Fix many body force jitter on small graphs - #255
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…jitter The many-body near field estimates dense cells' repulsion from a fresh random K=8 subset every tick, Horvitz–Thompson-weighted by count/sampled. The per-tick variance was designed to act as annealing jitter, which holds for pure repulsion: the dense clump that causes it disperses within ~60 ticks and the noise dies sub-pixel. This graph is the case where it doesn't hold — 163 countries and 642 border links, where link attraction keeps finest-cell occupancy at ~45 (far above the 8 sampling slots) for as long as alpha stays high, so the re-drawn sample turns into permanent visible shimmer: ~0.5 units/tick of pure random walk (~92° mean direction change between consecutive ticks) on top of a settled layout. The story runs the graph live with alpha held at 1 and a sliding-window meter of per-tick step, mean turn angle, and finest-cell occupancy versus the sampling slots — numbers that make the jitter, and any fix for it, verifiable rather than anecdotal. It yields on timers instead of requestAnimationFrame so it keeps running in hidden tabs (rAF pauses there, e.g. under browser automation). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Signed-off-by: Nikita Rokotyan <nikita@rokotyan.com>
…field sampling above The Monte-Carlo near field re-draws each finest cell's K=8 sample every tick and weights it by count/sampled. That per-tick variance was designed to act as annealing jitter, which holds for pure repulsion: the dense clump that causes the variance disperses within ~60 ticks and the noise dies with it, sub-pixel before anyone sees it. But when link attraction or gravity keeps density up while alpha stays high, cell occupancy never falls toward K and the re-sampling becomes permanent visible shimmer — see the country-borders story added in the previous commit: ~0.5 units/tick of pure random walk (92° mean direction change) on top of a settled layout. A CPU replica differing only in the per-tick re-sampling reproduced the GPU numbers exactly, ruling out integration and damping. The invariant this restores: a settled layout is still. Sampling noise may only exist where it is annealing something, never at equilibrium of a small or medium graph. - Graphs of at most ALL_PAIRS_MAX_POINTS (4,096) skip the grid and the sampler entirely: one exact O(n²) pass (force-allpairs.frag) with the same falloff and coincident-point kick, absent points excluded. Exact at any occupancy — and faster there, because depth peeling pays ~0.1ms of fixed cost per sequential slot pass while the n² texel loop is trivial at this scale (measured 1.8ms/step at 2k points vs 6.4ms for a 64-slot peel). - Above the threshold, the slot count adapts: 32 up to 16k points, 16 up to 65k, 8 beyond — more samples extend the exactly-covered occupancy range and shrink residual variance (amplitude ∝ occupancy/K · 1/√K), while the large-graph tier keeps its cost unchanged. - The slots moved from 8 hand-unrolled sampler2Ds to one sampler2DArray looped with a slotCount uniform, which is what makes K a runtime value instead of a shader-edit. Peeling ping-pongs two plain 2D targets and copies each pass into its array layer: pass k samples pass k−1, and sampling one layer of a texture while rendering to another layer of the same texture is a WebGL feedback loop. Result: the country graph settles to 0.02 units/tick (turn 0.8°); a synthetic dense clump matches a CPU exact all-pairs reference in every tick window; the repulsion benchmark is unchanged at and above 100k points (2k 1.81ms, 5k 3.80, 20k 2.29, 50k 3.90, 100k 6.63, 200k 13.79 ms/step). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Signed-off-by: Nikita Rokotyan <nikita@rokotyan.com>
…dd history entry The deep dive described a single code path and claimed the sampling noise always anneals — both now wrong, and the document even cited the dropped brute-force prototype as permanently unnecessary. Bring it in line with the fixed force: - New sections: the sustained-density failure mode that made the noise permanent (and the measurements that pinned it on re-sampling), and the exact all-pairs path that now serves graphs of at most 4,096 points, including why it is also the faster option there. - Step 3 rewritten for the sampler2DArray slots and the adaptive K (32/16/8), with the ping-pong-and-copy peeling explained via the feedback-loop constraint. - Comparison table, cost figures, and the "trade, honestly" section updated; stale Misc → Performance story reference fixed. - History entry records the why, the numbers before/after, and points to the country-borders reproduction story. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Signed-off-by: Nikita Rokotyan <nikita@rokotyan.com>
📝 WalkthroughWalkthroughThe PR adds exact all-pairs repulsion for graphs up to 4,096 points. Larger graphs use adaptive near-field sampling with layered textures and ping-ponged peel targets. Documentation, history, and a side-by-side country-border comparison story describe both paths. ChangesMany-body force computation
Estimated code review effort: 4 (Complex) | ~60 minutes Merge Risk: 🔵 Low · up to The performance story can break later force updates after its forced sampled-path setup removes required state, and one accompanying diagram mislabels alternating passes. The PR is otherwise mergeable, but these bounded issues should be corrected or explicitly accepted before merge. Possibly related PRs
Sequence Diagram(s)sequenceDiagram
participant Graph
participant ForceManyBody
participant AllPairsShader
participant NearFieldShader
participant VelocityFramebuffer
Graph->>ForceManyBody: Run force simulation
alt Point count <= 4096
ForceManyBody->>AllPairsShader: Render exact pairwise repulsion
AllPairsShader->>VelocityFramebuffer: Write velocity
else Point count > 4096
ForceManyBody->>NearFieldShader: Bind slotsTexture and slotCount
NearFieldShader->>VelocityFramebuffer: Write sampled near-field velocity
end
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Inline comments:
In `@src/modules/ForceManyBody/force-allpairs.frag`:
- Around line 80-93: Update the all-pairs shader’s velocity calculation after
the jitter adjustment to clamp the aggregate velocity magnitude to a maxStep
value, matching the near-field pass’s 2 * cellSize bound. Add and bind the
equivalent maxStep uniform from ForceManyBody, then use it before writing
fragColor so stacked positions cannot produce unbounded output.
