fix(matter): backend-stable seeded hash streams (MAT-92) - #131
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three's TSL hash() writes its PCG constants as bare numbers, so both code generators emit them as float literals. WGSL const-evaluates those at 64-bit precision and recovers the exact integers; GLSL rounds them into f32's 24-bit mantissa, so the WebGL2 fallback ran a structurally identical PCG with wrong constants and every seeded layout diverged between backends. Exact constants alone were not enough: the hash(x).mul(0xffffff) .toUint() chaining pattern crosses u32 -> f32 -> u32, the two compilers disagree by an ULP on the float leg, and truncation turns that ULP into a different integer, reseeding whole rows of cells. stableHash and stableHashUint run the same PCG with uint-typed constants, and voronoiCells, grain, metaballs, and ditherPattern now chain seeds integer-to-integer, taking floats only as final outputs. Verified by diffing WebGPU against WebGL2 captures of the voronoi component page: 97.7% of pixels identical, residual differences confined to antialiased cell vertices. Deriving seeds from the raw hash word re-rolls every seeded layout once, on both backends: visual baselines and posters regenerate with this change, documented in the changeset.
The u32-native seed chain re-rolls every seeded layout once, so the voronoi visual baselines and the voronoi, blobs, and grain posters change. Dither's demo and poster stay on the arithmetic bayer-8x8 map, so they are unchanged. grain and blobs baselines still need a scoped snap run.
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Included review availability: 7 reviews are currently available. Your included PR review attempts over the past 7 days set your current allowance at 10 reviews per hour. 📝 WalkthroughWalkthroughThe PR adds backend-stable PCG hash utilities, exports them publicly, and replaces Three.js hash usage in Voronoi, grain, metaballs, and dither primitives. It adds regression tests, usage guidance, and a release note. ChangesStable hash backend parity
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🔵 Low · up to The PR makes seeded layouts consistent across rendering backends and is otherwise mergeable, but the cap regression test does not prove that the clamp changes stableHash output, leaving a bounded correctness-validation gap for owner follow-up. Possibly related PRs
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In `@packages/matter/src/primitives/stable-hash/stable-hash.ts`:
- Around line 77-90: The stableHash implementation can produce 1.0 when the
uint-to-float conversion rounds near the maximum value, violating its [0, 1)
contract. Update stableHash to clamp the scaled result below 1, and add a
regression covering the 0xffffff80 boundary case.
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apps/docs-tests/visual/voronoi.spec.ts-snapshots/voronoi-default-chromium-darwin.pngis excluded by!**/*.pngapps/docs-tests/visual/voronoi.spec.ts-snapshots/voronoi-default-chromium-linux.pngis excluded by!**/*.pngapps/docs/public/posters/blobs.jpgis excluded by!**/*.jpgapps/docs/public/posters/grain.jpgis excluded by!**/*.jpgapps/docs/public/posters/voronoi.jpgis excluded by!**/*.jpg
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.changeset/stable-hash-backend-parity.mdAGENTS.mdpackages/matter/src/index.tspackages/matter/src/primitives/dither-pattern/dither-pattern.tspackages/matter/src/primitives/grain/grain.tspackages/matter/src/primitives/metaballs/metaballs.tspackages/matter/src/primitives/stable-hash/stable-hash.test.tspackages/matter/src/primitives/stable-hash/stable-hash.tspackages/matter/src/primitives/voronoi/voronoi-cells.ts
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Cap stableHash at 1 - 2^-24: toFloat() rounds hash words at or above 0xFFFFFF80 up to 2^32, which scaled to an exact 1.0 and broke the documented [0, 1) contract about once per 33 million draws. The cap moves only those out-of-contract draws, so rendered output and the regenerated baselines are unaffected.
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In `@packages/matter/src/primitives/stable-hash/stable-hash.test.ts`:
- Around line 51-61: Update the “caps the float output below 1” test to locate
the MathNode with method “min” and assert that its bNode has value 1 - 2 ** -24,
replacing the weaker standalone cap-value existence check while preserving the
existing stableHash(float(1)) graph traversal.
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Tighten the stableHash cap regression: assert a min() MathNode carries the 1 - 2^-24 constant as its second operand, instead of matching the constant anywhere in the graph. The old assertion would pass with the cap attached to anything at all.
Cap stableHash at 1 - 2^-24: toFloat() rounds hash words at or above 0xFFFFFF80 up to 2^32, which scaled to an exact 1.0 and broke the documented [0, 1) contract about once per 33 million draws. The cap moves only those out-of-contract draws, so rendered output and the regenerated baselines are unaffected.
Tighten the stableHash cap regression: assert a min() MathNode carries the 1 - 2^-24 constant as its second operand, instead of matching the constant anywhere in the graph. The old assertion would pass with the cap attached to anything at all.
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Why
The same
seedrendered a different Voronoi layout on the WebGL2 fallback than on WebGPU, so Safari users saw a different pattern than Chrome users. Every seeded primitive inherited the bug.What changes
Backend-stable hash primitives
three's TSL
hash()writes its PCG constants as bare numbers, so both code generators emit them as float literals. WGSL evaluates those at 64-bit precision during constant folding and recovers the exact integers. GLSL float literals are f32, whose 24-bit mantissa cannot hold the 30-32-bit constants, so WebGL2 ran a valid but different hash. The newstableHashandstableHashUintexports run the same PCG with uint-typed constants, which emit as integer literals in both languages. Full reasoning is documented instable-hash.tsand AGENTS.md gotcha 25.Integer-to-integer seed chaining
Exact constants were not enough. The old
hash(x).mul(0xffffff).toUint()pattern crosses u32 to f32 and back, the two compilers disagree by an ULP on the float leg for roughly a quarter of inputs, and truncation turns that ULP into a full reseed.voronoiCells,grain,metaballs, andditherPatternnow chain seeds throughstableHashUintand take floats only as final outputs. Verified by diffing WebGPU against WebGL2 captures of the voronoi page: 97.7% of pixels identical, residuals confined to antialiased cell vertices.One-time pattern re-roll
Seeds now derive from the raw hash word, so every seeded layout changes once, on both backends. The changeset flags it. Voronoi baselines and the voronoi, blobs, and grain posters are regenerated here.
Known limitations
The grain and blobs baselines still need a scoped
pnpm snap, so their visual jobs stay red until that lands. Negative seeds remain backend-defined, tracked in MAT-106.Summary by CodeRabbit
New Features
Improvements
Tests