Sylpheed/docs/re/f6-unit4-sweep-is-onscreen-early.md

F6 unit 4 — the sweep is on screen 41 frames after the title starts, long before the plate

Status: ✅ measured ⟨capture⟩ ×1. 2026-09-02.

Question: how many frames after the title starts drawing does the sweep first appear on screen? Look at: the frame its leading edge crosses into the viewport, minus the frame the title's first element draws. Not covered: alpha gating (unit 2, 🟡); the title's own clock origin.

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The measurement

Same capture as units 2 and 3. Counts only — no clock conversion.

title's first drawn element   frame 706
sweep first SUBMITTED         frame 746    (+40 frames)
sweep first ON SCREEN         frame 747    (+41 frames)   NDC x span [−2.380, −0.990]

The sweep's leading edge is inside the viewport 41 frames after the title begins drawing.

For scale: the plate reaches full alpha at declared t = 236. At the 1 unit/present the splash work established, that is frame ≈ 942 — 195 frames after the sweep is already on screen.

⚠️ That last comparison carries two assumptions and I am naming them rather than folding them in: that the title's first drawn frame is its t = 0, and that its clock runs at 1 unit/present. The 41 is a raw frame count and carries neither.

🔴 What this settles, and what it hands to unit 2

Position does not gate the sweep late. It is geometrically visible from very early in the title, not at the plate. So if a human sees it start when the plate appears, the gate cannot be where the quad is — it has to be how opaque it is.

That puts the whole of F6 on unit 2, which is 🟡: does the parent's alpha multiply into the leaf? The parent ramps 0→255 across t = 70→100, so:

if parent alpha multiplies in	the sweep is invisible until ≈ t 70 and dim to 100 — a real early gate
if it does not	the sweep is fully lit from the moment it enters at +41 frames — and the human's report has no mechanism yet

Neither is established. Unit 2's evidence was 14 sub-floor samples of 1754 and was rightly downgraded.

📌 But note the shape of the disagreement has narrowed usefully. Before this unit, "starts when the plate appears" had two candidate mechanisms — position or alpha. Position is now excluded by measurement. One candidate left, and it is the one already under test.

Refutation attempt

Target: my own unit 1 framing, that the human might be watching the leaf travel on screen rather than an alpha gate.

Result: REFUTED. I wrote that a quad can be "fully opaque and entirely off-screen" and offered that as the likely explanation. It is not: the sweep is on screen at +41 frames, nowhere near the plate. The travel explanation is dead, and I am recording it because I proposed it two units ago and it steered the port's reading of their own renderer.

Reach

⟨capture⟩ ×1, one boot, pteff03 only. The +41 is a frame count between two events in one stream and needs no clock. The comparison to the plate does need one, and is labelled. pteff03a does not appear in this capture.