cb084e0a39
A disc-wide census of the ARGB that keyless elements carry. Every full-screen *eff00* PRIMITIVE is pure black at its various alphas (ff000000, 7f000000, 40000000, b2000000, cc000000, d4000000, 00000000). Black at alpha a over content is exactly an alpha-over dim or fade, and an additive black quad would be a no-op nobody would author -- so this narrows the open blend question a long way. The only non-black primitive on the disc is pbafc.prm, RGB 00e8e0 cyan at alphas up to ff, and it is 844x600, NOT full-screen, so it sits outside forced_backdrop's geometry guard. It is now the sole additive candidate. The census also refutes my own argument for nearly half its verdicts. Of the 80 forced-first instances only 42 are .prm; 38 are .tbm carrying fade ffffffff. A SOLID white quad at alpha 255 painted first would make the screen white, and no screen is white -- so a .tbm is a white modulation on a texture, and element alpha does not establish its coverage. That is the .t32 error one file extension further out. I guarded that with el.sprite.is_some(), which fixed the symptom and not the cause: an element's alpha is not its texture's opacity, and only an untextured primitive makes the two the same fact. So 42 verdicts stay decoded and 38 drop to inferred -- still almost certainly right, since all are named *base*, all are full-screen, and pfbase.tbm's first position is measured in the running game, but that is a name-and-role argument which this page elsewhere calls the weaker kind. The code is deliberately unchanged. Restricting forced_backdrop to .prm would send eleven screens' backgrounds back to u32::MAX -- last -- which is the blank-screen bug the rule was written to fix. Downgrading the status is honest; reverting the position would be wrong. The 42/38 split is pinned by a test so anyone tightening the rule sees what it costs. Separately, on the port's black_hold_units ask: four more no-input boots yielded one usable log, which armed late and missed the publisher splash, so the sample is still two runs spanning 3 and 4 frames. Their 6.5-9.2 range stands. And a reason it may not be resolvable this way: the draw log DROPS frame numbers -- in the 3-frame run, frames 121 and 124 are absent entirely, so "frames with no sprite" and "span of frame numbers" are different quantities. Their statistical correction is taken: at n=3 the sample SD (3.893) is the estimator, not the population SD (3.179), making my run 1.88 sigma from the corpus mean rather than 2.31. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01QsEPXWVaEpyfudtR6re1Pd
200 lines
9.9 KiB
Markdown
200 lines
9.9 KiB
Markdown
# Where a keyless primitive paints, when the file forces it
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**Classification: decoded.** Derived from the keyframes and the element's own
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geometry, checked disc-wide, and validated against **both** primitives whose
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position was measured in the running game — one it must reproduce, one it must
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not disturb.
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## The question
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A `.prm` / `.tbm` primitive carries **no layer key**: the key is read from a
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sprite's header, and a primitive has no sprite.
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[`ui-prm-primitives.md`](ui-prm-primitives.md) established that the key is not in
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the bundle either — the declaration entry's unread words are constant, and the
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bundle has no RATC child for a primitive — so `implied_layer_key` is a **measured
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per-name table**, and anything not in it keeps `u32::MAX` and sorts **last**.
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"❔ Where an *unmeasured* primitive paints" has been that page's standing blocker.
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The port hit it: `build_12` / `build_15` composited to solid black at every
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instant of their declared life, because `pgloading_eff00.prm` — a full-screen
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opaque quad — sorted on top.
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## The rule
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> **An element that covers the screen and is fully opaque at some instant cannot
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> paint above anything visible at that instant.** Where the elements visible
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> during its opaque span are *all* of them, its position is forced to first.
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This is a constraint read off the file, not a preference, and it is not an
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analogy to a neighbouring screen.
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`pgloading_eff00.prm` is opaque for **39** instants, and **all 9** other elements
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on the loading screen are visible inside that span. **Forced first**, 4/4
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instances.
