Sylpheed/docs/re/f6-unit9-sweep-period-and-onset.md

F6 unit 9 — the sweep's loop period, and when it starts

Question (one sentence): does the title's sweeping glow start when the plate appears, and how long is one pass?

What the human looks at: boot the title, watch the white sweep. Pass = it first enters the screen shortly before the "PRESS Ⓐ" plate fades in — under a second before, not at the very start of the title.

What this does NOT cover: which declared keyframe drives the sweep, whether the artwork and the plate share one clock (that is F4), and F5 entirely.

Instrument: ⟨capture⟩ — two independent Canary runs, f6 and f6b.

⚠️ First, the thing that invalidates frame counts across captures

The same animation took 1168 frames in one run and 600 in the other — 1.947× apart. Every other interval in those captures scales by the same factor (the baseline below: 1.953×). So the runs differ in present rate by ~1.95×, and any timing quoted in captured frames is specific to its run.

This is why the numbers below are all ratios. It also means a frames-based rate carried from one capture to another is wrong by up to 2×.

Measured (phase-free, reproduced across both runs)

Baseline interval = from the x=-0.740 element appearing to the plate appearing. Chosen because it is long; a short alpha ramp quantises to ±1 frame and gave an 8% spread where this gives 0.35%.

	f6	f6b	agreement
sweep loop period	1168 fr	600 fr	— (not comparable)
baseline	84 fr	43 fr	—
period / baseline	13.905	13.953	0.35%
sweep starts before plate	0.798	0.791	0.9%

The cycle boundary is unambiguous: the sweep enters at x=-1.540 and wraps from x=+1.840 back to x=-1.540, so first-appearance→wrap is one whole period.

🔴 Do not convert these to title units. Calibrating via the plate's declared 12-unit ramp gives a title clock that puts title start → plate at 75 units against a declared 238 — see below. The ratios stand alone; the conversion does not.

What this says about F6

The sweep starts about HALFWAY between the title's first drawn element and the plate's appearance. Clock-free, and the quantity to implement against:

	f6	f6b	agreement
(onset − title start) / (plate − title start)	0.489	0.507	3.7%

So it starts neither at t=0 (the port's behaviour) nor just before the plate.

🔴 This corrects an earlier version of this page, which said "≈40 title units, well under a second" and "at t=0 the port is ~200 units early". Both came from converting the ratio into title units through the plate's declared 12-unit ramp — the same calibration this page already flags as conflicted. The conflict is worse than stated: title start → plate measures 75 title units through that calibration, where the declared data puts the plate at t=238. 3.2× out, not 1.75×.

Every title-unit figure on this page is withdrawn. The ratios never needed a clock and are unaffected.

⚠️ Note the two ratios describe the same fact and do not conflict: the baseline (-0.740 → plate) is only ~64% of title start → plate, so 0.79 baselines and 0.51 title-spans are the same interval measured against different rulers.

🟡 A conflict I did not smooth

Unit 8 measured the title clock at 1.0000 units/frame in f6 from ptcopyright's 22-unit ramp taking 22 frames. In the same capture, the plate's declared 12-unit ramp takes 21 frames — 0.571 units/frame, 1.75× apart.

One of three things is true: ptcopyright's ramp is not 22 units, the plate's is not 12, or the two elements are not on one clock — which is exactly the premise (clock: "shared") the port's flow.json is built on. Not resolved here; it needs F4.

Because of this, the title-unit conversions above are 🟡. The ratios are ✅ — they need no clock at all.

Not settled

• Still one wrap per capture, so the period is start→wrap, not wrap→wrap. It is a full cycle by the x-boundary argument, but a second wrap would be better. The title exits before one arrives; f6b ran 504 frames past the wrap and the next was ~100 short.
• Which declared keyframe the sweep's leaf hangs off.
• F5, untouched.