Kinematic modeling and interference-analysis tooling for STEP-based X-ray spectrometer stage stacks. Open Cascade reads the CAD hierarchy and Drake handles motion, visualization, and collision queries.

The current reviewed model is subassembly *43841 from drawing DSG-000040389. It contains an EPIX detector stage, three crystal stacks, and three polycapillary stacks with 22 controllable joints.

Set up already? Open the repository in VS Code, open a terminal with Ctrl+Shift+`, and start the collision viewer:

uv run slac-collision cad/DSG-000040389/reviews/43841-stage-stack.inventory.yaml

It opens a Meshcat page at http://localhost:7000 in your browser. Everything it can do is in Collision detection. First time here? Start at Setup instead.

Everything below happens in a single VS Code window. Step 1 is Windows-only groundwork; on Linux, open this repository in VS Code and start at step 2. From there on, every command goes into the VS Code integrated terminal, which you open with Ctrl+Shift+`.

Run every unlabelled command block, in order. Anything that is not part of the normal path is labelled in the text introducing it, or at the top of its section: Only if / Only when for something you run in one specific case, Optional for something you can skip, Fallback for a workaround, and Reference for a command listed for lookup rather than for following along.

Do this before anything else. Twin Lab is a Linux project: Drake publishes no Windows wheels, and every command in this README is a bash command. VS Code on Windows opens PowerShell by default, and PowerShell cannot run them. A copy-paste of sudo apt update into PowerShell fails with sudo : The term 'sudo' is not recognized, and source ~/.bashrc fails the same way, because those are Unix shell commands, not Windows ones. WSL 2 gives you a real Ubuntu system where they work.

The goal is one VS Code window whose terminals, extensions, file explorer, and search all run inside Ubuntu. Getting there costs one elevated command and one reboot; after that you never open a separate terminal application again. You need VS Code installed on Windows, not inside WSL.

a. Install WSL. wsl --install requires administrator rights and VS Code's terminal cannot elevate itself, so for this one step start VS Code elevated: close it, press the Windows key, type code, and choose Run as administrator on Visual Studio Code. Open the integrated terminal with Ctrl+Shift+`. It is PowerShell, which is the right shell here and nowhere else. Run:

wsl --install -d Ubuntu

b. Reboot. WSL does not work until Windows restarts. From the same terminal:

Restart-Computer

c. Create your Linux user. Reopen VS Code normally this time; the administrator rights were only for step a. Open a terminal, which is PowerShell again, and start Ubuntu inside it:

wsl -d Ubuntu

The first launch asks for a UNIX username and password. These are new credentials for Linux, unrelated to your Windows login, and nothing appears on screen while you type the password. You need it for sudo later, so pick something memorable. When it finishes, the prompt becomes something like you@LCLS-PC12345:~$: that is bash, running in Ubuntu, inside VS Code. Type exit to return to PowerShell.

d. Connect the whole window to Ubuntu. Open the Extensions view with Ctrl+Shift+X, search for WSL, and install it. Then press Ctrl+Shift+P, run WSL: Connect to WSL, and wait for the status bar at the bottom left to read WSL: Ubuntu.

That is the whole point of the setup. Every terminal you open from now on (Ctrl+Shift+`) is Ubuntu bash rather than PowerShell, and everything else the editor does — editing, search, extensions, debugging — happens on the Linux side too. You should not need PowerShell again. Confirm in a fresh terminal:

uname -srm

It should print something like Linux 5.15.167.4-microsoft-standard-WSL2 x86_64. A PS C:\> prompt or a not recognized error means the window is not connected; check the status bar and re-run WSL: Connect to WSL.

One thing still leaves the window: the viewers open a localhost URL in your normal Windows browser. WSL forwards the port for you, so no Linux desktop or X server is involved.

The viewers open that page in your browser themselves on startup, and print the URL as well so there is still something to click if the browser cannot be launched. Under WSL there is no Linux browser to hand it to, so open_in_browser() in src/twin_lab/meshcat_ui.py passes the URL to the Windows default browser through explorer.exe. Closing the tab does not stop the viewer; reopen it from the URL, or from the Ports panel next to the terminal, where the port is listed as Meshcat viewer.

