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blit modulated by tint only, which is 0xffffffff on essentially every keyframe, so the fade word was decoded, stored and then discarded. Applying it as an ARGB modulate takes the title composite's edge correlation against the framebuffer capture of the running game from 0.4597 to 0.9538, both at zero shift. The white wordmark with its blue outline, the trademark, the copyright and the orange exploding planet all appear -- the planet because a full-screen blue effect that rests at alpha 0 had been painting over it at full opacity. ARGB is measured: across a fade-in the high byte walks 0x00 -> 0x80 -> 0xc0 -> 0xe0 -> 0xff while the low three stay ffffff, and the low 24 bits are 0xffffff on 5276 of the disc's 5453 resting keyframes. A modulate can only remove pixels, so the risk is a blank screen. Measured: no-op on 4060 of 5200 sprite elements, partial on 453, hides 687 -- transient HUD indicators that should not be lit at rest -- and NO build is left with nothing visible. A disc test asserts that, and that the no-op share stays high, which also guards the resting rule against drifting onto ramp frames. Both changes are in one commit because the second is invisible without the first. Applying fade erased the word PAUSE, which the running-game capture plainly shows: pgptitle.rat has three runs of two identical keyframes -- pre-roll, hold, exit -- and the 'later run wins' tie-break grabbed the exit. A group carries the screen's entry animation AND its exit, so a run ending on the last keyframe is now excluded unless it is the only one. The title correlation is unchanged.
156 lines
7.3 KiB
Markdown
156 lines
7.3 KiB
Markdown
# A keyframe is the start of a ramp, not a pose that is held
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**Status:** ✅ `CONFIRMED` against the framebuffer capture of the running title
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screen — the new rule aligns at **zero shift**, the old one had to be moved.
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🟡 the fallback for groups that never hold is unverified. ❔ interpolation
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between keyframes is still not implemented, only the resting pose.
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## The rule that was wrong
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`Element::rest()` answers "where is this element when the screen is just sitting
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there", and every composite the port draws depends on it. It used to pick the
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keyframe with the **largest gap to the next keyframe's time** — the frame that
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"dwells longest".
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That reads a keyframe as a value held until the next one. It is not: a keyframe
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is the **start of a ramp toward the next one**. So a long gap after keyframe *k*
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means the screen spends that whole time *arriving at* `k+1` — the settled pose is
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at the **far** end of the gap, not the near one.
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The title wordmark makes it concrete. `ptlogo1.t32` zooms in from off-screen:
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```
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kf0 150% (-116, -7) a=0x00 t=26 off-screen, invisible
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kf1 150% (-116, -7) a=0x00 t=34
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kf2 112% ( 109, 123) a=0x80 t=38
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kf3 103% ( 165, 171) a=0xc0 t=40
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kf4 101% ( 179, 186) a=0xe0 t=42 ← old rule picked this
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kf5 100% ( 184, 193) a=0xff t=251 ← the settled pose
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kf6 100% ( 184, 193) a=0xff t=264 ← held here
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kf7 100% ( 184, 193) a=0x00 t=None fades out
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```
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The gap 42 → 251 is by far the largest, so the old rule picked **kf4** — 1 % too
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large and 5 px up-left, a frame from mid-zoom. The pose the screen actually holds
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is kf5–kf6.
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## The rule that is right
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**The resting pose is the hold: the longest run of consecutive keyframes with an
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identical pose.** Ties go to the later run, matching the in → hold → out shape.
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Groups that ramp through every frame and never hold fall back to longest-dwell.
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## The measurement
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`docs/re/captures/title-screen-oracle.png` is a framebuffer capture of the
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running title screen. It is a **1:1 crop** of the 1280×720 frame (1279×675) —
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verified by the copyright line landing on row 669 in the capture and in both
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composites — so frame coordinates map directly and a shift is meaningful.
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Edge-correlated over the wordmark box (x 150–1150, y 200–400) with
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`tools/re-capture/align_to_capture.py`. Gradient magnitude, not colour: the
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capture's planet is mid-explosion and orange while ours is blue, and the wordmark
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materials are undecoded, so a pixel diff would measure everything except the
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question being asked.
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| resting rule | best correlation | at shift | at (0,0) |
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|---|---|---|---|
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| **plateau** (landed) | **0.4597** | **(0, 0)** | 0.4597 |
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| longest dwell (old) | 0.1511 | (+3, +8) | 0.1268 |
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The old composite peaks 3× lower **and only after being moved** — displaced by
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about the (−5,−7) that kf4-instead-of-kf5 predicts. The new one is already where
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the game puts it.
