Fix iOS Motion accel: match Android units + sign convention#70
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`Mob.Motion` documents `accel` as m/s² with gravity included, but the iOS NIF emitted `userAcceleration + gravity` straight from CoreMotion — which is in G (~1.0, not ~9.81) AND uses iOS's own sign convention: the gravity vector points down, so at rest the up-axis reads -g. Android's SensorManager reports specific force (proper acceleration), a_coord - g_field, so at rest the up-axis reads +g. The two were off by both a scale factor and a sign, so any tilt- or shake-driven UI (e.g. a mascot whose pupils follow gravity) barely moved on iOS and moved backwards when it did. Emit `(userAcceleration - gravity) * 9.80665` instead. That is exactly Android's a_coord - g_field: +g on the up-axis at rest, m/s², correct for both the static tilt term and the dynamic linear term (not merely a rest-time negation). gyro (rad/s) and mag (µT) already matched and are unchanged. Also document the `accel` convention explicitly in the moduledoc so it's a stated cross-platform contract, not folklore. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
The accel normalization is ObjC inside the CoreMotion callback — no host seam to unit-test, and the iOS Simulator delivers no motion data, so it's not integration-testable off a physical device. Guard it the way Mob.NifDeclarationTest guards the NIF tables: parse ios/mob_nif.m and assert each accel axis SUBTRACTS gravity (Android specific-force sign, not iOS's inverted `+ gravity`) and scales G→m/s². Fails on a revert to the old form. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Ships #70: Mob.Motion iOS `accel` now matches Android's m/s² units and specific-force sign convention — (userAcceleration - gravity) * 9.80665 instead of the raw CoreMotion userAcceleration + gravity (G, inverted). Fixes tilt/shake UIs that barely moved and moved backwards on iOS. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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What
Mob.Motiondocumentsaccelas m/s² with gravity included, but the iOS NIF was emitting CoreMotion'suserAcceleration + gravityverbatim. That value is:SensorManager.TYPE_ACCELEROMETER; anda_coord − g_field, so at rest the up-axis reads +g.So iOS
accelwas off from Android by both a scale factor and a sign. Any tilt- or shake-driven UI barely moved on iOS, and moved backwards when it did (found via a mascot whose pupils follow gravity — nearly still on iOS, then inverted once scaled).Fix
Emit
(userAcceleration − gravity) × 9.80665. That is exactly Android'sa_coord − g_field: +g on the up-axis at rest, m/s², correct for both the static tilt term and the dynamic linear term — not merely a rest-time negation of the old value.gyro(rad/s) andmag(µT) already matched Android and are untouched.Verified against known rest orientations:
(0, 0, +9.81)az = (0 − (−1.0))×9.81 = +9.81✓(0, +9.81, 0)ay = (0 − (−1.0))×9.81 = +9.81✓The underlying diagnosis (units + sign) was confirmed on a physical iPhone via an app-level patch that made a tilt UI track correctly on both axes; iOS Simulator has no motion hardware, so device verification of this exact NIF happens when a consuming app rebuilds
--nativeagainst a release carrying it.Also documents the
accelconvention explicitly in theMob.Motionmoduledoc so it's a stated cross-platform contract.Note on
mix.lockMaster's committed
mix.lockis out of sync withmix.exs— the dev-tooling deps (recon,ex_ast,reach,sourceror) are declared but weren't locked, somix format/mix compile(and the pre-push hook) can't run on a fresh checkout of master. The 4-linemix.lockaddition here is justmix deps.getre-locking them; it's required for this branch to build and is unrelated to the accel change.