WSL gives you two separate filesystems, and knowing which one you are standing in is most of what makes WSL confusing at first.

Ubuntu has its own disk with its own root, /. Your account lives at /home/<your-linux-username>, which bash abbreviates as ~. Linux paths use forward slashes, have no drive letters, and are case-sensitive, so Source.stp and source.stp are different files. Reference, for whenever you lose track of where you are:

pwd     # print the directory you are in
cd ~    # go back to your Linux home

Your Windows drives are still reachable, mounted under /mnt. C:\ appears as /mnt/c, so a browser download at C:\Users\you\Downloads\DSG-000040389.stp is /mnt/c/Users/you/Downloads/DSG-000040389.stp from Ubuntu. That is how you hand a Windows file to a Linux command without copying it anywhere.

Clone into the Linux side, not /mnt/c. Step 3 below uses ~/src/Twin-Lab, which is on Ubuntu's own disk. Working under /mnt/c instead means every file read crosses a Windows-to-Linux translation layer: builds and Git operations run many times slower, and Linux file permissions do not survive the trip. The rule of thumb is that Linux tools want Linux files.

Because the window is connected to WSL, VS Code's own File > Open Folder dialog browses the Ubuntu filesystem, so the repository is reachable from the GUI like any other project. If you ever need it from Windows itself, File Explorer can browse to \\wsl$\Ubuntu\home\<your-linux-username>.

In the VS Code terminal, which is now Ubuntu bash:

sudo apt update
sudo apt install -y git git-lfs pipx
git lfs install
pipx install uv
pipx ensurepath
source ~/.bashrc

Three things happen here. git lfs install must run before you clone: the 88 MiB STEP files are stored in Git LFS, and a clone made without it silently gives you small text pointers instead of CAD. pipx install uv installs uv, the tool that manages this project's Python version and packages. source ~/.bashrc reloads the shell so uv is on your PATH right away instead of only in the next terminal you open.

Confirm before continuing:

uv --version

uv is not packaged in the Ubuntu repositories, and the uv documentation's curl ... | sh line pipes a downloaded script straight into a shell, so a hijacked host or bad DNS answer would run arbitrary code as your user. pipx installs the official PyPI release into its own isolated environment, records a version you can audit with pipx list, and removes cleanly with pipx uninstall uv.

Optional, only if you intend to model a stack other than the XCS polycapillary assembly. Fork first and clone your fork, so your catalog entries, inventories, and STEP files stay yours to change and the reviewed 43841 model is not in your way. Open github.com/slaclab/Twin-Lab, click Fork, and create the fork under your own account or organisation. Then use your fork's URL in place of the slaclab one in the clone below.

What to edit lists the two reviewed inputs that describe an assembly: config/stage-catalog.yaml for the stage models themselves, and a per-drawing inventory under cad/<drawing>/reviews/. The tooling is not specific to 43841; that inventory is simply the one assembly reviewed so far.

Clone:

mkdir -p ~/src && cd ~/src
git clone https://github.com/slaclab/Twin-Lab.git
cd Twin-Lab

~/src is just a folder for checkouts inside your Linux home, so the clone lands at ~/src/Twin-Lab.

Only if you cloned a fork, keep a link back to the original so you can still pull fixes:

git remote add upstream https://github.com/slaclab/Twin-Lab.git
git fetch upstream

Check that the CAD came down as real geometry rather than an LFS pointer:

ls -lh cad/DSG-000040389/source.stp

The size should be about 88M, in which case carry on. A few hundred bytes means Git LFS was not active for this clone. See Large files for the background.

