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re: the 11.5 px residual was the alpha-only fit s resolution, and it closes

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Left open earlier as do-not-fit. Closed by adding observables rather than tuning
a parameter.

The draw s vertex buffer carries positions and colours at the same instant, so
all four quantities must agree on one t. Solving independently: quad A x gives
357.88, quad B x 357.58, quad A alpha 355.75, quad B alpha 354.09. The alphas are
about 50x less precise per unit of time because alpha is a byte changing by only
0.27 to 0.33 levels per keyframe unit, so a single level of quantisation is worth
1.5 to 1.9 units, which at 4 px per unit is 6 to 8 px of sweep. The 11.5 px was
that.

At the position-derived t = 357.7 every observable lands: centres within 0.70 and
0.48 px, both alphas inside one level, and the parent alpha is 0 throughout. And
there is no pivot correction to find -- the leaf pivot is (200,90) against a
399x180 sprite, so the pivot is the sprite centre and rotation displaces it by
nothing.

The methodological point is the one this exchange started with, inverted. Earlier
a rule looked confirmed because it was checked against alpha, the insensitive
field. Here the same insensitivity manufactured an apparent 11.5 px error. The
insensitive quantity does not merely fail to falsify, it invents residuals. Solve
on the fastest-moving field and check the slow one, never the reverse.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QsEPXWVaEpyfudtR6re1Pd
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sylph-decoder
2026-08-29 18:23:28 +00:00
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15
docs/port/HANDOFF.md
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@@ -792,9 +792,18 @@ over the same span**. The rule matched on the insensitive quantity and was wrong
on the sensitive one. **Check a new interpretation against the fastest-moving
field you have, not the one that happens to agree.**
❔ Left open: the residual **11.5 px** between 980.5 and 992.0. A rotation about
a declared pivot rather than the centre would displace by roughly that, and
nothing here measures it — so do not fit to close it.
✅ **CLOSED — the 11.5 px was my fit's resolution, not geometry.** Closed by
adding observables rather than tuning. The vertex buffer carries positions *and*
colours at the same instant, so all four must agree on one `t`. Solved
independently: quad A x → **357.88**, quad B x → **357.58**, alphas → 355.75 and
354.09. ⚠️ **The alphas are ~50× less precise per unit** (0.27–0.33 levels/unit,
so one byte of quantisation is worth 1.5–1.9 units = 6–8 px of sweep). At
**t = 357.7** everything lands: centres **−0.70** and **−0.48 px**, alphas within
one level, parent alpha **0** throughout.
✅ **And there is no pivot correction to look for**: the leaf pivot is
**(200, 90)** against a 399×180 sprite — the pivot *is* the centre, so rotation
displaces it by nothing.
## Status

44
docs/re/structures/ui-leaf-vs-parent-alpha.md
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@@ -108,9 +108,47 @@ t=355 is inside a long segment where a one-keyframe shift barely moves it, while
can look confirmed on the insensitive quantity and be wrong on the sensitive
one.**
❔ The residual **11.5 px** between 980.5 and 992.0 is not explained by this and
is left open — a rotation about a pivot rather than the centre would displace by
about that much, and nothing here measures it.
## ✅ The 11.5 px residual is CLOSED — it was the alpha-only fit's resolution
Left open above as *"do not fit to close it"*. It is closed by **adding
observables, not by tuning a parameter**.
The draw's vertex buffer carries positions *and* colours at the **same instant**,
so all four quantities must agree on one `t`. Solving for `t` from each
independently:
| observable | solved t | sensitivity | its own precision |
|---|---|---|---|
| quad A x | **357.88** | 4.00 px/unit | ±0.12 units |
| quad B x | **357.58** | 4.06 px/unit | ±0.12 units |
| quad A alpha | 355.75 | **0.326 levels/unit** | **±1.54 units** |
| quad B alpha | 354.09 | **0.265 levels/unit** | **±1.89 units** |
⚠️ **The alphas are ~50× less precise per unit of time**, because alpha is a byte
changing by only ~0.3 levels per keyframe unit — so a single level of
quantisation is worth **1.5–1.9 units**, which at 4 px/unit is **6–8 px of
sweep**. The 11.5 px was that, not geometry.
At the position-derived **t = 357.7**, every observable lands:
| | predicted | measured | diff |
|---|---|---|---|
| quad A centre x | 991.30 | 992.0 | **−0.70 px** |
| quad B centre x | 466.72 | 467.2 | **−0.48 px** |
| quad A alpha | 195.64 | 195 | +0.64 |
| quad B alpha | 182.95 | 182 | +0.95 |
✅ **Sub-pixel on both positions, inside one byte on both alphas** — while the
parent's alpha is **0** throughout. And the leaf **pivot is (200, 90)** against a
399×180 sprite, i.e. the pivot *is* the sprite centre, so rotation displaces the
centre by nothing. There is no pivot/rotation correction to find.
⚠️ **The methodological point is the same one this exchange started with,
inverted.** Earlier, a rule *looked confirmed* because it was checked against
alpha — the insensitive field. Here the same insensitivity produced a spurious
11.5 px residual. **The insensitive quantity does not just fail to falsify; it
manufactures apparent error.** Solve on the fastest-moving field and check the
slow one, never the reverse.
---