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## The controls — the rule has to survive both, and it does
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| primitive | measured in the game | opaque instants | forced below | rule says |
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|---|---|---|---|---|
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| `palogo_eff0.prm` (entry 11, developer) | **paints FIRST** | 211 | **6 of 6** | ✅ forced first |
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| `palogo_eff0.prm` (entry 10/13, publisher) | **paints FIRST** | 256 | 2 of 2 | ✅ forced first |
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| `pteff00.prm` (title) | **paints LAST** | 2 | 3 of 23 | ✅ permitted on top |
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| `pteff00.prm` (menu) | **paints LAST** | 2 | 7 of 15 | ✅ permitted on top |
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🔴 **The first row is the one that matters.** `palogo_eff0.prm` is named like an
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overlay, and a rule keyed on the *name* would sort it last — against a measured
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order. Occlusion gets it right. `pteff00.prm` is opaque only for two instants, at
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its screen's entry and exit, so the constraint never binds it: it is the fade
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cover, and it belongs on top.
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## The span, and what it costs to get wrong
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The rule quantifies over "every instant the primitive is opaque" and "every
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element visible then", so it depends on where a screen's timeline ends and on what
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an element does after its own last keyframe. The port asked, having got **256**
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opaque instants for `palogo_eff0.prm` where this page said 211.
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⚠️ **That pair was a bundle mismatch, not a definitional one** — `palogo_eff0.prm`
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appears on both splashes, and the publisher (entries 10, 13) runs to t=255 while
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the developer (11, 14) runs to t=210. 256 and 211 are both right, for their own
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screen. The definitions already agreed.
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**The definition:** the span is `0 ..= max keyframe time over every element in the
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build`, and an element **holds its final pose** past its own last keyframe — which
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is what `pose_at` does, and it is decoded rather than assumed: a group holds at its
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last keyframe rather than looping
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([`ui-keyframe-time-unit.md`](../ui-keyframe-time-unit.md)), and
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[`ui-record-loop-length.md`](ui-record-loop-length.md) shows the declared length
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never falls short of the last keyframe, the slack being exactly that hold.
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**How much rests on it — 130 keyless full-screen primitives with an opaque
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interval, and how many verdicts change:**
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| alternative convention | verdicts changed |
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|---|---|
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| span = the bundle header's declared `+0x08` | **0** |
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| span = the primitive's **own** last keyframe | 72 |
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| elements counted **gone** after their last keyframe (no hold) | **72** |
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🔴 **The hold decides 55 % of the verdicts, and dropping it is refuted by a
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measured order.** `palogo_eff0.prm` is a *single* keyframe at t=0. Without the
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hold it would be opaque for one instant, no other element would be up yet, and the
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rule would call it **free** — against the order measured in the running game,
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which paints it first. Pinned by
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`the_hold_after_a_final_keyframe_is_required_by_a_measured_order`.
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✅ **And the header length is interchangeable with the elements' maximum**: zero
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disagreements disc-wide. Either may be used.
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✅ **The verdicts that matter are convention-independent.**
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`pgloading_eff00.prm` comes out **first** under all four conventions;
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`pteff00.prm` comes out **free** under all four. Only `palogo_eff0.prm` moves, and
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only under the convention its own measured order rules out.
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## Disc-wide
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Keyless **full-screen** primitives with an opaque interval:
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| | count |
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|---|---|
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| position **forced first** | **80** |
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| constrained below some, not all | 50 |
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| occluding nothing | 0 |
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The split falls almost exactly along the names — every `*base*` is forced, every
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`*eff00*` is not — with three families crossing it: `palogo_eff0.prm`,
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`pgloading_eff00.prm` and `pzeff00.prm` are named like overlays and are forced
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first. **That is precisely why the name is not the rule.**
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✅ **And it explains a symptom the corpus had recorded without a cause.**
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`ui-prm-primitives.md` notes 36 builds that "come out one colour ... wiped by
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`pzeff00.prm` and `pceff00.prm`, whose positions have never been measured".
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`pzeff00.prm` is forced first in **32 of 32** instances. Those builds were wiped by
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our own sort, not by the game.
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## 🔴 The rule's real limit, found by its own test
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An earlier version applied to any element. Its disc-wide test asserted that no
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*keyed* element is ever forced — and that assertion failed, on **22** of them:
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`pneff01.t32` (key `0xd850`, paints #8 of 13) and `pbfriendly.t32` (key `0x9230`,
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#17 of 49).