Only if the size is wrong:

git lfs install
git lfs pull

Now point the window at the repository so the file explorer, search, and every new terminal start there:

code -r ~/src/Twin-Lab

-r reuses the current window rather than opening a second one. VS Code reloads with the project open and its terminal already at the repository root, which is where the remaining commands expect to run. File > Open Folder does the same thing through the GUI.

uv sync --all-extras

That one command reads .python-version and uv.lock, downloads the exact Python this project is tested against, creates .venv, and installs the pinned CAD, Drake, collision, and dev dependencies. It takes a few minutes the first time. Drake only publishes wheels for specific Python versions, which is why the interpreter is pinned rather than taken from the system.

Install the Python extension from the Extensions view (Ctrl+Shift+X). Because the window is connected to WSL, install it into WSL: Ubuntu rather than locally; VS Code offers the right target automatically. Then reload the window with Ctrl+Shift+P > Developer: Reload Window.

That is the whole step. The repository ships a .vscode/settings.json that pins the interpreter to ${workspaceFolder}/.venv/bin/python and turns on terminal activation, so from now on opening this folder is enough: the status bar shows the .venv interpreter, and every new terminal (Ctrl+Shift+`) opens with (.venv) already in the prompt. There is nothing to activate by hand, in this shell or any future one.

Confirm in a fresh terminal:

which python

It should print /home/<your-linux-username>/src/Twin-Lab/.venv/bin/python. If it prints /usr/bin/python3, or nothing at all, the pinned interpreter has not been applied: run Python: Select Interpreter from the Command Palette and choose the one at ./.venv/bin/python.

What this buys you is editor-side: working imports, go-to-definition, and inline errors for twin_lab and Drake. Commands stay written as uv run … throughout this README, which works whether or not the environment is active, so there is only ever one form to copy.

uv run pytest -q

Expect 53 passed in roughly 15 seconds. The suite exercises the CAD manifest, inventory remap, SDF compiler, and collision plumbing without opening a viewer. If this passes, setup is done.

Every command below is prefixed with uv run. Step 5 already puts you in the environment, but the prefix is kept everywhere so a copied line also works in a plain terminal, on a machine without the Python extension, or in a script — and so there is never a question of whether the right Python is selected. Run them in the VS Code terminal, which already opens at the repository root.

This is the point of the model. Quick start, from the repository root:

uv run slac-collision cad/DSG-000040389/reviews/43841-stage-stack.inventory.yaml

The command opens a http://localhost:7000 Meshcat page in your normal browser, and prints the URL too in case it needs reopening. First load takes about 20 seconds, and the window stays blank until the console says the geometry is loaded. Note that the very first run also has to build the collision hulls, which takes far longer; see what the first build costs. Drag any joint slider and the background colour tells you the state of the pose immediately.

The collision viewer drives a Drake plant compiled from the same reviewed inventory, so the geometry drawn on screen and the geometry checked for interference are the same kinematics.

Checking can be switched off at any time with the Collision detection: ON (click to disable) toggle button, which turns the window into a plain slider-driven viewer with Drake's normal sky background. The same command therefore covers both jobs; clicking the toggle back on repaints the state for the current pose.

The Animation: OFF (click to start) toggle and the two Auto motion sliders described under animated motion are available here too, so a whole sweep can be checked for interference without touching a slider. With checking left on, each animation frame is evaluated, which is the fastest way to find the poses that actually collide.

The offending parts light up in place with the same code: yellow inside the warning band, red where they touch. The highlight is drawn from the convex hulls Drake actually tested, so it wraps the reviewed part and stays visible through the transparent enclosure. It follows the part as the sliders move and clears itself as soon as the pair separates, which makes a crowded stack searchable without reading the pair list.

The worst three part pairs are listed by reference ID in the Meshcat controls panel, for example TOUCHING 1: P844 <-> P850, so the offenders are identifiable without leaving the browser. Distances are deliberately omitted there; a live number would rebuild the panel on every slider step. The terminal carries the numbers, one line per state change, and Log clearance report dumps the worst 25 pairs with both the part IDs and their owning links.

Only interference is a hard finding. close depends entirely on the Clearance warning band (mm) slider, so it is a design-review aid rather than a pass/fail. At the reviewed home pose the assembly is close at the default 5 mm band and only goes clear below about 0.9 mm; the stack really is that tightly packed, so home is reported as close rather than clean.

