Keyframes: auto-Bezier spatial tangents follow After Effects

Measured against After Effects 26.3 with the original
examples/ae-spatial-tangents.jsx (three paths, ten keys, even and uneven
timing): an auto-Bezier key's out tangent is (next - previous) / 6 and
the in tangent its mirror, whatever the key times; an end key uses its
one neighbour, (neighbour - key) / 6. EffectCraft gave end keys none and
scaled middle tangents by segment length, so paths with auto-Bezier keys
ran up to 28 px from AE's. All ten tangents now match; path samples are
within 0.05 px.

Tests that relied on end keys having no tangent now expect AE's: Spatial
Bezier keeps auto keys' handles (linear keys still get a third), the
motion-path handle is drawn at a sixth, and a straight loopOut path is
compared within 1e-9. docs/fidelity records the method and scores.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/fidelity/README.md

@@ -50,12 +50,30 @@ time curve. Its closed-form inverse and a parameter-based convergence check now
 mathematical regression test at times within `1e-12` of the midpoint; that test is additional to
 the 168 live AE samples.
 
+## Auto-Bezier spatial tangents
+
+Measured on 10 October 2026 with After Effects 2026 (26.3) by running the original
+`examples/ae-spatial-tangents.jsx` in both hosts (`AfterFX.exe -r` and `effectcraft-cli script`):
+three original Position paths, ten keys, with even and uneven key timing. The script pins spatial
+interpolation to auto-Bezier, since an AE preference can default new keys to linear.
+
+AE gives each auto-Bezier key an out tangent of (next key − previous key) / 6 and the mirrored in
+tangent, whatever the key times; an end key uses its one neighbour, (neighbour − key) / 6. That is a
+Catmull-Rom spline. EffectCraft gave end keys no tangent and scaled middle keys' tangents by the
+lengths of their segments, so all ten tangents differed. Sampled Position along an auto-Bezier path
+with Easy Ease was up to 25.1 px off, and a roving path up to 27.7 px.
+
+After the fix, all ten tangents equal AE's to the printed 0.001 px, and the two path cases are within
+0.05 px over 61 samples each; what remains is the arc-length table's sub-pixel speed. The regression
+test `spatial_auto_bezier_tangents_follow_after_effects` checks the rule on other original keys,
+including a timing change. AE's output stays local; only the script and these scores are tracked.
+
 ## Still unmeasured
 
 Rendered frame comparisons now have a deterministic `effectcraft_effects::fidelity::compare_frames`
 helper. Corpus runners can use it with locally generated host/reference frames and fail on either a
 mean or maximum channel error without committing proprietary renders.
 
-Mixed interpolation sides, non-unit durations, automatic/continuous Bezier, spatial speeds and paths,
+Mixed interpolation sides, non-unit durations, temporal automatic/continuous Bezier, spatial speeds,
 time remapping, expressions, actual rendered motion and other effects need separate corpus cases.
 The initial probe makes no claim about those features.

docs/fidelity/ae-2026-acceptance.md

@@ -12,7 +12,7 @@ platform, fonts, color settings, source files, and project parameters.
 | --- | --- | --- |
 | Work projects | Representative projects recreated or imported, edited, saved, reopened and delivered without lost state | Native AE project import and third-party AE plug-in execution remain gaps |
 | Workflow | Pointer and keyboard cases for all panels, tools, shortcuts, docking, undo, dialogs and selections | Current pass covers 34 panel layouts and selected interaction regressions; every control is not verified |
-| Animation | AE-sampled values for scalar, spatial, multidimensional and expression-driven properties, timing and interpolation | Eight scalar Rotation/ease cases, 168 samples verified against AE 26.3x87; other cases remain open |
+| Animation | AE-sampled values for scalar, spatial, multidimensional and expression-driven properties, timing and interpolation | Eight scalar Rotation/ease cases, 168 samples verified against AE 26.3x87; auto-Bezier spatial tangents (ten keys, three paths) match AE 26.3; other cases remain open |
 | Render fidelity | Original test scenes compared at 8/16/32 bpc, specified color spaces and alpha conventions; edges, impulses, gradients and temporal sequences | Most AE-rendered comparisons remain open; CPU/GPU agreement alone is insufficient |
 | Media and delivery | Real footage import, seek, frame rates, audio sync, relinking, image sequences, codec/container metadata and exports checked in downstream tools | Actual work fixtures still required |
 | Performance | Matched AE/EffectCraft input-to-present and export timings at 1080p/4K, warm/cold caches, memory/VRAM and long sessions | No claim of beating AE or measured blur response yet |