USD Portal

Any-to-any geometry portal between Houdini, ZBrush, Blender and Maya — over USD.

One click sends geometry from any of the four apps to any other, preserving:

• object / subtool structure (names survive round trips)
• UVs
• vertex colour (ZBrush polypaint ⇄ Houdini Cd ⇄ Blender color attributes ⇄ Maya colour sets)
• assigned diffuse textures (auto-assigned on arrival: Principled Shader in Houdini, Principled BSDF in Blender, Texture Map in ZBrush, file texture in Maya)
• visibility — hidden objects / subtools / prims never travel
• real-world scale (meters convention; Maya's cm handled automatically)

The interchange artifact is a real .usd stage you can open in any USD-aware app.

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Tested with

Application	Version	Notes
Houdini	22.0.368 (Python 3.13, PySide6)	SOPs and Solaris/LOPs
ZBrush	2026.2	driven via generated ZScript, no plugin install
Blender	5.2.0 LTS	add-on (4.2+ API-compatible)
Maya	2026 + mayaUsd 0.32.0	agent script + USD Portal menu
OS	Windows 10 / 11	paths and deploy script are Windows-oriented

Older/newer versions may work but are untested. Houdini must have its Python 3.13 build (python3.13libs is where the module lives).

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Install

git clone https://github.com/baborub/usd-portal.git
cd usd-portal
powershell -ExecutionPolicy Bypass -File deploy\deploy.ps1

The deploy script:

1. Houdini — generates Documents\houdini22.0\packages\usd_bridge.json pointing at your clone (the Python/shelf load live from the repo — edit & restart, no re-copy). Edit the generated json to change the ZBrush path or cache dir.
2. Blender — copies the add-on to %APPDATA%\Blender Foundation\Blender\5.2\scripts\addons\usd_bridge_blender.py. Enable it once: Edit ▸ Preferences ▸ Add-ons ▸ "USD Portal". Preferences hold the cache dir and ZBrush path.
3. Maya — copies the agent to Documents\maya\2026\scripts\usd_bridge_maya.py and appends an autostart line to userSetup.py (idempotent).

Then restart the apps. You get:

• Houdini: a USD Portal shelf (+ tab ▸ Shelves ▸ USD Portal) with 6 tools
• Blender: View3D ▸ Sidebar (N) ▸ USD Portal panel
• Maya: a USD Portal menu in the main menu bar
• ZBrush: nothing to install — it is driven remotely via ZScript

Blender add-on and Maya agent are copies — re-run deploy.ps1 after pulling updates. The Houdini package loads live from the repo.

Requirements

• ZBrush must be running for any ZBrush leg (it executes generated ZScripts).
• Blender/Maya must be running with the add-on/agent active for their legs (a background watcher answers commands automatically).
• All apps share one cache directory (default C:\usd_bridge_cache) — make sure every app's config points at the same one.

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Usage

From Houdini (USD Portal shelf)

Tool	What it does
Get from ZBrush	Exports every visible subtool (GoZ + textures), authors a USD stage, imports it into SOPs (polygons with uv+Cd+Principled Shaders) or sublayers it into your LOP network if a LOP net is active.
Send to ZBrush	Selected SOP (split into subtools by name/path) or LOP stage → subtools in ZBrush, with vertex colour and textures assigned to Texture Map.
Send to Blender / Send to Maya	Selected SOP/LOP → USD → auto-imported on the other side.
Get from Blender / Get from Maya	The other app exports its visible selection (or all visible meshes) → imported into SOPs or LOPs.

From Blender (N-panel ▸ USD Portal)

Get from ZBrush, Send to ZBrush (OBJ + #MRGB polypaint + textures), Get from Maya, Send to Maya (USD). Houdini legs are driven from Houdini's shelf — the add-on answers automatically.

From Maya (USD Portal menu)

Get from ZBrush, Send to ZBrush, Get from Blender, Send to Blender, and Viewport: Un-tone-mapped Colors — switches Maya's view transform off ACES tone mapping so colours match ZBrush/Houdini (display only; data is untouched).

Selection & visibility rules (all apps)

• Export pool = visible objects only. A visible selection narrows it; no selection = all visible.
• ZBrush: subtool eye icon (folders honoured). Blender: viewport visibility. Maya: visibility incl. parents. Houdini: LOP invisible prims are pruned, SOP _3d_hidden_primitives group is dropped.

