[EMSUSD-3926] Exporting skinned mesh with hidden bones creates mismatched skel:joints list and invalid jointIndices #39

/*
    recreate_issue39_scene.ms

    Rebuilds the scene from GitHub issue Autodesk/3dsmax-usd#39:
    a skinned mesh whose *deforming* bones are hidden.

    Hierarchy that gets created:

        SkelRootNode                 (Point helper, visible)
        |-- BoneA                    (bone, VISIBLE, NOT skinned)
        |   |-- BoneB                (bone, HIDDEN, skins the left/red half)
        |   +-- BoneC                (bone, HIDDEN, skins the right/white half)
        +-- SkinMesh                 (Editable Mesh + Skin bound to BoneB, BoneC)

    The point of the repro: BoneB and BoneC are hidden but drive the Skin
    modifier. Exporting with "Hidden Objects" OFF used to drop them from
    skel:joints while jointIndices still referenced them (issue #39).

    Run: MAXScript > Run Script, or drag into a viewport.
*/

(
resetMaxFile #noPrompt
units.SystemType = #Centimeters

-- --- geometry constants --------------------------------------------------
local meshLen    = 120.0   -- length along X
local meshWidth  = 24.0    -- width along Y
local meshThick  = 1.5     -- thickness along Z
local widthSegs  = 24      -- segments along X so the weight gradient is smooth
local halfLen    = meshLen / 2.0

-- --- root helper -----------------------------------------------------------
local skelRoot = Point name:"SkelRootNode" pos:[0,0,0] \
    size:30 box:true cross:true axistripod:false centermarker:false wirecolor:(color 180 180 180)

-- --- bones -----------------------------------------------------------------
-- BoneA: vertical, pointing straight down from the mesh centre. Visible,
--        acts as the parent / root joint but is NOT part of the skin.
local boneA = BoneSys.createBone [0,0,0] [0,0,-45] [0,1,0]
boneA.name   = "BoneA"
boneA.width  = 12.0
boneA.height = 12.0
boneA.wirecolor = color 226 200 96

-- BoneB: runs from centre out to the left (-X). Skins the left (red) half.
local boneB = BoneSys.createBone [0,0,0] [-halfLen,0,0] [0,0,1]
boneB.name   = "BoneB"
boneB.width  = 8.0
boneB.height = 8.0

-- BoneC: runs from centre out to the right (+X). Skins the right (white) half.
local boneC = BoneSys.createBone [0,0,0] [halfLen,0,0] [0,0,1]
boneC.name   = "BoneC"
boneC.width  = 8.0
boneC.height = 8.0

-- --- parenting (world transforms are preserved) ---------------------------
boneA.parent = skelRoot
boneB.parent = boneA
boneC.parent = boneA

-- --- mesh ------------------------------------------------------------------
-- Long thin strip centred on the origin, extended along X.
local skinMesh = Box name:"SkinMesh" length:meshWidth width:meshLen height:meshThick \
    lengthsegs:1 widthsegs:widthSegs heightsegs:1 pos:[0,0,(-meshThick/2.0)]
convertToMesh skinMesh                 -- collapse to "Editable Mesh"
skinMesh.parent = skelRoot

-- --- skin ------------------------------------------------------------------
local skinMod = Skin()
addModifier skinMesh skinMod

-- Add ONLY the deforming bones (BoneB, BoneC). BoneA is intentionally left out.
select skinMesh
max modify mode
modPanel.setCurrentObject skinMod node:skinMesh
skinOps.addBone skinMod boneB 1        -- bone index 1
skinOps.addBone skinMod boneC 1        -- bone index 2

-- Paint an explicit red->white gradient along X:
--   left  end (x = -halfLen) -> fully BoneB
--   right end (x = +halfLen) -> fully BoneC
local numVerts = skinMesh.mesh.numverts
for v = 1 to numVerts do
(
    local p  = getVert skinMesh.mesh v            -- object space ~= world (mesh at origin)
    local t  = (p.x + halfLen) / meshLen          -- 0 at left, 1 at right
    t = amin (amax t 0.0) 1.0
    local wB = 1.0 - t
    local wC = t
    skinOps.replaceVertexWeights skinMod v #(1, 2) #(wB, wC) node:skinMesh
)

-- --- visibility: hide the two deforming bones -----------------------------
boneB.isHidden = true
boneC.isHidden = true
-- BoneA, SkinMesh, SkelRootNode stay visible.

-- --- leave it looking like the screenshot ---------------------------------
select skinMesh
max modify mode
modPanel.setCurrentObject skinMod node:skinMesh
try ( subobjectLevel = 1 ) catch ()   -- enter Skin > Edit Envelopes (cosmetic)

format "Issue #39 repro scene created: SkelRootNode / BoneA / (hidden BoneB, BoneC) / SkinMesh\n"
)