USD Asset Pipeline

When to use

• Catalog USD assets from a folder tree (sizes, shaders, prim counts).
• Replace placeholder geometry (cubes/spheres/bboxes) with real assets.
• Build set-dressed scenes from modular libraries.
• Validate headless rendering compatibility (MDL vs UsdPreviewSurface).
• Cube-prototype to real-asset swap workflows.

Core Concepts

The Placeholder-to-Asset Pipeline

Real-world USD scene building follows this pattern:

1. Prototype with cubes — Layout spatial zones using colored UsdGeom.Cube meshes
2. Catalog assets — Measure every candidate USD asset (bbox, shaders, prim count)
3. Map blocks to assets — Match placeholder types to appropriately-sized real assets
4. Place with offset correction — Reference assets at block positions, correcting for asset bbox center offset
5. Validate renders — Vision model + domain expert scoring
6. Iterate — Fix overshoot, corridor intrusion, scale mismatches

Why Bbox Offset Correction Matters

Most USD assets are NOT centered at origin. A rack asset might have its bbox center at (46.7, 104.6, 4.5) — if you place it at the target position (112.0, 20.0, 0.0) without correction, it lands 46.7m east and 104.6m north of where you want it.

Formula:

translate_x = target_x - asset_bbox_center_x
translate_y = target_y - asset_bbox_center_y
translate_z = -asset_bbox_min_z  (puts asset base on ground plane)

Phase 1: Asset Discovery & Measurement

Script Pattern (Kit Python — No Renderer Needed)

from pxr import Usd, UsdGeom, UsdShade
import os

def measure_asset(path):
    """Measure a USD asset: bbox, shader type, prim count."""
    if not os.path.exists(path):
        return None

    stage = Usd.Stage.Open(path)
    dp = stage.GetDefaultPrim()
    if not dp:
        children = list(stage.GetPseudoRoot().GetChildren())
        dp = children[0] if children else None
    if not dp:
        return {"error": "no default prim"}

    bc = UsdGeom.BBoxCache(Usd.TimeCode.Default(), [UsdGeom.Tokens.default_])
    r = bc.ComputeWorldBound(dp).ComputeAlignedRange()
    mn, mx = r.GetMin(), r.GetMax()

    if mn[0] > 1e30:
        return {"error": "invalid bbox"}

    mpu = UsdGeom.GetStageMetersPerUnit(stage)
    w, d, h = (mx[0]-mn[0])*mpu, (mx[1]-mn[1])*mpu, (mx[2]-mn[2])*mpu
    cx = (mn[0]+mx[0])/2
    cy = (mn[1]+mx[1])/2
    cz = mn[2]  # base of asset

    # Check for UsdPreviewSurface (headless-compatible)
    has_preview_surface = False
    for p in stage.Traverse():
        if p.IsA(UsdShade.Shader):
            sid = p.GetAttribute("info:id")
            if sid and sid.Get() and "Preview" in str(sid.Get()):
                has_preview_surface = True
                break

    prims = sum(1 for _ in stage.Traverse())

    return {
        "width": w, "depth": d, "height": h,
        "center": (cx, cy, cz),
        "mpu": mpu,
        "prims": prims,
        "dual_shader": has_preview_surface,  # True = renders everywhere
        "shader_tag": "dual" if has_preview_surface else "MDL-only"
    }

Batch Discovery Pattern

import glob

def catalog_assets(root_dir, extensions=(".usd", ".usda", ".usdc")):
    """Recursively find and measure all USD assets in a directory tree."""
    results = {}
    for ext in extensions:
        for path in glob.glob(f"{root_dir}/**/*{ext}", recursive=True):
            info = measure_asset(path)
            if info and "error" not in info:
                name = os.path.basename(path).replace(".usd", "").replace(".usda", "")
                results[name] = {**info, "path": path}
    return results

Key Learnings

• Use Usd.Stage.Open() not Sdf.Layer.FindOrOpen() — Sdf fails silently on binary .usd crate files, returns default mpu=1.0
• Always check mpu — Some assets use cm (mpu=0.01), some use meters (mpu=1.0). Scale measurements accordingly.
• Invalid bbox (min > 1e30) means the asset didn't compose — usually missing references or payloads
• Kit Python (kit/python/bin/python3) is fastest for measurement — no renderer startup needed
• For bbox in Kit runtime (SimulationApp), define a temp prim with reference, update a few frames, then compute bbox — more reliable for complex compositions

Phase 2: Shader Compatibility Check

The MDL Problem on Headless arm64

Rule: For headless rendering pipelines, ONLY use assets with UsdPreviewSurface fallback (dual-shader) or native UsdPreviewSurface.

