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re: forced_backdrop DECIDES 62 of its 80 instances, not 6

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The port agent raised that every check this corpus ran on the rule measured
its STABILITY -- that no verdict moved -- and never its NECESSITY. It is
right, and the distinction is load-bearing.

New probe: recompute derived_paint_order with the forced_backdrop fallback
removed and diff the orders, over every dat/*.pak.

  80 forced instances = 62 the rule DECIDES + 18 it merely AGREES with.

The 80 reproduces the page's own census exactly, which is the check that the
probe sees the same set. Every one of the 62 deciders is keyless; no keyed
element is ever moved.

Of the 18 that agree, 14 have their own key -- and that includes the
palogo_eff0.prm 'control', whose implied key is 0x00000000 and would sort it
first regardless. So that agreement is the rule reproducing our crate, not the
game confirming the rule. The port saw this before I did. The remaining 4 are
keyless but inert: every element on those two builds is forced, so the
tie-break gives the same order either way.

Confirms the port's GP_TITLE finding from the other side: entries 10/11/13/14
unchanged without the rule, entries 12/15 decided by it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Wuu56cE8vJGTBtn1ppsk8v
This commit is contained in:
sylph-decoder
2026-08-30 06:53:16 +00:00
parent 89d51449e7
commit 18e880a430
4 changed files with 448 additions and 0 deletions
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88
crates/sylpheed-formats/examples/forced_backdrop_necessity.rs Normal file
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@@ -0,0 +1,88 @@
//! Which screens does `forced_backdrop` DECIDE, and which does it merely agree with?
//!
//! Every check this corpus has run on the rule measured its **stability** — that
//! no verdict moved when something else changed. That is a different property
//! from **necessity**: an element whose position is already fixed by a read or an
//! implied key is confirmed by the rule, not decided by it.
//!
//! So: compute `derived_paint_order` with the rule, and again with the
//! `forced_backdrop` fallback removed, and report every entry whose order moves.
//! Where nothing moves, the rule is decorative on that screen; where it moves,
//! the rule is the only thing holding the order up.
//!
//! Raised by the port agent 2026-08-30. Reach note in
//! `docs/re/structures/ui-forced-backdrop.md`.
use std::path::PathBuf;
use sylpheed_formats::{pak::PakArchive, ui_layout};
fn order_without_rule(build: &ui_layout::UiBuild, bundle: &[u8]) -> Vec<usize> {
let mut idx: Vec<usize> = (0..build.elements.len()).collect();
idx.sort_by_key(|&i| {
let el = &build.elements[i];
(
ui_layout::sprite_layer_key(build, bundle, el)
.or_else(|| ui_layout::implied_layer_key(&el.name))
.unwrap_or(u32::MAX),
i,
)
});
idx
}
fn main() {
let root = PathBuf::from(std::env::var("SYLPHEED_DISC").expect("SYLPHEED_DISC"));
let path = std::env::args()
.nth(1)
.unwrap_or_else(|| root.join("dat/GP_TITLE.pak").to_string_lossy().into());
let ar = PakArchive::open(&path).expect("pak");
println!("# {path}");
println!("# entry forced decides elements note");
let mut decides = Vec::new();
let mut agrees = Vec::new();
for (i, e) in ar.entries().iter().enumerate() {
let Ok(by) = ar.read(e) else { continue };
let Some(b) = ui_layout::parse_build(&by) else {
continue;
};
let with = ui_layout::derived_paint_order(&b, &by);
let without = order_without_rule(&b, &by);
// Which elements does the rule fire on, and of those, which have no key
// of their own to fall back on?
let mut forced = Vec::new();
let mut keyless = Vec::new();
for el in &b.elements {
if !ui_layout::forced_backdrop(&b, el) {
continue;
}
forced.push(el.name.clone());
let own = ui_layout::sprite_layer_key(&b, &by, el)
.or_else(|| ui_layout::implied_layer_key(&el.name));
if own.is_none() {
keyless.push(el.name.clone());
}
}
if forced.is_empty() {
continue;
}
let moved = with != without;
if moved {
decides.push(i);
} else {
agrees.push(i);
}
println!(
" {i:>5} {:>6} {:>7} {:>8} forced=[{}] keyless=[{}]",
forced.len(),
if moved { "YES" } else { "no" },
b.elements.len(),
forced.join(","),
keyless.join(","),
);
}
println!("\n# rule DECIDES the order on entries {decides:?}");
println!("# rule merely AGREES on entries {agrees:?}");
}