In `@src/stories/performance/country-borders-jitter.ts`:
- Around line 4-17: Update the country-borders jitter story to stop describing
or measuring near-field sampling, since its 163-point dataset uses the all-pairs
path via drawAllPairsForce(). Either replace the fixture with one exceeding
ALL_PAIRS_MAX_POINTS to exercise sampled forces, or revise the story comments,
occupancy meter, and linked documentation to describe an all-pairs regression
scenario.
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…ison — both paths visible at HEAD After the two-path fix, the country-borders repro story ran on the exact all-pairs path: its meter reported a still layout and its comments described sampling machinery that no longer executes at 163 points. The shimmer it was built to demonstrate was only reachable by checking out the pre-fix commit — evidence for bisection, invisible in Storybook. Replace it with a side-by-side comparison that keeps both worlds visible at HEAD: the same graph, data, and seed run twice — left today's exact path (settled layout, still), right the pre-fix configuration (grid + Monte-Carlo near field, K = 8), where the layout shimmers with the same numbers that motivated the fix (~0.46 u/tick step, ~94° mean turn). - The pre-fix side is forced through a story-only patch of ForceManyBody internals via the repo's src alias — the engine deliberately exposes neither the path choice nor the slot count, and the patch keeps it that way: per-instance via a config marker key, failing loudly if the internals move. None of it resolves against the published package, and the story header says so before the code does. - Each side adds a trajectory panel: one dense-cell point's path over the last 360 ticks with a unit scale bar — a smooth drift arc today, a random-walk tangle before. The shapes make the turn-angle metric legible at a glance. - The deep dive's story pointer follows the rename; the original repro story remains observable at its own commit for bisection. The story now teaches what the fix changed rather than describing machinery the engine no longer runs at this size — each pane's title says which half is history. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Signed-off-by: Stukova Olya <stukova.o@gmail.com>
…ntry The entry's Example still pointed at the single-pane repro story, which 'feat(stories): replace the jitter repro with a fixed-vs-before comparison' (2fe05a6) removed — after the fix that story ran on the exact path and could no longer show the shimmer it documented. The Example now describes the side-by-side story (and how its pre-fix pane is forced via a story-only internals patch), keeps the pointer to the bisectable original at its own commit, and the Commits line gains the new commit with the stale pre-rebase hashes refreshed to the branch's current ones. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Signed-off-by: Stukova Olya <stukova.o@gmail.com>
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Inline comments:
In `@src/stories/performance/country-borders-comparison.ts`:
- Around line 73-77: Update the pointsNumber override around
originalCreateSlotTargets to capture the existing property descriptor before
calling Object.defineProperty, then restore that descriptor in finally instead
of deleting the property. Preserve the temporary 100_000 value during
originalCreateSlotTargets.apply.
