Sylpheed-Godot/docs/DECISIONS.md

# Decisions

One entry per decision that outlives the container it was made in. Newest last.
A decision that lives only in an agent's context is lost when that container
dies, which is what this file is for.

---

## P0 — the exporter, 2026-08-28

### The exporter reads one authored file, and stamps its provenance into the output

`export/` is derived and `authored/` is hand-written, and the natural reading of
that is that the exporter never touches `authored/`. But a screen has to be
*called* something, and the disc does not name its builds — the identification of
build 5 as the main menu is HANDOFF Q2, **measured against a live capture**, not
a field.

Two ways to handle that:

1. the exporter emits `build_05.json` and the runtime renames it from
   `authored/screen_names.json`;
2. the exporter reads that map and writes `main_menu.json` directly.

Chose **2**, with a condition: every name it applies carries `name_source:
"authored"` and a `name_why` quoting the evidence, and `check` **rejects** an
authored name with no `why`. The file that lands in `export/` is therefore still
honest about which of its fields is a measurement — which is the property the
derived/authored split exists to protect — while a human opening the tree sees
`main_menu.json` rather than having to resolve a rename in their head. A build
nobody has identified exports as `build_NN` with `name_source: "index"`, which is
a locator and not a claim.

This is the **only** authored input the exporter takes. Everything else in
`authored/` is applied by the runtime over `export/`.

### Sprites are per screen, not a flat pool

`main_menu` and `extras` both ship a `ptbase.t32` and they are different
pictures. A flat `sprites/` directory would have silently collided; whichever
screen exported second would have won, and the loser would have drawn the wrong
background with no error anywhere. `sprites/<subdir>/<screen>/<name>.png`.

### The format is executable

`sylpheed-export check --out export` validates a tree against `docs/FORMAT.md`
with no disc in hand. It exists because "the export is correct" is otherwise an
assertion, and because the P0 gate is *"validates against FORMAT.md"* — which is
not a thing anyone can confirm by reading.

It reads the tree the way Godot will: as a stranger, with no access to the disc,
the decoders, or the exporter's internals. It deliberately does **not** check the
export against the disc — that is what `sylpheed-cli screen render` is for, at P1.

Checked that it bites, rather than assuming: five mutations of a valid
`main_menu.json` — a broken `paint_order` permutation, a dangling
`focus_sprite`, a reversed `buttons` list, a `#rrggbbaa` colour, an invented
`name_source` — are each caught with a specific message.

### The highlight sprite pairs by name; `opt ` is exported but not believed

FORMAT v1 said `focus_sprite` came from the element's `opt ` link. That reading
was **measured and refuted** by the RE agent, and this export shows why plainly:
on the main menu, `opt ` chains `ptloop01 → ptloop02 → ptbtn01` — two decorations
and then a button. It is a linked list of something, and it is not focus.

The highlight is paired by **sprite name** instead (`ptbtn01.t32` ↔
`ptbtn01f.t32`), which is HANDOFF's convention and holds for all 54 real pairs on
the disc. It resolves all five main-menu buttons. The raw link is still exported
as `opt_link`, renamed so that nothing downstream mistakes it for navigation, and
so that whoever eventually decodes it has the data.

Note this is 🟡 a naming convention, not a decoded field. It is authored in
effect, and lives in the exporter only because it is a rule over disc data rather
than a value we chose.

### The paint order is exported, not authored

Q3 decoded it — a `u16` layer key at `+0x0A` of each `T8aD` sprite header,
stable-sorted with declaration index. So it is read in the exporter, per the
contract's own rule for a decoded answer, and `paint_order` in `export/` is a
derived field. `"paint_order"` is gone from `unresolved`; **`paint_order_ties`
replaces it**, because the tie-break is still unknown and costs one element's
blend on one screen.

Where an element has no `T8aD` header the key comes from the decoders' table of
keys **measured off the running game**. That is a different kind of fact, so it
is labelled: `layer_source` is `"sprite"`, `"implied"` or `"none"`, and a
consumer that needs to know whether a layer is read or measured can tell.

### Colours are exported as two fields with the byte order in the name

There are two modulate colours and they multiply: `tint` is RGBA, `fade` is
**ARGB** and its high byte is the alpha that ramps. v1's single `"#ffffffff"`
could not carry both and silently discarded the ramping alpha. They are exported
as `tint_rgba` and `fade_argb`, raw hex, byte order in the key — because getting
it backwards is silent and looks like an art bug rather than a parse bug.