32
docs/port/HANDOFF.md
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@@ -2529,6 +2529,38 @@ whatever it returns — the same reason a truncated log and a t=0 render both lo
fine from inside. Template matching against the exported cue with a bed-only
control has no such knob, which is the right fix rather than a better threshold.
## ✅ 2026-08-30 — your forced-backdrop correction is RIGHT, and disc-wide it is bigger than you said
You told me "stability is not necessity" and that removing `forced_backdrop`
leaves the four splashes byte-identical while `build_12`/`build_15` go black.
**Confirmed independently — from my crate, not yours.** I recomputed
`derived_paint_order` with the fallback removed and diffed the orders:
| `GP_TITLE` entry | rule decides? | forced element |
|---|---|---|
| 10, 11, 13, 14 (splashes) | **no** — order unchanged | `palogo_eff0.prm`, which has its own implied key `0x00000000` |
| **12, 15** (loading) | **YES** | `pgloading_eff00.prm` — no read key, no implied key |
Two renderers, same answer. Your point about the agreement not being independent
support is also right and I have written it into the page: `palogo_eff0.prm` would
sort first from its implied key anyway, so the rule reproducing it is the rule
reproducing my crate.
🔴 **Disc-wide it is not 2 of 6, it is 62 of 80.** Over every `dat/*.pak`, the rule
**decides** the order on **62** forced instances and merely agrees on 18. Every one
of the 62 is keyless; no keyed element is ever moved. The 80 reproduces this
corpus's own earlier census exactly, which is the check that the probe sees the
same set.
**What that means for you:** nothing on your five screens beyond the two you
already identified — but it does mean the rule is not a decoration anywhere, and if
the blend-mode assumption under it ever fails, 38 `.prm` instances go with it (the
other 24 are `.tbm`, whose pixels this corpus cannot locate, so those are
"correct or inert" either way).
[`structures/ui-forced-backdrop.md`](../re/structures/ui-forced-backdrop.md) ·
[census](../re/data/forced-backdrop-necessity.txt)
## 2026-08-30 — the Ⓐ blocker is SOLVED, and it was the emulator, not the game
**This supersedes the section below, which stands as the record of the wrong

