sylph-decoder 5744f379b2 formats: a keyframe's time comes before its pose, and none of them was missing ...

The placement region is `frames` records of `{u32 time; 36-byte pose}` after an
8-byte header, so the time word PRECEDES the pose it belongs to. Our parser's
40-byte window opened at the pose, four bytes into the record, and then read the
word at its `+36` as that pose's time -- which is the NEXT pose's. Every pose
field was right; only the time association slipped by one.

Two things the corpus has carried for weeks are that off-by-one and nothing
else: "a group's data stops 4 bytes short of its final block's time slot", and
"the last keyframe carries no time". The group is not short (8 + frames*40 is
exact) and no time is missing -- the first pose's time is the lead-in word at
`header + 8` that `parse_placements` skipped without asking what it was.

Disc-wide, 33 archives, 13 991 groups, each test with a control:

  A  lead-in prepended to the shifted times is non-decreasing  13991/13991
  B  a non-zero lead-in is strictly below the next time         5058/5058
     control (another group's lead-in, same bundle)            70.9%
  C  multi-segment alpha ramp at a constant rate, corrected    857/1540
     the same, under the old reading                             0/1042

C is the one that cannot be argued with: interpolation between keyframes is
linear, and under the old reading not one multi-keyframe ramp on the disc comes
out at a constant rate.

Adoption is free on every static composite, which is what the corpus previously
declined it over. `SYLPHEED_KF_TIME_SHIFT=1` moved GP_TITLE build 7 by 13.1% of
its pixels because it left pose 0 untimed; with the lead-in restored, all 12
GP_TITLE builds render byte-identically, and across 217 builds in six archives
only two elements pick a different rest pose -- both times between two poses
that are equally invisible.

`SYLPHEED_KF_TIME_SHIFT` is gone; `SYLPHEED_KF_TIME_LEGACY=1` restores the old
reading for A/B work.

ui_header_time_disc needed one line: 546 bundles whose every group is a single
static pose now report max_time = 0 where they previously reported no time at
all. Excluding them, the result it guards strengthened -- the bound holds over
2 859 bundles instead of 2 313, still with zero violations.

Not established: the executable's own parser. Reach is written down.

docs/re/ui-keyframe-record-layout.md

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Nsxw1A9JseUw99Yw1ZRQzY

2026-08-29 14:01:39 +00:00

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