Shallow interference is not automatically real. Drake collides with the convex hulls, which are always slightly larger than the parts, so a penetration of a millimetre or two can be the decomposition's own error. See how wrong are the hulls? for the tool that measures that error budget per part, and press Verify contact against CAD to take that error back off the pairs in front of you.

The viewer builds convex geometry for you, so nothing extra is required.

Optional, only when you want a collision-enabled SDF package rather than the viewer:

uv run slac-compile-sdf \
  cad/DSG-000040389/reviews/43841-stage-stack.inventory.yaml \
  --with-collisions --collision-mode convex

Drake's proximity queries do not see a concave mesh. The signed-distance support table states it outright:

Meshes are represented by the convex hull of the mesh, therefore the results for Mesh are the same as for Convex.

Every clearance number this tool reports comes from that query, so handing Drake the enclosure as a single mesh means handing it a solid block: the interior fills in and anything inside it reports contact. Splitting each part into convex pieces and declaring them with <drake:declare_convex/> is the only way to get honest distances.

Drake ships no decomposition tool: pydrake.geometry provides the Convex shape, which consumes a piece and takes the hull of whatever it is given. That is deliberate, since decomposition is slow, offline, and wants caching. So the question is which external tool to use, not whether to use one.

The cold run is genuinely expensive. Measured on the reviewed 43841 inventory, which is 187 sub-parts totalling 1.0 M triangles:

Do not expect more cores to rescue this. The median part is only about 1,400 triangles, while spawning a worker, importing CoACD, and running its size-independent tree search costs the equivalent of roughly 15,000. Per-part overhead dominates the run, not geometry.

A progress bar reports percent complete and an ETA weighted by that setup cost plus triangle count. Weighting by triangles alone under-predicted the real build by nearly 4x, because the largest parts are dispatched first.

Workers are sized automatically from CPU count and free memory. CoACD parallelizes internally with OpenMP, so one worker is not one core; two threads per worker measured fastest, and the run keeps workers times threads inside the machine.

Optional, only if you want the machine back while the build runs:

uv run slac-collision cad/DSG-000040389/reviews/43841-stage-stack.inventory.yaml \
  --decomposition-workers 2

Interrupting a build is safe. Every finished sub-part writes a marker, so a re-run resumes; only the parts in flight when you killed it are repeated. Because those are dispatched largest-first, they are also the most expensive ones, which is a good reason to let a nearly-finished build finish.

Results are cached under .cache/twin_lab/convex-collision/, keyed on the source mesh mtime and size plus the decomposition settings (threshold, max_hulls, seed). Later runs start immediately, and the cache is worth keeping across branches.

It is invalidated only when the STEP is updated and the meshes are re-tessellated, or when --threshold or --max-hulls changes. A re-tessellation that produces byte-identical output is recognised by hash, so rebuilding the viewer cache alone does not force a re-decomposition.

The compiled package under exports/ carries a build stamp recording the scene meshes, the collision mode, and the inventory decomposition block it was built from. slac-collision recompiles whenever that stamp no longer matches, so editing a per-part override reaches the viewer without --rebuild.

A convex piece can only ever be at least as big as the material it wraps, so every decomposition overshoots: bolt holes fill in, small concavities get spanned, and the hull surface sits slightly proud of the CAD surface. Drake collides with that overshoot, not with the part, so a reported contact of a millimetre or two may be entirely the decomposition's own error. This command measures that error per part, which is what turns a red readout into either a false positive or a real finding:

uv run slac-hull-audit

It reads the decomposition cache directly, so it needs no viewer and no compiled package — only a build that has already run. Each cached hull is compared against the tessellated part it replaces, and the parts are printed worst fit first.

Bulge is signed against the CAD surface, so hull vertices buried inside solid material count as zero rather than as error; without that, a well-fitted part reads as badly bulged. Volume overlap is estimated by sampling, so vol x moves in the third decimal between runs.