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Both are `.t32` **sprites**, and that is the flaw: a sprite's *element* alpha
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being 255 says nothing about whether its **texture** covers the screen. Most of it
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may be transparent. The disagreements were the rule overreaching, not the keys
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being wrong.
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`forced_backdrop` is now restricted to elements with **no sprite** — untextured
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primitives, which are solid quads and do occlude what they cover. That is also the
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only case `derived_paint_order` consults it for.
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## 🔴 Self-refutation: the argument is sound for only 42 of the 80
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A disc-wide census of the *colour* these elements carry breaks the rule's premise
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for nearly half its verdicts.
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| the 80 forced-first instances | count | fade ARGB |
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|---|---|---|
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| `.prm` — untextured solid quads | **42** | pure black (`ff000000`, `7f000000`, `40000000`, …) |
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| `.tbm` | **38** | **`ffffffff`** — white at full alpha |
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**A solid white quad at alpha 255 painted first would make the screen white.** No
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screen is white. So a `.tbm` is not a solid quad: `ffffffff` is a white
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*modulation* on a texture, which is exactly what a background bitmap carries.
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🔴 **And that means element alpha does not establish coverage for them** — the same
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error the `.t32` guard already caught, one file extension further out. I fixed that
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symptom (`el.sprite.is_some()`) rather than its cause: **an element's alpha is not
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its texture's opacity, and only an untextured primitive makes the two the same
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thing.**
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**What this does and does not change:**
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* ✅ The **42 `.prm`** verdicts stand as decoded. For a solid colour quad the fade
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*is* the pixel, so opacity and coverage are the same fact.
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* 🟡 The **38 `.tbm`** verdicts are **not** decoded. They are almost certainly still
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right — every one is named `*base*`, is full-screen, and one of them
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(`pfbase.tbm`) has its first position **measured in the running game** — but that
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is a name-and-role argument, which this page elsewhere argues is the weaker kind.
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* ⚠️ **The code is deliberately unchanged.** Restricting `forced_backdrop` to
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`.prm` would send `pcbase`, `pnbase`, `pqbase`, `pubase`, `pvbase`, `pjbgbase2`,
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`po_menu_base` and the four `px_*_base` back to `u32::MAX` — last — which is the
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blank-screen bug this rule was written to fix. Downgrading their *status* is
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honest; reverting their *position* would be wrong.
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## Reach
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⚠️ **Assumes straight alpha-over blending.** Blend mode is ❔ on
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[`ui-prm-primitives.md`](ui-prm-primitives.md): an *additive* quad at alpha 255
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would not occlude, and the rule would then be placing it wrongly. The
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`palogo_eff0.prm` control is evidence the assumption holds at least there.
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✅ **The colour census narrows this a long way.** Every full-screen `*eff00*`
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primitive on the disc is **pure black** at its various alphas — `ff000000`,
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`7f000000`, `40000000`, `b2000000`, `cc000000`, `d4000000`, `00000000`. Black at
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alpha *a* over content is exactly what an alpha-over dim or fade looks like, and an
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*additive* black quad would be a no-op, so a designer would not author one. The
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single non-black primitive on the disc is **`pbafc.prm`**, RGB `00e8e0` (cyan) at
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alphas up to `ff` — and it is **844×600, not full-screen**, so it is outside this
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rule's geometry guard entirely. ❔ Whether *it* is additive is still open, and it is
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now the only candidate.
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⚠️ **It gives a lower bound, not an ordering.** It settles the 80 instances where
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occlusion forces the position, and says nothing about the 50 where the primitive
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is opaque only part of the time — including `pteff00.prm`, whose place on top is
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still a *measured* per-name entry, not a decoded one.
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⚠️ **No new oracle measurement.** The two controls are orders measured previously;
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nothing here was captured from a running game. A draw capture of a loading screen
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would confirm it directly, and the loading screens are not reachable from the
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title path.
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## Reproducing
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```bash
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cargo run -p sylpheed-formats --example prm_forced_first
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cargo run -p sylpheed-formats --example prm_occlusion_check
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SYLPHEED_DISC=/disc cargo test -p sylpheed-formats --test ui_forced_backdrop_disc
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```
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