Xvfb Discovery

Running Isaac Sim under DISPLAY=:99 (Xvfb virtual framebuffer) instead of the locked real display :0 produces non-black renders for some MDL assets. Not fully reliable but worth trying:

# Start Xvfb
Xvfb :99 -screen 0 1920x1080x24 &>/dev/null &

# Run with virtual display
DISPLAY=:99 isaac-sim.sh --exec script.py

Identifying Dual-Shader Assets

Look for these patterns in USD:

• info:id = "UsdPreviewSurface" on any Shader prim → headless-safe
• info:mdl:sourceAsset without UsdPreviewSurface sibling → MDL-only, headless-unsafe
• Lightspeed-processed assets typically = MDL-only
• "Collected" Dematic assets often = dual-shader

Phase 3: Placeholder-to-Asset Mapping

Strategy

1. Group placeholder cubes by name prefix (e.g., CvL001→CvL, BRk045→BRk)
2. For each prefix, find the best-fit asset by:
   • Similar function (racks→rack assets, conveyors→conveyor assets)
   • Compatible size (asset shouldn't massively overshoot the placeholder zone)
   • Dual-shader compatibility (headless rendering requirement)
3. Document the mapping table before building

Mapping Table Format

| Block Prefix | Count | Placeholder Size | Asset | Asset Size | Shader | Notes |
|---|---|---|---|---|---|---|
| BRk | 352 | 3.5×1.2×5.0m | ASRS_Racks_Center | 2.36×2.82×8.29m | dual | Per-block, no scaling |
| CvL | 190 | 2.0×4.0×1.5m | Conveyor_09 | 0.94×6.72×1.66m | dual | Roller conveyor |

Size Philosophy

Use natural asset sizes, NOT scaled-to-cube. Scaling assets to match cube dimensions destroys visual density and realism. Place at the block's XY position with the asset's natural dimensions.

Exception: If an asset is dramatically larger than its zone (e.g., 83m assembly in a 20m zone), use smaller modular pieces instead.

Phase 4: Placement Script Pattern

SimulationApp Runtime Placement

from isaacsim import SimulationApp
app = SimulationApp({"headless": True, "width": 1920, "height": 1080,
                      "renderer": "RayTracedLighting"})

import omni.usd
from pxr import Gf, UsdGeom, Usd
from collections import defaultdict

stage = omni.usd.get_context().get_stage()

# 1. Compute asset bbox in Kit runtime (more reliable than offline)
def get_asset_bbox(stage, asset_path, app):
    """Reference asset temporarily to get accurate bbox."""
    bc = UsdGeom.BBoxCache(Usd.TimeCode.Default(), [UsdGeom.Tokens.default_])
    name = os.path.basename(asset_path).replace(".", "_")
    test = stage.DefinePrim(f"/BBoxTest_{name}", "Xform")
    test.GetReferences().AddReference(asset_path)
    for _ in range(5): app.update()
    r = bc.ComputeWorldBound(test).ComputeAlignedRange()
    mn, mx = r.GetMin(), r.GetMax()
    stage.RemovePrim(f"/BBoxTest_{name}")
    if mn[0] > 1e30:
        return None
    return {"cx": (mn[0]+mx[0])/2, "cy": (mn[1]+mx[1])/2, "cz": mn[2]}

# 2. Collect visible cube blocks by prefix
def collect_blocks(stage):
    xf_cache = UsdGeom.XformCache(Usd.TimeCode.Default())
    blocks = defaultdict(list)
    for prim in stage.Traverse():
        if not prim.IsA(UsdGeom.Cube): continue
        img = UsdGeom.Imageable(prim)
        if img.ComputeVisibility(Usd.TimeCode.Default()) == "invisible": continue
        name = prim.GetName()
        prefix = ""
        for c in name:
            if c.isdigit(): break
            prefix += c
        mtx = xf_cache.GetLocalToWorldTransform(prim)
        pos = mtx.ExtractTranslation()
        blocks[prefix].append({
            "name": name, "path": str(prim.GetPath()),
            "x": pos[0], "y": pos[1], "z": pos[2]
        })
    return blocks

# 3. Place assets with offset correction
def place_assets(stage, blocks, asset_map, asset_bboxes, module_name):
    root = stage.DefinePrim(f"/World/{module_name}", "Xform")
    placed = 0

    for prefix, asset_path in asset_map.items():
        block_list = blocks.get(prefix, [])
        if not block_list: continue
        bb = asset_bboxes.get(asset_path)
        if not bb: continue

        group = stage.DefinePrim(f"/World/{module_name}/{prefix}", "Xform")

        for b in block_list:
            prim = stage.DefinePrim(
                f"/World/{module_name}/{prefix}/{b['name']}", "Xform")
            xf = UsdGeom.Xformable(prim)
            # OFFSET CORRECTION — the key insight
            tx = b["x"] - bb["cx"]
            ty = b["y"] - bb["cy"]
            tz = -bb["cz"]  # ground the asset
            xf.AddTranslateOp(UsdGeom.XformOp.PrecisionDouble).Set(
                Gf.Vec3d(tx, ty, tz))
            prim.GetReferences().AddReference(asset_path)
            placed += 1