The practical rule when triaging a clearance report: a touching pair whose penetration is smaller than the sum of the two parts' bulge values, plus the 2 mm tessellation deflection already in the meshes, is inside the model's error budget and is a candidate for ignored_pairs rather than an interference finding. A pair that penetrates well past that budget is real.

The table says how badly a part fits; the three viewers say where. They are flags on the same command, and each opens a http://localhost:7000 page in your browser, exactly like the collision viewer. Press Escape in the browser or Ctrl-C in the terminal to stop.

The ten worst parts, one per click, which is the review to run if you run only one:

uv run slac-hull-audit --tour

No index and no slider: the audit already knows which parts fit worst, so it picks them and shows them in order, one at a time, framed by the camera. Next part (right arrow) advances and wraps round at the end; Previous part (left arrow) goes back. The arrow keys do the same, so a whole review is ten keystrokes. The panel repeats the numbers for the part on screen — its rank, its volume ratio and hull count, its bulge, and its gap — so the shape and the row that flagged it are read together. --tour-count changes how many parts are toured, and --sort decides what "worst" means, so --tour --sort gap tours the parts with the most missing material instead.

Every part where it actually sits, for a whole-assembly overview:

uv run slac-hull-audit --assembly

The whole assembly is drawn with the translucent hulls layered over the grey CAD mesh, worst fit first by index.

The tour carries the same two hull buttons, so bulge shading is available there too. Both colourings are uploaded once and swapped by visibility, so the toggle is instant. The red end of the ramp defaults to 2 mm of outward bulge and moves with --bulge-scale-mm. The grey end is deliberately held out to a quarter of that scale: every hull vertex bulges a little at the tessellation deflection, so a ramp anchored at zero paints the whole assembly amber and separates nothing.

One part at a time by index, when you already know the row you want:

uv run slac-hull-audit static_A003 --view

The Part index slider steps through the worst parts in isolation, framing the camera on each, and the hull button cycles solid, wireframe, hidden. Use --view-limit to change how many parts are uploaded (12 by default).

The measurement itself is the slow part, so each part's distances are cached in an audit.npz beside the hulls they judge, keyed to the source mesh digest and the decomposition settings. A re-run costs about a second, and only re-decomposed parts are re-measured. Reach for --refresh only after the metrics themselves change.

Knowing the hulls are proud is one thing; taking that error back off a specific reported contact is another. The collision viewer can do it on demand. Press Verify contact against CAD and every touching pair in the current pose is re-checked against geometry closer to the CAD than the hulls Drake collided, and the result is printed to the terminal you launched from. This is the real output at the reviewed home pose:

--- CAD re-check of touching pairs ---
  P1355 <-> P1091: hulls -2.69 mm; CAD meshes intersect -> CONTACT
  P1355 <-> P1048: hulls -2.37 mm; CAD meshes intersect -> CONTACT
  P1355 <-> P1090: hulls -2.10 mm; CAD meshes 1.18 mm apart -> explained by hull proudness
  P1355 <-> P1054: hulls -1.59 mm; CAD meshes 0.18 mm apart -> explained by hull proudness
  P759 <-> P784: hulls -1.34 mm; CAD meshes intersect -> CONTACT
  P056 <-> P1089: hulls -1.15 mm; CAD meshes intersect -> CONTACT

Two other line shapes appear when the exact distance cannot be had: local proudness 0.98 mm -> +0.56 mm when it falls back to the audited numbers, and nothing cached to check it against -> unverified when it has neither.

Note what that output actually says: four of the six are real. The proudness correction does not explain most of the home-pose contacts, so they are genuine touching in the assembled CAD rather than decomposition artefacts. Home is an assembled state, so by-design contact there is expected — but it has to be confirmed part by part and recorded, not assumed to be noise.

The button is a button rather than a live readout on purpose. The check costs about 13 ms per pair once the meshes are cached, and roughly 95 ms on the first press while they are parsed. That is fine for a review step and not fine for every frame of the 20 Hz detector. It is only ever run on pairs already flagged as touching. Pairs merely inside the warning band are left alone: the correction exists to tell a decomposition artefact from an interference, and a pair with clearance is neither.