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…ld scale The exact all-pairs pass wrote its velocity unbounded. The discrete tick makes that a real failure: the softened falloff still diverges at near-zero separations, and a coincident stack sums n−1 same-direction random kicks — measured on a 1,000-point single-position start, tick one flung points 2,896 units, straight into the space corners. Before the two-path split every graph ran the near-field pass, whose 2 × cellSize clamp bounded exactly this case — the small-graph path had lost it. The invariant: both paths bound the same thing. Pairs within the grid path's near-field scale are jittered and capped at 2 × the finest cell size it would use at this point count; farther pairs pass through unbounded, as the level passes do. A stacked start expands; it never teleports — and dynamics stay continuous across the 4,096-point threshold. - Clamping the whole sum would have been simpler but wrong: the grid path leaves its far field unclamped, so a total-sum cap would throttle legitimate multi-clump expansion below the threshold only — a behavioral seam of exactly the kind this force just got rid of. The shader splits pairs at the maxStep radius and bounds only the near sum. - The finest-grid formula moved into getFinestGridSize(), shared by the pyramid allocation and the new maxStep uniform, so the two paths cannot drift apart on the scale they bound with. - Drop the dead commandEncoder.destroy() in the peel copy loop — luma.gl's finish() destroys the encoder itself (verified in webgl-command-encoder: finish() calls this.destroy()). Measured: the stacked start steps 108.8 u on tick one (the 128 u bound × 0.85 friction), decaying 33.6 → 16.7 → 9.5 — a controlled dispersal; the country graph's settled numbers are unchanged (0.02 u/tick, 0.6° turn exact; 0.46 u/tick, ~92° with sampling forced). Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Signed-off-by: Stukova Olya <stukova.o@gmail.com>
…the data module The forced pane's patch asserted only that the two private members it replaces still exist — not that they still mean the same thing. If the slot tiers are retuned so 100k points no longer maps to 8 slots (or the count stops flowing through data.pointsNumber), the pane would keep rendering under a silently false 'K = 8' header. The wrapper now checks the effective nearFieldSlots after the original call and throws — the story's fail-loudly contract now covers meaning, not just existence. Also list country-borders-data.ts in the story's source panel, matching how the other multi-module performance stories show their companions — the displayed source is otherwise uncompilable and hides the dataset the example is built on. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Signed-off-by: Stukova Olya <stukova.o@gmail.com>
…for the two-path force The deep dive documents the algorithm; the story of the near-field jitter — what shimmered, how it was measured, what the fix changed — lived only in commit messages and review threads. jitter-fix.html tells that story on its own: problem, mechanism, measured trajectories, fix, results. It is fully self-contained (every figure inlined, the live trajectory captures embedded as data URIs), so it opens from a checkout in any browser with no build and no network. The diagrams had fossilized on the K = 8 design, exactly what the generator's header warns against: - c-depth-peeling and d-gpu-pipeline now speak in terms of K (32/16/8 by graph size), note the sampler2DArray layers, and say where the annealing argument stops holding; the pipeline gains the line that graphs of ≤ 4,096 points skip all three passes for one exact one. - Three new generated diagrams: f-resample-jitter (the failure mode — same cell, fresh sample, re-rolled force), g-two-paths (the decision and both pipelines), h-pingpong-peel (why peeling renders to plain 2D targets and copies into array layers). Embedded in the README sections they illustrate, and inlined in the new page. - README: pointer to the new page, the new figures, and a note that the all-pairs path bounds its near-range sum the way the near-field pass does while leaving far pairs unclamped. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Signed-off-by: Stukova Olya <stukova.o@gmail.com>
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Inline comments:
In `@docs/many-body-force/gen-diagrams.cjs`:
- Line 376: Correct the pass-parity label in the diagram data near the
pass-3-through-K−1 entry: represent the alternating read/write targets rather
than labeling every pass as “reads A · writes B”. Ensure pass 3 uses A→B, pass 4
uses B→A, and subsequent passes continue alternating through K−1.
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Problem
The many-body near field estimates dense cells' repulsion from a fresh random K = 8 subset every tick, Horvitz–Thompson-weighted by
count/sampled. The per-tick variance was designed to act as annealing jitter — valid under pure repulsion, where the dense clump that causes it disperses within ~60 ticks and the noise dies with it. But when link attraction or gravity keeps density up while alpha stays high (long-running or reheated layouts), cell occupancy never falls toward K and the re-sampling becomes permanent visible shimmer: on the 163-country border graph every point wandered ~0.5 units/tick with ~92° mean direction change — a pure random walk on top of a settled layout. A CPU replica differing only in the per-tick re-sampling reproduced the GPU numbers digit-for-digit, pinning the cause on the sampling rather than integration or damping.The invariant this restores: a settled layout is still. Sampling noise may only exist where it is annealing something, never at equilibrium of a small or medium graph.
Fix
force-allpairs.frag) replaces the grid pyramid and the sampler — same falloff and coincident-point kick, exact at any occupancy, so zero sampling noise. Also faster there: ~1.8 ms/step at 2k points vs ~6.4 ms for a 64-slot peel, because depth peeling pays ~0.1 ms of fixed cost per sequential pass while the n² texel loop is trivial at this scale.sampler2Ds to onesampler2DArraylooped with a runtimeslotCount.Results
Example
Performance → Repulsion Jitter: Fixed vs Before — the country graph run twice with identical data, seed, and settings: left today's exact path, right the pre-fix sampled configuration (K = 8) forced back on through a story-only patch of
ForceManyBodyinternals (repo-only; fails loudly if the internals move). Each side reports step/turn over a sliding window and traces one dense-cell point's trajectory.Screen.Recording.2026-08-20.at.12.47.54.mov
Notes
No config or public-API change;
simulationRepulsionbehaves as before andmigration-notes.mdis intentionally untouched. The deep dive (docs/many-body-force/README.md) documents the two-path structure; the engineering record ishistory/2026/2026-08-14-nearfield-jitter.md.🤖 Generated with Claude Code
Summary by CodeRabbit
New Features
Bug Fixes
Documentation