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* [composited with the plateau rule](../captures/ui-layout/title-composited-plateau-rest.png)
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* [composited with the old longest-dwell rule](../captures/ui-layout/title-composited-longest-dwell-rest.png)
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## What else it fixed, on the same screen
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The old rule systematically picked the **invisible** end of a fade-in. On the
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title screen it rested these at alpha `0x00`, where the capture plainly shows
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them:
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`ptlogo_tm` (the ™), `ptcopyright`, `ptlogo_back2`, `ptlogo_back2eff`,
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`ptloop01`/`ptloop02`.
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And `pteff00.prm` — the full-screen fade quad that paints **last** — rested at
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**opaque black**. That is the whole screen, and it is why `.prm` compositing was
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blocked ([`ui-prm-primitives.md`](ui-prm-primitives.md)).
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None of that was visible before, because `compose` never applied the `fade`
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alpha at all: `blit` modulates by `tint` only, and `tint` is `0xffffffff` on
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essentially every keyframe. The wrong keyframes were being chosen and then their
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one distinguishing field was ignored. That is worth stating as its own finding —
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see below.
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## The `fade` alpha, applied (same day)
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`blit` modulated by `tint` only — `0xffffffff` on essentially every keyframe — so
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the `fade` word was decoded, stored, and then thrown away. Applying it as an
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**ARGB** modulate on top of `tint` is what turns the resting pose from an
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academic result into a picture:
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| composite | best correlation vs the capture | at shift |
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|---|---|---|
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| plateau rest, `fade` applied | **0.9538** | (0, 0) |
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| plateau rest, `fade` ignored | 0.4597 | (0, 0) |
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| longest-dwell rest | 0.1511 | (+3, +8) |
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0.95 against a framebuffer capture of the running game.
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[The composite](../captures/ui-layout/title-composited-fade-applied.png) now has
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the white wordmark with its blue outline, the ™, the copyright, and the orange
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exploding planet — the last of which appeared because a full-screen blue effect
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that rests at alpha 0 had been painting over it at full opacity.
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**ARGB is measured, not assumed.** Across a fade-in the *high* byte walks
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`0x00 → 0x80 → 0xc0 → 0xe0 → 0xff` while the low three stay `ffffff`, and the low
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24 bits are `0xffffff` on 5 276 of the disc's 5 453 resting keyframes (with
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`0x000000` on 165 — the `.prm` blacks — and `0x5dc9ff` on 12).
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Risk checked, because a modulate can only ever remove pixels: it is a **no-op on
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4 060 of 5 200** sprite elements, partial on 453, and hides 687 — which are
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transient HUD indicators (`pb_emergency`, `pb_refilling`, `pbcm1_arrow1`) that
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should not be lit on a resting screen. **No build is left with nothing visible**,
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and a disc test asserts it.
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### And it caught a bug in the resting rule
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With `fade` applied, the pause menu lost the word **PAUSE**, which the capture of
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the running game plainly shows
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([`pause-tutorial-real-vs-rebuilt.png`](../captures/ui-layout/pause-tutorial-real-vs-rebuilt.png)).
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`pgptitle.rat` has **three** runs of two identical keyframes — invisible,
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visible, invisible:
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```
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kf0 a=0x00 t=5 kf1 a=0x00 t=13 pre-roll
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kf2 a=0xff t=23 kf3 a=0xff t=28 the hold
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kf4 a=0x00 t=30 kf5 a=0x00 t=None the exit
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```
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A group carries the screen's entry animation **and its exit**. The tie-break
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"later run wins" grabbed the exit. Fixed: a run ending on the last keyframe is
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excluded unless it is the only one. The title's correlation is unchanged at
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0.9538, and [PAUSE is back](../captures/ui-layout/pause-composited-fade-applied.png).
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This is why the two changes landed together: the trailing-run defect is
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invisible — in the literal sense — until `fade` is applied.
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## What is not settled
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* ❔ **Blend mode.** Everything above is straight alpha-over. The near-white
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flash quads and the coloured ones may well be additive, and nothing has been
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measured; the title capture cannot separate the two because its resting
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elements are all `0xffffff`.
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* 🟡 **The fallback.** Groups with no two adjacent keyframes alike still use
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longest-dwell. How many there are, and whether the correct answer for them is
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the *last* keyframe instead, is unmeasured.
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* ❔ **Interpolation.** Only the resting pose is decoded; nothing tweens. A
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viewer that animates these screens needs the ramp, and whether it is linear is
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unknown.
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* 🟡 One-shot flashes (`ptlogoall_eff`, `ptlogoall_eff2`) ramp 0 → 0x80 → 0x4b →
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0 and never hold at a visible value, so the plateau rule rests them at alpha 0
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— invisible. That is *probably* right for a settled screen, but the capture
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cannot confirm it while `fade` is unapplied.
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