Two corrections sit behind it, and the stronger one wins:

The first needs slac-hull-audit to have been run at least once, since it reads the audit.npz the audit writes. Without it the pair comes back unverified rather than silently uncorrected. The second needs nothing but the cached tessellations, which the decomposition already depends on, so in practice it is the one that answers.

Run against each other on the home-pose contacts, the two agree on four of six pairs, and on the other two the audited proudness is the more pessimistic: it calls contact where the exact distance finds 1.18 mm and 0.18 mm of real clearance. That is the expected direction and the safe one. Bulge is sampled at hull vertices, and a hull face stands off the CAD by more in its interior than at its corners, so the correction systematically understates itself. Read a Tier 0 explained as trustworthy and a Tier 0 contact as "not yet ruled out".

The exact distance is a true triangle-to-triangle minimum over the triangles within 20 mm of the two witness points Drake reported, capped at 400 triangles a side so the pair arrays stay bounded. Both limits are comfortable: raising them to 2000 triangles and 80 mm on the home-pose contacts does not move a single result. Interpenetration is tested for separately, because the closest-feature minimum between two triangles is only the distance when they are disjoint — an edge passing clean through a face has candidate distances that are all positive.

Read the verdicts as:

explained is evidence, not a clearance measurement. It says the flag came from the collision geometry rather than the hardware, which is a prompt to look at the pair in the viewer and, if it is genuinely by design, to add it to ignored_pairs with the reason recorded. It is not a statement that the parts clear each other by the printed amount: that number still inherits the 2 mm tessellation deflection, and a mesh built by inscribing facets is slightly undersized on convex surfaces. Treat sub-millimetre results as "too close to call from the model" and go to the CAD.

If a pair keeps coming back CONTACT and the CAD says otherwise, the decomposition is too coarse there rather than too proud, and the lever is preprocess_resolution on that part — see convex decomposition above.

Drake automatically ignores pairs that share a body, sit either side of one joint, or belong to the same welded subgraph, so a reported pair is a real finding rather than bookkeeping noise. Parts that are in contact by design go in the ignored_pairs block of the stage inventory, which already exists and looks like this:

ignored_pairs:
  - pair: [P1091, P1355]
    reason: touching at reviewed CAD home (-2.69 mm)

That block is currently seeded from the home-pose report. The reviewed CAD home is an assembled state, so contact there is pre-existing rather than something motion caused; baselining it is what keeps the indicator off red at home and reserves red for interference the stages actually create. Those seven pairs have not been individually validated as by-design, so re-review them if a stack is re-modelled. Run Verify contact against CAD at home before adding to this block: a pair that comes back explained is an artefact of the collision geometry, and recording it as expected contact hides a decomposition problem instead of a design one.

Optional, only when the pairs you want to exclude live elsewhere: pass --ignore-file to read the block from another YAML file instead.

Optional. This viewer is an alternative to the collision viewer, not a step after it. Reach for it when you want kinematics without the collision plant, since it is lighter and starts faster:

uv run slac-stage-cad \
  cad/DSG-000040389/reviews/43841-stage-stack.inventory.yaml

The first run builds meshes under .cache/twin_lab/stage-cad/. Later runs reuse that cache. Use the North/Middle/South Crystal and Polycap controls in Meshcat; Reset to home restores every reviewed home position.

Both viewers carry the same controls. Besides the per-joint sliders, one toggle button and two sliders drive a continuous demo animation of the whole stack:

Each joint swings sinusoidally about its reviewed home. Joints are phase-staggered around the cycle so the stack does not translate as one block and stage-to-stage interactions are visible. The manual sliders track the animation live, so you can stop on any frame by clicking the toggle off and then nudge individual joints from there. Reset to home also switches the toggle back off.

The range slider is a travel heuristic, not a clearance guarantee. To check an animated pose for real interference, run the animation inside the collision viewer above, which evaluates every frame.