        # Hide original cubes
        for b in block_list:
            orig = stage.GetPrimAtPath(b["path"])
            if orig:
                UsdGeom.Imageable(orig).MakeInvisible()

    return placed

Hierarchy Convention

/World/
  Module1/          # Racks
    VNA/
      VNA001        # Individual asset reference
      VNA002
    BRk/
      BRk001
  Module2/          # Conveyors, sorters
    CvL/
      CvL001
    Sort/
      Sort001
  Module3/          # Safety, humans

Phase 5: Animation & Articulation

Animation Baking

Use bake_waypoints() from spatial-reasoning skill to animate robots, humans, or objects along paths.

# In spatial-reasoning skill:
def bake_waypoints(xform_op, waypoints, speed_mps, fps=30, mpu=1.0):
    """Create keyframed animation for XformOp along a list of (x,y,z) points."""
    time_count = len(waypoints)
    time_samples = [i/fps for i in range(time_count)]

    for i, wp in enumerate(waypoints):
        t = time_samples[i]
        xform_op.Set(Gf.Vec3d(wp[0]*mpu, wp[1]*mpu, wp[2]*mpu), t)

Articulation Builder

For robot USD:

from pxr import Usd, UsdGeom, UsdPhysics, Gf, Sdf

# Creates a nested articulation tree with joints
stage = Usd.Stage.CreateNew("robot.usd")

root = UsdGeom.Xform.Define(stage, "/Robot")
UsdPhysics.ArticulationRootAPI.Apply(root.GetPrim())

# Base link
base = _create_box(stage, "/Robot/base", size=(2.5, 1.2, 0.4), mass=2000.0)
UsdPhysics.RigidBodyAPI.Apply(base)

# Mast base (fixed to chassis front)
mast_base = _create_box(stage, "/Robot/base/MastBase", size=(0.1, 1.0, 2.0), mass=200.0)
UsdPhysics.RigidBodyAPI.Apply(mast_base)
_create_fixed_joint(stage, "/Robot/base/MastBase/FixedJoint",
        body0="/Robot/base", body1="/Robot/base/MastBase",
        local_pos=Gf.Vec3f(1.3, 0, 0.3))

# Inner Mast (prismatic lift)
inner_mast = _create_box(stage, "/Robot/base/MastBase/InnerMast", size=(0.08, 0.9, 1.8), mass=100.0)
UsdPhysics.RigidBodyAPI.Apply(inner_mast)
_create_prismatic_joint(
    stage, "/Robot/base/MastBase/InnerMast/LiftJoint",
    body0="/Robot/base/MastBase", body1="/Robot/base/MastBase/InnerMast",
    axis="Z", lower_limit=0.0, upper_limit=3.0,
    drive_stiffness=1e6, drive_damping=1e4)

UV Mapping & Materials

For textures:

• Use sphereUV mapping for global assets
• Use linear for flat planes (floors, walls)
• Always use UsdPreviewSurface as fallback
• Never use MDL-only materials for headless

Phase 6: Validation

Color Key for Placeholder Flow

Format Validation Checklist

• All assets have .usd, .usda, or .usdc extension
• All paths use forward slashes /
• No .. relative paths
• mpu = 1.0 for all assets (script validates scale)
• No ./ or ../ syntax in references
• Topology: artifacts must not be nested under Xform if empty

Asset Recommendation (Based on Scan)

First, scan all assets with catalog_assets(), then:

• Filter: dual_shader is True
• Sort: prims < 5000
• Assign: Match bbox dimensions within 20% tolerance of placeholder cube
• Reject: All blockpallet_a*, palletstack_a* — these are not real assets

Hard-Won Lessons

1. Never scale assets to match cube dimensions — destroys visual density. Use natural sizes.
2. Large assemblies (>20m) rarely fit block clusters — use smaller modular pieces instead.
3. Always correct for bbox center offset — most assets aren't origin-centered.
4. Lightspeed-processed assets = MDL-only = BLACK on headless arm64. Only use "Collected" dual-shader variants.
5. Tote_01, Pallet_Pile, AMR_Table = MDL-only — they'll place but render black. Use Scissor_Lift or Electrical_Panel as functional placeholders.
6. Kill ALL kit processes before new Isaac Sim launch — zombie processes cause 90-170s cold starts.
7. Clean /dev/shm/carb-* between restarts to prevent SIGKILL.
8. SimulationApp headless requires explicit DomeLight + DistantLight — GUI adds viewport lights automatically, headless does NOT.
9. Xvfb (DISPLAY=:99) can improve MDL rendering over locked desktop (:0), but not a universal fix.