253
docs/re/data/forced-backdrop-necessity.txt Normal file
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@@ -0,0 +1,253 @@
# Does forced_backdrop DECIDE a screen's order, or merely AGREE with it?
#
# Produced by: cargo run -p sylpheed-formats --example forced_backdrop_necessity -- <pak>
# over every dat/*.pak, 2026-08-30. SYLPHEED_DISC=/disc.
#
# 'decides' = derived_paint_order() differs from the same sort with the
# forced_backdrop fallback removed. 'no' = the element's own read or implied
# key already puts it there, OR every element on the screen is forced so the
# declaration-index tie-break gives the same order either way.
#
# TOTALS: 80 forced instances = 62 decides + 18 agrees.
# The 80 reproduces the census in ui-forced-backdrop.md exactly.
# Every one of the 62 deciders is keyless; no keyed element is ever moved.
#
# /disc/dat/GP_BUNK.pak
# entry forced decides elements note
0 1 YES 21 forced=[px_bunk_base.tbm] keyless=[px_bunk_base.tbm]
2 1 YES 21 forced=[px_bunk_base.tbm] keyless=[px_bunk_base.tbm]
4 1 YES 19 forced=[pvbase.tbm] keyless=[pvbase.tbm]
6 1 YES 19 forced=[pvbase.tbm] keyless=[pvbase.tbm]
# rule DECIDES the order on entries [0, 2, 4, 6]
# rule merely AGREES on entries []
# /disc/dat/GP_CHALLENGE.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_DEBRIEFING_PILOTLOG.pak
# entry forced decides elements note
118 1 YES 6 forced=[px_deb_base.tbm] keyless=[px_deb_base.tbm]
130 1 YES 6 forced=[px_deb_base.tbm] keyless=[px_deb_base.tbm]
131 1 YES 13 forced=[pjbgbase2.tbm] keyless=[pjbgbase2.tbm]
134 1 YES 10 forced=[px_deb_base.tbm] keyless=[px_deb_base.tbm]
150 1 YES 13 forced=[pjbgbase2.tbm] keyless=[pjbgbase2.tbm]
165 1 YES 10 forced=[px_deb_base.tbm] keyless=[px_deb_base.tbm]
# rule DECIDES the order on entries [118, 130, 131, 134, 150, 165]
# rule merely AGREES on entries []
# /disc/dat/GP_DIALOG.pak
# entry forced decides elements note
2 1 YES 15 forced=[pcbase.tbm] keyless=[pcbase.tbm]
3 1 YES 15 forced=[pcbase.tbm] keyless=[pcbase.tbm]
9 1 YES 34 forced=[pzeff00.prm] keyless=[pzeff00.prm]
10 1 YES 46 forced=[pzeff00.prm] keyless=[pzeff00.prm]
11 1 YES 38 forced=[pzeff00.prm] keyless=[pzeff00.prm]
12 1 YES 32 forced=[pzeff00.prm] keyless=[pzeff00.prm]
13 1 YES 32 forced=[pzeff00.prm] keyless=[pzeff00.prm]
14 1 YES 34 forced=[pzeff00.prm] keyless=[pzeff00.prm]
15 1 YES 26 forced=[pzeff00.prm] keyless=[pzeff00.prm]
16 1 YES 20 forced=[pzeff00.prm] keyless=[pzeff00.prm]
17 1 YES 30 forced=[pzeff00.prm] keyless=[pzeff00.prm]
18 1 YES 38 forced=[pzeff00.prm] keyless=[pzeff00.prm]
19 1 YES 30 forced=[pzeff00.prm] keyless=[pzeff00.prm]
20 1 YES 36 forced=[pzeff00.prm] keyless=[pzeff00.prm]
21 1 YES 38 forced=[pzeff00.prm] keyless=[pzeff00.prm]
22 1 YES 46 forced=[pzeff00.prm] keyless=[pzeff00.prm]
23 1 YES 34 forced=[pzeff00.prm] keyless=[pzeff00.prm]
24 1 YES 36 forced=[pzeff00.prm] keyless=[pzeff00.prm]
25 1 YES 14 forced=[pzeff00.prm] keyless=[pzeff00.prm]
26 1 YES 16 forced=[pzeff00.prm] keyless=[pzeff00.prm]
28 1 YES 14 forced=[pzeff00.prm] keyless=[pzeff00.prm]
29 1 YES 16 forced=[pzeff00.prm] keyless=[pzeff00.prm]
30 1 YES 18 forced=[pzeff00.prm] keyless=[pzeff00.prm]
31 1 YES 18 forced=[pzeff00.prm] keyless=[pzeff00.prm]
32 1 YES 10 forced=[pzeff00.prm] keyless=[pzeff00.prm]
33 1 YES 16 forced=[pzeff00.prm] keyless=[pzeff00.prm]
34 1 YES 14 forced=[pzeff00.prm] keyless=[pzeff00.prm]
35 1 YES 18 forced=[pzeff00.prm] keyless=[pzeff00.prm]
36 1 YES 16 forced=[pzeff00.prm] keyless=[pzeff00.prm]
37 1 YES 18 forced=[pzeff00.prm] keyless=[pzeff00.prm]
38 1 YES 26 forced=[pzeff00.prm] keyless=[pzeff00.prm]
39 1 YES 20 forced=[pzeff00.prm] keyless=[pzeff00.prm]
40 1 YES 18 forced=[pzeff00.prm] keyless=[pzeff00.prm]
41 1 YES 20 forced=[pzeff00.prm] keyless=[pzeff00.prm]
86 1 YES 2 forced=[esrb_base.prm] keyless=[esrb_base.prm]
130 1 YES 2 forced=[esrb_base.prm] keyless=[esrb_base.prm]
# rule DECIDES the order on entries [2, 3, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 86, 130]
# rule merely AGREES on entries []
# /disc/dat/GP_GAMEOVER.pak
# entry forced decides elements note
4 1 YES 13 forced=[pnbase.tbm] keyless=[pnbase.tbm]
7 1 YES 13 forced=[pnbase.tbm] keyless=[pnbase.tbm]
