.skin, skeleton)
 	var torso_bone := _torso_map(mi.skin, skeleton)
+	var hips_slot := _hips_bind(mi.skin, skeleton)
 	var surfaces: Array = []
 	var snapped := 0
 	var removed := 0
 	for s in range(mi.mesh.get_surface_count()):
 		var arrays: Array = mi.mesh.surface_get_arrays(s)
-		var r := _repair_surface(arrays, side, torso_bone, knee, chain_l, chain_r, limb_radius)
+		var r := _repair_surface(arrays, side, torso_bone, hips_slot, knee, chain_l, chain_r, limb_radius)
 		snapped += r[0]
 		removed += r[1]
 		surfaces.append({
@@ -153,6 +167,18 @@ static func _repair_mesh(mi: MeshInstance3D, skeleton: Skeleton3D, knee: float,
 	return [snapped, removed]
 
 
+## SKIN BIND index of the hips, which is what draping cloth should hang from.
+static func _hips_bind(skin: Skin, skeleton: Skeleton3D) -> int:
+	for b in skin.get_bind_count():
+		var n := skin.get_bind_name(b)
+		if n == "":
+			var bone := skin.get_bind_bone(b)
+			n = skeleton.get_bone_name(bone) if bone >= 0 else ""
+		if n.findn("hips") != -1 or n.findn("pelvis") != -1:
+			return b
+	return -1
+
+
 ## Is each bind a torso bone? Keyed by SKIN BIND index, like _side_map.
 static func _torso_map(skin: Skin, skeleton: Skeleton3D) -> Array:
 	var out: Array = []
@@ -197,7 +223,7 @@ static func _side_map(skin: Skin, skeleton: Skeleton3D) -> PackedInt32Array:
 
 
 static func _repair_surface(arrays: Array, side: PackedInt32Array,
-		torso_bone: Array, knee: float,
+		torso_bone: Array, hips_slot: int, knee: float,
 		chain_l: PackedVector3Array, chain_r: PackedVector3Array,
 		limb_radius: float) -> Array:
 	var verts: PackedVector3Array = arrays[Mesh.ARRAY_VERTEX]
@@ -290,7 +316,63 @@ static func _repair_surface(arrays: Array, side: PackedInt32Array,
 				weights[v * per + k] /= sum
 		snapped += 1
 
-	# Step 3 — drop triangles that still span the legs below the knee.
+	# Step 3 — draping cloth hangs from the hips. A skirt pulled by both thighs
+	# at a split stride collapses flat; taking it off the legs keeps its volume.
+	if hips_slot >= 0:
+		for v in verts.size():
+			var leg_total := 0.0
+			for k in per:
+				if side[bones[v * per + k]] != 0:
+					leg_total += weights[v * per + k]
+			if leg_total <= MAX_DRAPE_LEG:
+				continue
+			# ABOVE THE KNEE ONLY. Distance from the bone chain alone is not a
+			# safe test down here: chunky footwear sits further from the ankle
+			# than the limb radius, so a boot reads as "drape" and taking its
+			# leg weight away makes it trail off the hips. A skirt hem is above
+			# the knee; boots and stockings are not.
+			if verts[v].y <= knee:
+				continue
+			# Limb vertices were already handled above; this is only for cloth
+			# that hangs clear of BOTH leg bone chains.
+			if minf(_dist_to_chain(verts[v], chain_l),
+					_dist_to_chain(verts[v], chain_r)) <= limb_radius:
+				continue
+			var k_hips := -1
+			for k in per:
+				if bones[v * per + k] == hips_slot:
+					k_hips = k
+					break
+			if k_hips < 0:
+				# No hips influence yet: take over the weakest leg slot.
+				var weakest := -1
+				var weakest_w := 2.0
+				for k in per:
+					if side[bones[v * per + k]] != 0 and weights[v * per + k] < weakest_w:
+						weakest_w = weights[v * per + k]
+						weakest = k
+				if weakest < 0:
+					continue
+				bones[v * per + weakest] = hips_slot
+				weights[v * per + weakest] = 0.0
+				k_hips = weakest
+				leg_total -= weakest_w
+				if leg_total <= MAX_DRAPE_LEG:
+					continue
+			var keep: float = MAX_DRAPE_LEG / leg_total
+			for k in per:
+				if side[bones[v * per + k]] != 0:
+					weights[v * per + k] *= keep
+			weights[v * per + k_hips] += leg_total - MAX_DRAPE_LEG
+			var total2 := 0.0
+			for k in per:
+				total2 += weights[v * per + k]
+			if total2 > 0.0:
+				for k in per:
+					weights[v * per + k] /= total2
+			snapped += 1
+
+	# Step 4 — drop triangles that still span the legs below the knee.
 	var removed := 0
 	if not idx.is_empty():
 		var keep_idx := PackedInt32Array()