Every viewer carries a view cube in the bottom left corner, the same navigation every CAD package puts there. Click a face to look straight down an axis, an edge to look down the diagonal between two, or a corner for the three-quarter view; the camera swings round to it and keeps the zoom it already had. The red, green, and blue arrows through the cube are the model's own X, Y, and Z axes, so a Twin-Lab view and a CAD view can be checked against each other by their axes rather than by eye. Face names follow the CAD package: the front of the assembly faces -Y, right is +X, top is +Z.

Two keys reframe the whole model rather than just turning it:

Chrome keeps Ctrl-T for its own new tab and never passes it to the page, which is why the unmodified keys do the same thing.

Both look in from the near left corner, the same corner the CAD package presents, so putting the two side by side is a like-for-like comparison. The camera sits at +X -Y +Z.

The framing is measured, not fixed: the key takes the bounding box of everything on screen, aims at its centre, and stands back a little over twice the box radius. Move the joints and press it again and it reframes around wherever the stack has got to.

Isometric view in the collision viewer does the same from the panel, measured from the collision hulls rather than the render. Expect it to think for about a second on the 43841 stack, since it walks all 5000-odd hulls.

None of this touches the model or the clearance checking, only the camera, so the mouse still works normally afterwards. For a clean plate in a screenshot, clear Collision detection first so no parts are lit yellow or red.

Only when a new STEP revision arrives. Nothing in this section is part of first-time setup. For this reviewed polycap assembly, use the dedicated helper instead of running manifest refresh, remap, and cache rebuild manually. Pass the path to the new STEP file and the helper copies it into the repository for you:

uv run slac-refresh-43841 \
  /path/to/DSG-000040389.stp \
  --rebuild-viewer-cache

Omit the path if the replacement STEP is already sitting at cad/DSG-000040389/source.stp. Under WSL, a file downloaded on the Windows side is reachable at /mnt/c/Users/<your-user>/Downloads/DSG-000040389.stp.

What this command does:

1. Copies the replacement STEP into cad/DSG-000040389/source.stp when you pass a path.
2. Backs up the previous manifest to cad/DSG-000040389/manifest.previous.json.
3. Regenerates cad/DSG-000040389/manifest.json from the new STEP.
4. Remaps cad/DSG-000040389/reviews/43841-stage-stack.inventory.yaml using cad/DSG-000040389/reviews/43841-stage-stack.aliases.yaml.
5. Rebuilds the cached viewer scene, because --rebuild-viewer-cache was passed.

After it finishes, verify the result with:

uv run slac-stage-cad \
  cad/DSG-000040389/reviews/43841-stage-stack.inventory.yaml

Optional, only when you need to hand the model to someone outside this repository. Build the portable SDF package:

uv run slac-compile-sdf \
  cad/DSG-000040389/reviews/43841-stage-stack.inventory.yaml

The shareable ZIP is written to:

exports/DSG-000040389.43841-stage-stack.sdf-package.zip

Unzip it in MATLAB and run load_in_matlab. See SDF sharing for details.

cad/DSG-000040389/source.stp is roughly 88 MiB. GitHub rejects any single file over 100 MiB in ordinary git storage, and the limit applies to every revision in a push rather than only the current one, so an oversized blob that reaches history blocks all later pushes until the history is rewritten.

Git LFS avoids that. .gitattributes routes every STEP file to LFS:

*.stp filter=lfs diff=lfs merge=lfs -text

What git commits is a small pointer, while the bytes live in LFS storage:

version https://git-lfs.github.com/spec/v1
oid sha256:69dc3dc0b1b64932f25fde6b65b36c38442e9caa34eec7bfe0646d026418734a
size 92523231

Because the rule is a pattern rather than a per-file entry, a replacement STEP is handled automatically: copy it into place and commit as usual.

Only when you have committed a new STEP, confirm it landed in LFS rather than in git proper:

git lfs ls-files

Both STEP files should be listed. A file missing from that output was committed as a normal blob, which means git lfs install never ran in this clone. Undo that commit before pushing rather than after.