### `t` stays raw

HANDOFF Q1 is answered — linear ramp, 2 units per rendered frame, working
conversion 1 unit = 1/60 s — but that conversion is **measured off the running
game, not read from the file**, and the finding itself flags the 27.6 present-
frames/second measurement as the part worth re-testing. If the game turns out to
present at 60 Hz, every duration halves.

So `t` is exported exactly as the disc spells it, `keyframe_time_unit` stays in
`unresolved`, and the conversion will live in one authored place at P2. One
constant to change, in a file that says it is a decision.

### The final keyframe has no `t`, and `check` enforces that

The disc has no time slot on the last keyframe of a group. A file that carries
one there has invented it. `check` rejects it — this is the one place where the
temptation to emit a plausible number is strongest and the resulting error is
completely invisible.

---

## P1 — Godot draws the screen, 2026-08-28

### The Godot side reads the manifest, not a path

`ExportTree` is the only class that knows where `export/` is: `SYLPHEED_EXPORT`
if set, otherwise `<project>/../export`. Screens are addressed by their manifest
**name** (`main_menu`), never by a file path, so the runtime never encodes the
archive's subdirectory and a re-export that moves a file does not break it. It
also checks `format` on both the manifest and each screen, and refuses a tree it
was not built to read rather than half-drawing one.

Textures are read as bytes and decoded with `load_png_from_buffer` at runtime.
They are deliberately **not** Godot-imported resources: `export/` is gitignored
and regenerated wholesale, and a `.import` sidecar per sprite would be derived
state living next to derived state, invalidated on every re-export.

### One CanvasItem draws the whole screen

`ScreenView._draw` walks `paint_order` and draws each element itself, rather
than making a node per element and leaning on `z_index`. The export's
`paint_order` is already back-to-front, so honouring it is a loop; expressing
the same order through sixteen nodes' z-indices would hide the one thing that is
still unresolved about it — the **ties** — behind Godot's own sibling rules,
where a change in the export would silently become a change in Godot's tree
order instead of a visible change in the draw sequence.

### P1 draws `rest` and nothing else

Every element is drawn at its resting pose. No keyframe interpolation: that is
P2, and it depends on the keyframe time unit, which is **measured** rather than
decoded. A milestone whose gate is a pixel diff must not have a measured
constant inside it, or the diff stops being evidence about the port.

For the same reason `focused_id` is empty at P1. Initial focus was measured as
unstable boot to boot (HANDOFF Q5), so choosing one is an authored decision and
it belongs to P5, where a human is pressing keys.

### Nearest-neighbour, and why that is not a preference

`TEXTURE_FILTER_NEAREST`. The export is a 1:1 copy of the disc's own texels and
elements draw at up to 500 %; a bilinear filter invents detail the disc does not
have. It is also what the reference renderer does — `ui_layout::blit` maps
destination to source by integer division — so a filter difference cannot
masquerade as a placement difference in the diff.

### The capture is the SubViewport, not the window

The screen is drawn into a `SubViewport` sized to the export's own `design`
rectangle and shown through a container that scales it to the window. The first
attempt captured `get_viewport()` and got **1235×695**: there is a window manager
on the Xvfb display and its title bar had eaten 45×25 px of a screen the export
declares as 1280×720. A gate that compares a rescaled 1235×695 capture against a
1280×720 composite measures the compositor.

So `--capture` grabs the SubViewport texture: exactly the design rectangle,
independent of the window, directly comparable with `screen render` with no crop
and no resample. The windowed run is still worth doing — it is what proves a
human sees the screen — but it is not what the numbers come from.