# rule DECIDES the order on entries [4, 7]
# rule merely AGREES on entries []
# /disc/dat/GP_HANGAR_ARSENAL.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_LEADERBOARD.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_MAIN_GAME_D.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_MAIN_GAME_D2D.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_MAIN_GAME_E.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_MAIN_GAME_E2D.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_MAIN_GAME_F.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_MAIN_GAME_F2D.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_MAIN_GAME_I.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_MAIN_GAME_I2D.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_MAIN_GAME_J.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_MAIN_GAME_J2D.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_MAIN_GAME_S.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_MAIN_GAME_S2D.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_MISSION_LOG.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_MISSION_SELECT.pak
# entry forced decides elements note
3 1 YES 15 forced=[px_mission_base.tbm] keyless=[px_mission_base.tbm]
5 1 YES 15 forced=[px_mission_base.tbm] keyless=[px_mission_base.tbm]
# rule DECIDES the order on entries [3, 5]
# rule merely AGREES on entries []
# /disc/dat/GP_MOVIE_THEATER.pak
# entry forced decides elements note
0 1 YES 12 forced=[px_movie_base.tbm] keyless=[px_movie_base.tbm]
1 1 YES 12 forced=[px_movie_base.tbm] keyless=[px_movie_base.tbm]
# rule DECIDES the order on entries [0, 1]
# rule merely AGREES on entries []
# /disc/dat/GP_OPTIONS.pak
# entry forced decides elements note
0 2 no 2 forced=[po_menu_base.tbm,po_menu_base.tbm] keyless=[po_menu_base.tbm,po_menu_base.tbm]
2 2 no 2 forced=[po_menu_base.tbm,po_menu_base.tbm] keyless=[po_menu_base.tbm,po_menu_base.tbm]
# rule DECIDES the order on entries []
# rule merely AGREES on entries [0, 2]
# /disc/dat/GP_PAUSE_MENU.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_READY_ROOM.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_SAVE_LOAD.pak
# entry forced decides elements note
37 2 no 13 forced=[pfbase.tbm,pfbase.tbm] keyless=[]
40 2 no 13 forced=[pfbase.tbm,pfbase.tbm] keyless=[]
46 1 YES 10 forced=[pgloading_eff00.prm] keyless=[pgloading_eff00.prm]
58 1 no 9 forced=[pfbase.tbm] keyless=[]
69 1 YES 10 forced=[pgloading_eff00.prm] keyless=[pgloading_eff00.prm]
78 1 no 9 forced=[pfbase.tbm] keyless=[]
89 1 YES 7 forced=[px_replay_base.tbm] keyless=[px_replay_base.tbm]
93 2 no 13 forced=[pfbase.tbm,pfbase.tbm] keyless=[]
98 1 YES 7 forced=[px_replay_base.tbm] keyless=[px_replay_base.tbm]
99 2 no 13 forced=[pfbase.tbm,pfbase.tbm] keyless=[]
# rule DECIDES the order on entries [46, 69, 89, 98]
# rule merely AGREES on entries [37, 40, 58, 78, 93, 99]
# /disc/dat/GP_STAGE_CLEAR.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/GP_SYSTEM.pak
# entry forced decides elements note
0 1 YES 21 forced=[pqbase.tbm] keyless=[pqbase.tbm]
1 1 YES 21 forced=[pqbase.tbm] keyless=[pqbase.tbm]
# rule DECIDES the order on entries [0, 1]
# rule merely AGREES on entries []
# /disc/dat/GP_TITLE.pak
# entry forced decides elements note
10 1 no 3 forced=[palogo_eff0.prm] keyless=[]
11 1 no 7 forced=[palogo_eff0.prm] keyless=[]
12 1 YES 10 forced=[pgloading_eff00.prm] keyless=[pgloading_eff00.prm]
13 1 no 3 forced=[palogo_eff0.prm] keyless=[]
14 1 no 7 forced=[palogo_eff0.prm] keyless=[]
15 1 YES 10 forced=[pgloading_eff00.prm] keyless=[pgloading_eff00.prm]
# rule DECIDES the order on entries [12, 15]
# rule merely AGREES on entries [10, 11, 13, 14]
# /disc/dat/GP_TUTORIAL.pak
# entry forced decides elements note
0 1 YES 18 forced=[pubase.tbm] keyless=[pubase.tbm]
1 1 YES 18 forced=[pubase.tbm] keyless=[pubase.tbm]
# rule DECIDES the order on entries [0, 1]
# rule merely AGREES on entries []
# /disc/dat/fonts.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/sound.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []
# /disc/dat/tables.pak
# entry forced decides elements note
# rule DECIDES the order on entries []
# rule merely AGREES on entries []

