Sylpheed/docs/re/focus-ring-spin-measured.md

✅ The main menu's focus ring spins continuously — period 2.18 s, measured

Status: ✅ measured (not on the disc as a period; the disc declares the ramp, the running game supplies the rate). Taken 2026-08-29 against Xenia Canary with the disc mounted at /disc.

Question this closes: the port asked whether ptbtneff01 — the 42×46 ring on the focused button — is animated while a button sits focused, or drawn once and held. It had shipped the ring at 0° and marked that known-wrong. structures/ui-button-focus-record.md already said "the ring SPINS" from one frame showing it at a large angle; that is consistent with a continuous spin and with a static draw at a fixed angle, so it did not answer the question asked.

What the ring actually does

Five single frames from one run, 4 s apart, focus held on TUTORIAL throughout:

The ring carries a bright head, and the head is at a different angular position in every frame. It is still moving 16 s in, so it does not ramp once and stop.

⚠️ The 20 s mean of the same run is a uniform circle (ring-20s-mean-uniform.png) — that is the spin smearing itself out, and it is why an averaged frame must never be read as a single frame. A human looking at the live game sees the head; the average does not have one.

The measurement, and why it is not an angle

🔴 No angle is estimated anywhere. The corpus's centroid estimator fails its own control by up to 19.8°, and a 360-bin angular cross-correlation written for this measurement also failed its control — a synthetic 30° rotation of a live frame came back as 0° (peak 0.596), while 90/180/270° came back exactly (peak 1.000), i.e. the estimator only resolves the exact pixel permutations. It was therefore not used.

What was used needs no angle. Two observables separate rotation from a brightness pulse, and both were taken in the same run:

observable	rotation predicts	pulse predicts	measured
total annulus brightness	conserved	varies	0.4 % spread over 16 s (5 frames); 0.53 % over 359 frames
per-angular-bin brightness	varies (a travelling feature)	varies together	per-bin sd 24.2, max 80.3, against a per-frame angular sd of 42.2

Brightness moves around the annulus while the total holds. A pulse is excluded.

The temporal standard deviation over 103 frames is an annulus and nothing else — dark inside, dark outside, peaking exactly on the ring's stroke (radial std: r 0–4 → 1.08, r 10–13 → 36.75, r 20–26 → 1.18):

⚠️ A positional jitter would smear variation outside the stroke. It does not: variation falls to ~1 both inside and outside, so the ring is not moving, it is turning.

The period

A dense 359-frame filmstrip (24 s at 15.03 fps against a requested 15 fps — the consumer kept up exactly, so these timestamps are not backlogged) gives the annulus's 360-bin profile per frame, correlated against frame 0. A rotating ring returns to itself once per revolution, so the trace's period is the spin period — again with no angle estimated.

Autocorrelation local maxima, in seconds:

2.18  4.36  6.52  8.70  10.86  13.02  15.22  17.42
spacings: 2.18 2.16 2.18 2.16 2.16 2.20 2.20     mean 2.177 s

Eight consecutive evenly-spaced peaks over nine revolutions. A drifting instrument cannot produce even spacing, which is the internal check on the number.

Raw trace committed at data/focus-ring-period-corr.npy (rows: t, correlation-with-frame-0, annulus mean).

What the period is in the game's own units

⚠️ 2.18 s is wall-clock under this emulator, and the emulator is not running the game at 30 Hz. The corpus measures 27.6–28.8 fps here. ptbtneff01 declares its first keyframe at t = 120, and under the settled reading (1 unit = 1/60 s, 2 units per rendered frame) 120 units is 60 rendered frames — which at 27.6–28.8 fps spans 2.08–2.17 s. The measurement sits at the top of that band.

So the spin is one revolution per 120 units = 60 frames = 2.00 s at a true 30 Hz, and no new constant is needed to account for it. 🟡 The 2.18 s is consistent with the declared 120 rather than a re-derivation of it: the guest frame rate was not measured in this same run, so the agreement is consistency, not closure.

Two other things the same run measured

• ✅ The focus ring is the ONLY moving thing on the settled main menu. Over 103 frames / 20 s untouched, temporal std is exactly 0.000 on every unfocused button box, on the NEW GAME label, and on the ptmsg footer. Only the focused button's box moves (std 4.46 against a background noise floor of 0.906). A port that draws the main menu statically plus a spinning ring is drawing everything that moves.
• ✅ The ring is ptbtneff01, positionally confirmed. Its centre was located from the temporal-std map at game (520.7, 339.7). The declared leaf offset applied to button 3's rest position (542, 322) predicts (521, 340). That is a sub-pixel agreement between a decoded declaration and a live measurement, and it is what ties the annulus to the record rather than to "a circle near the cursor".

Reach

• One run, one emulator, English locale, GP_TITLE build 5.
• The period is measured on one focused button (OPTIONS, button 4) and the spin is shown on a second (TUTORIAL, button 3). Not checked on all five, and not checked on EXTRAS.
• Says nothing about the direction of rotation — the estimator that would give a signed angle failed its control and was not used.