## P1 gate — the diff, and what it found

`tools/verify-screen` renders every screen in the manifest both ways and reports
the largest per-channel difference anywhere in the frame. Both renderers are held
to the same inputs: the reference CLI built by `build-reference-cli` from the
revision the exporter is **pinned** to (not `/reborn/target/`, which is a live
mount that moves mid-iteration), `--black` because the screen carries its own
background, and `--primitives --animated` because those are what make the CLI
draw the same element set the port draws at rest.

| screen | build | max per-channel Δ | |
|---|---|---|---|
| `main_menu` | 5 | **3** | the P0/P1 gate screen |
| `main_menu_jp` | 8 | 3 | |
| `extras` / `extras_jp` | 6 / 9 | 4 / 3 | |
| `press_start` / `press_start_jp` | 2 / 3 | 1 | |
| `build_00` / `build_01` | 0 / 1 | 3 | |
| `build_10` / `build_11` | 10 / 11 | **0** | byte-identical |
| `title` | 4 | 6 | paint-order tie, below |
| `title_jp` | 7 | 154 | sampling phase, below |

`main_menu` — the milestone's own gate — agrees to **≤3/255 on every channel of
every pixel**, RMSE 0.38 %, with **no** pixel differing by more than 4 %. 3/255
is what integer-truncating compositing in the CLI and float rounding on a GPU
differ by; there is no structural disagreement anywhere in the frame.

Three screens exceed that, and each has a named cause rather than a threshold.

### `title`: a tie in the paint order — neither renderer is wrong

Build 4 is the one screen where the CLI uses a paint order **measured off the
running game** instead of deriving it. Compared against the order this port
exports, every single disagreement is **inside a tie** — the two orders differ
only among elements carrying *identical* layer keys (`0x8083`, the `back2` glow
group, and `0x80a0`):

```
derived : … 15, 16, 17, 18, 0, 1, 2, 3, 4, 5, 7, …
measured: … 15, 18, 16, 17, 0, 2, 4, 7, 1, 3, 5, …
```

That is exactly the residual HANDOFF Q3 documents and this export already
declares in `unresolved: ["paint_order_ties"]`. It is worth stating what it
costs: **904 px** in the glow band at (445,117)–(1195,313), all of them 4–6/255.
The port keeps the stable sort, per HANDOFF's own recommendation. Nothing to fix,
and nothing to tune — a "fix" here would be fitting the port to one screen's
capture.

Two of the reordered indices (`0x80a0`) are `kind & 0x4` template instances that
both renderers skip, so the only real reorder outside the glow group is
`ptlogo2` against `ptlogo_tm`, which do not overlap.

### `title_jp`: nearest-neighbour sampling phase — the CLI is the one I would call wrong

`title_jp` is the **only** screen in the export with a drawn element at a scale
that is not a whole multiple of 100 %: `ptlogo_eff2` at 125 %. It is also the
only screen with a difference above 6/255. The two facts are the same fact.

At a non-integer ratio the two renderers pick different source texels:

* `ui_layout::blit` samples the source at the destination pixel's **top-left
  corner** — `sxi = col * sw / dw`.
* A GPU samples at the destination pixel's **centre** — `floor((col+0.5)·sw/dw)`.

At 125 % those disagree on one column in five, which is why the differing pixels
are ~30 above 100/255 strung along thin diagonal edges rather than a shifted
region. At every whole multiple of 100 % they agree exactly, which is why the
other eleven screens are clean.

**Which is wrong:** the CLI, I think. Corner-sampled nearest is a half-
destination-pixel bias toward the top-left that no rasteriser produces, and the
Xenon GPU that drew this screen sampled at pixel centres. But I have no
framebuffer capture of `title_jp` and the disagreement is sub-pixel on one glow,
so this is a reading, not a measurement — recorded in `docs/BLOCKED.md` rather
than acted on. **The port is not changing to match**, because matching the CLI
here would mean deliberately reproducing a half-pixel offset in order to make a
number smaller.

### `extras`: two pixels

Two pixels at 4/255. Rounding.

### What the diff cannot tell us

The pivot question in `docs/BLOCKED.md` predicted that a P1 diff could not
distinguish "anchor scale to the declared pivot" from "anchor to half the
texture", because both renderers use the declared pivot. That prediction held:
the port and the CLI agree on every scaled element, and that agreement is **not
evidence** about which anchor the game uses. It stays open.

### `pteff05.t32` and `pteff04.t32` have no sprite, and that is correct

Both renderers skip them. The bundle declares them and carries **zero** RATC
children for either, so there is no texture on the disc to export — this is a
property of the disc, not a gap in the exporter, and `ScreenView` reports it as
`no sprite in the export` rather than dropping it silently.