75
docs/re/structures/ui-forced-backdrop.md
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@@ -190,6 +190,81 @@ claim with the same consequence. Distinguishing them needs a capture of a screen
that carries one, which is behind the Ⓐ fault
([`ui-clock-freezes-at-settle.md`](ui-clock-freezes-at-settle.md)).
## 🔴 Stability is not necessity — 62 of the 80 are DECIDED by this rule (2026-08-30)
**Raised by the port agent, and it is right.** Every check this page has run — and
every re-check after a change elsewhere — measured whether a verdict *moved*. That
is the rule's **stability**. It never measured its **necessity**: an element whose
position is already fixed by a read `T8aD` key or an `implied_layer_key` is
*confirmed* by the rule, not *decided* by it, and on those screens removing the
rule entirely costs nothing.
So the question was asked directly. `derived_paint_order` was recomputed with the
`forced_backdrop` fallback removed and the two orders compared, over every
`dat/*.pak`:
| | instances |
|---|---|
| the rule **DECIDES** the order — it moves without it | **62** |
| the rule merely **AGREES** — order unchanged | 18 |
| **total forced** | **80** |
The 80 reproduces this page's own census exactly, which is the check that the probe
is looking at the same set. Data:
[`../data/forced-backdrop-necessity.txt`](../data/forced-backdrop-necessity.txt) ·
instrument: `crates/sylpheed-formats/examples/forced_backdrop_necessity.rs`.
**Every one of the 62 deciders is keyless — no keyed element is ever moved.** That
is the rule behaving as designed: it is a fallback, and it only ever fires where
nothing else can speak.
The 18 that merely agree split two ways, and the distinction matters:
* **14 have a key of their own** — `pfbase.tbm` (10) and `palogo_eff0.prm` (4). Here
the sort already had the answer and the rule reproduces it. ⚠️ **This is the part
that is not independent support.** `palogo_eff0.prm` being forced first agrees
with its measured order, but its `implied_layer_key` is `0x00000000` and would
have put it first anyway — so that agreement is the rule reproducing the crate,
not the game confirming the rule.
* **4 are keyless but inert** — `po_menu_base.tbm`, twice in each of two 2-element
builds. *Every* element on those screens is forced, so all keys collapse to the
same value and the declaration-index tie-break gives the identical order either
way.
### What actually holds the 62 up
Not a key — there is none — and not the `palogo_eff0.prm` control, which as above
is only decisive under a convention its own key already satisfies. **It is the
impossibility argument alone**: a full-screen quad that is opaque at some instant
cannot paint above everything visible then, or the screen is blank at that instant.
That argument is doing all the work on 62 instances, and its assumptions are
exactly the ones listed under *Reach* below — which is now a larger exposure than
this page previously implied.
By element, the 62:
| | count | kind |
|---|---|---|
| `pzeff00.prm` | 32 | `.prm`, colour census says pure black |
| `pgloading_eff00.prm` | 4 | `.prm` — 2 in `GP_TITLE` (entries 12/15), 2 in `GP_SAVE_LOAD` |
| `esrb_base.prm` | 2 | `.prm` |
| ten `*base*.tbm` families | 24 | `.tbm`, ❔ pixels never located |
⚠️ **24 of the 62 are `.tbm`**, and this page already records that a `.tbm`'s
pixels cannot be found anywhere on the disc. For those the two readings — "the rule
places it correctly" and "the element draws nothing, so its position is inert" —
remain indistinguishable, and both leave the composite right. The 38 `.prm`
deciders have no such escape: those are real quads with real colour, and the rule is
load-bearing on them in the full sense.
### For the port specifically
On the five menu screens the exposure is **two**: `GP_TITLE` entries **12 and 15**,
the dressed loading bundles, where `pgloading_eff00.prm` has neither a read nor an
implied key. Entries 10, 11, 13 and 14 — the four splashes — are unchanged with the
rule removed, exactly as the port measured on its own side. Two renderers, same
answer.
## Reach
⚠️ **Assumes straight alpha-over blending.** Blend mode is ❔ on