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What travels, exactly

Data	ZBrush	Houdini	Blender	Maya
Mesh + names	subtools	name/path attrs, prim per subtool	objects	transforms
UVs	✓	uv (vtx) ⇄ primvars:st	st	st / map1
Vertex colour	polypaint	Cd ⇄ primvars:displayColor (linear)	color attributes	colour sets (sRGB, converted)
Diffuse texture	Texture Map	Principled Shader / UsdPreviewSurface	Principled BSDF image	file texture / usdPreviewSurface
Units	—	meters	meters	cm (×100 auto)

Not carried (yet): normal/displacement maps, polygroups→GeomSubset, subdiv creases, animation, full material graphs beyond the diffuse texture.

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Colour & unit conventions (why round trips don't drift)

• USD displayColor is linear; Houdini Cd and Blender color attributes are linear — direct mapping.
• Maya colour sets are display-referred (sRGB) — mayaUsd itself imports displayColor with an sRGB encode. The agent keeps everything coherent: ZBrush #MRGB bytes go in/out raw, and colour sets are decoded back to linear when promoted to displayColor on export.
• ZBrush reads OBJ vertex colours as sRGB → senders pre-encode linear Cd.
• Scale: 1 unit = 1 meter everywhere; the Maya agent bakes ×100/÷100 on the ZBrush OBJ leg, mayaUsd converts the USD legs.
• Winding: authored USD is true right-handed (Houdini's opposite winding is reversed at authoring time, faceVarying primvars included), so Maya/Blender see correct normals. Sends to ZBrush auto-correct winding by a signed-volume test — geometry truth, regardless of where the mesh came from.

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Troubleshooting

Symptom	Fix
"X did not respond within 120 s"	The target app isn't running, or its add-on/agent isn't active, or its cache dir differs from the sender's.
Texture arrives upside-down	Flip toggles at the top of the relevant agent file (SEND_FLIP_TEXTURE_V / GET_FLIP_TEXTURE_V).
Normals inverted in ZBrush	SEND_FLIP_NORMALS in usd_bridge.py (should be automatic — see winding note above).
Maya viewport looks washed out	USD Portal menu ▸ Viewport: Un-tone-mapped Colors (Maya's default ACES view transform tone-maps the display).
Vertex colours invisible in Maya but the set has data	The mesh is bound to a usdPreviewSurface material whose VP2 shader ignores CPV — the agent rebinds textureless meshes to the default shading group automatically; for old meshes: cmds.sets(shape, e=True, forceElement='initialShadingGroup').
ZBrush: "Interface item could not be found"	Usually means the button is disabled in the current state (e.g. SubTool:Duplicate outside Edit mode), not that the path is wrong.
Everything black in Solaris after Get from Blender	Fixed automatically (Blender's world DomeLight is no longer exported; displayColor is wired into materials) — make sure you're on the current version.

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Architecture (short version)

                     ┌──────────────── C:/usd_bridge_cache ────────────────┐
 ZBrush ◄─ ZScript ─►│  goz/  send/  tex/        canonical .usd stages     │
   ▲                 │  blender/{cmd,done}.json   maya/{cmd,done}.json     │◄─► Maya agent
   └─ OBJ+#MRGB ────►│        (atomic JSON command protocol)               │    (QTimer)
                     └──────────────────────────────────────────────────────┘
 Houdini package ◄────────── pxr authoring / usdimport / sublayer ─────────► Blender add-on
 (shelf, live)                                                               (watcher, panel)

• ZBrush has no scripting API for file-targeted USD, so its legs run over GoZ/OBJ driven by generated ZScript, and the USD is authored by the peer app.
• Houdini⇄Blender⇄Maya legs are pure USD + a tiny cmd.json/done.json protocol.
• Every producer normalizes its output (colour primvars → displayColor, empty materials stripped, invisible prims pruned) so consumers stay simple.

Repository layout

package/usd_bridge/            Houdini package (python3.13libs, shelf, icons)
package/blender/addons/        Blender add-on
package/maya/scripts/          Maya agent
deploy/deploy.ps1              one-shot installer for all three apps
tests/test_usd_writer.py       head-less round-trip suite (hython)

Tests

Head-less round-trip suite (no ZBrush needed), run with Houdini's hython:

"C:/Program Files/Side Effects Software/Houdini 22.0.368/bin/hython.exe" tests/test_usd_writer.py

61 checks: USD authoring, materials, promotion/wiring fixes, winding, visibility, command protocol. Expected: RESULT: PASS.

Roadmap

• Normal / displacement maps (ZBrush Multi Map Exporter)
• Polygroups ⇄ USD GeomSubsets
• Update-existing-subtool-by-name on send (true sculpt iteration)
• Linux/macOS deploy scripts

License

MIT © 2026 Eugene Fokin