The pointer is also what you see in a diff, so git show on a STEP revision reports an oid and size instead of attempting to render 88 MiB of CAD text.

The inventory and catalog are the important reviewed inputs. Do not edit cache or export meshes; regenerate them instead.

.gitattributes               routes *.stp to Git LFS
.python-version              interpreter this project is tested against
uv.lock                      pinned dependency set installed by `uv sync`
cad/
  DSG-000040389/
    source.stp                 original full assembly
    manifest.json              generated CAD occurrence facts
    reviews/
      43841-stage-stack.inventory.yaml
  DSG-000046520/               earlier single-polycap review fixture
config/
  stage-catalog.yaml           reusable stage definitions
docs/
  cad-review.md                STEP hierarchy and constraint-review workflow
  sdf-sharing.md               portable SDF and MATLAB handoff
src/twin_lab/
  constraints_wizard.py        STEP import, manifest, tree, and preview
  cad_geometry.py              shared Open Cascade traversal/mesh helpers
  stage_cad_viewer.py          current full-stack cached motion viewer
  cad_motion.py                provisional rigid-group motion viewer
  sdf_compiler.py              reviewed scene to portable SDF package
  convex_collision.py          cached CoACD convex decomposition of part meshes
  collision.py                 clearance queries and reviewed pair filtering
  collision_viewer.py          slider-driven viewer with live clearance reporting
  scene.py                     generic Drake SDF/URDF loader and queries
  paths.py                     repo, cache, and export path resolution
tests/
  fixtures/                    small proxy models used only by tests
.cache/twin_lab/               generated previews, viewer meshes, convex hulls (ignored)
exports/                       generated share and collision packages (ignored)

Reference. Nothing in this section needs to be run in order. Console-script entry points, all installed with the package. Prefix each with uv run:

Inspect a STEP tree or generate a focused preview:

uv run slac-cad-manifest cad/DSG-000040389/source.stp --show-tree --manifest-only
uv run slac-cad-manifest cad/DSG-000040389/source.stp --view --focus A037 --manifest-only

Fallback, only if you need to debug the refresh helper. The low-level remap command it wraps is still available:

uv run slac-cad-manifest cad/DSG-000040389/source.stp \
  --refresh-manifest --manifest-only --no-preview \
  --remap-stage-inventory cad/DSG-000040389/reviews/43841-stage-stack.inventory.yaml \
  --previous-manifest cad/DSG-000040389/manifest.previous.json \
  --alias-map cad/DSG-000040389/reviews/43841-stage-stack.aliases.yaml

Only before committing changes, validate the code and reviewed data:

uv run pytest -q
uv run ruff check .
uv run ruff format --check .

• Motion and rigid attachment ownership have been visually reviewed.
• The enclosure, two camera assemblies, shield cone, chamber, long-jet assembly, and detector stage 7948 are included as named world-fixed environment groups.
• Viewer and SDF materials distinguish each stage model, adapters, crystal/holder payloads, polycap/holder payloads, and transparent enclosure geometry.
• SDF joint coordinates are zero at the reviewed CAD pose. North and South RA stages have a documented 180-degree logical display offset.
• The normal SDF share package is visual/kinematic only, so it stays fast in Drake and portable to MATLAB.
• Collision geometry is opt-in and lives in a separate package. Aggregate CAD meshes make poor convex hulls, so useful interference analysis needs the convex mode, which splits each part into CoACD hulls before Drake sees it.
• The 43841 assembly has 22 scalar joints and 4798 convex collision hulls. Hull mode reports contact almost everywhere because the enclosure hull is solid; convex mode is the mode to trust.
• Clearance is reported as a three-state readout: clear, close, and interference. Only interference is a hard finding; close tracks the warning band slider. Per-interface margins and a fully validated ignored_pairs list are still open; the current list is baselined from the home pose, not reviewed pair by pair.
• The animation range slider is a travel heuristic and has not been clearance-verified; treat animated poses as candidates to check, not as cleared motion.