---

## P2 — keyframe animation, 2026-08-28

### The time unit is authored, in one file, and says loudly that it is not on the disc

`authored/timing.json`. HANDOFF Q1 is answered — linear ramp, 2 units per
rendered frame, 1 unit = 1/60 s — but that conversion is **measured off the
running game**, not read from a file, which is exactly the case the
derived/authored split exists for. It is expressed as
`keyframe_units_per_second: 60` rather than seconds-per-unit so the value is
exact instead of a repeating decimal, and it carries the two independent lines
that support it. `t` stays raw everywhere in `export/`; seconds appear only
where this file is applied, which is one line of `boot.gd`.

`exit_ramp_seconds` is deliberately **null**. See below.

### The timeline stops at the last *timed* keyframe, and never plays the exit

The last keyframe of every group carries **no `t`** — the disc has no time slot
there. Across this export that final frame is an *exit* pose: for 116 of 134
elements it differs from the last timed keyframe **in alpha only** (a fade-out),
for 12 it is the loading splash's scale-and-slide exit, and for 6 it is
identical (no exit animation at all).

So the group is `pre-roll → ramp in → hold → [exit]`, and the port plays it up to
the hold and stops. Playing into the exit would mean **inventing how long the
ramp takes**, because the disc does not say. That duration is the screen
transition — HANDOFF Q7 measured it at ~0.4 s — and it belongs to P3, with its
own evidence. This is why `exit_ramp_seconds` is null rather than 0.4: P2 has no
business holding it.

### The interpolation is checked by where it lands, not by inspection

For **8 of the 12** screens the settled timeline is **byte-identical** to the
`--pose=rest` render. That is the useful assertion: the port walks the keyframes
with an authored time unit and arrives, to the pixel, at the pose the pinned
decoders independently identify as the resting one. `tools/screen-strip` reports
this per screen, so a change to the interpolation that drifts by one unit shows
up as a diff rather than as nothing.

The four that differ do so for two distinct reasons, below.

## `rest` misidentifies six elements, and the running game says so

On `main_menu`, the settled timeline and `rest` differ in exactly one region:
**400×470 at (440,108)** — the bounding box of `ptframe1` and `ptframe2`, and
nothing else on the screen.

`rest` puts both at their **first** keyframe: off-position and fully
transparent. The keyframes say they slide (620,108)→(440,108) and (403,267)→
(583,267) while fading 0x00→0xff, and then hold that pose for their last three
keyframes including the untimed one.

`/reborn/docs/re/captures/main-menu-oracle.png`, a capture of the running game,
**shows them**: the bright circuit-frame bracket around the menu, with a ring at
the bottom right. Cropping the same 250×180 region from the capture and from
both renders puts the ring and its elbow trace in the port's timeline render
**pixel-aligned with the game's**, and absent from the `rest` render. That is
geometry, not luminance, so it does not depend on the capture's gamma or on the
fact that it was taken with `NEW GAME` focused.

### Why the decoders get it wrong, precisely

`ui_layout::rest_plateau` excludes a run of identical keyframes that **ends the
group**, because that run is normally the exit — the comment cites the pause
menu, where taking the trailing run erased the word PAUSE. That exclusion is
right in general and wrong for an element with **no exit animation**, where the
trailing run *is* the hold. The rule then falls back to an earlier run, which
for a slide-in is the invisible pre-roll.

The condition that identifies the affected elements exactly, with no false
positives in this export, is:

> the final untimed keyframe has the **same pose** as the last timed keyframe

Six elements match it and `rest` misses all six: `ptframe1`/`ptframe2` on
`main_menu` and `main_menu_jp`, and `pteff02` on `title` and `title_jp`. This is
a **finding for the RE agent** about `sylpheed-formats`, not something this port
fixes: the decoders are pinned and must not be reimplemented here. The port
simply does not use `rest` — it derives the arrived pose from the keyframes,
which needs no heuristic — and `verify-screen` still asks for `--pose=rest` so
that renderer-vs-renderer diffing compares like with like.

Note what this says about P1: the port and the reference renderer **agreed** on
`main_menu` to 3/255, and both were missing two elements the game draws. Two
renderers reading the same field through the same decoder agreeing is not
evidence that the field is right. `docs/BLOCKED.md` had already said that about
the pivot; here it bit for real.

## The title is not settled, and P2 does not claim it

`title` and `title_jp` differ between the two modes by much more (max 142 and
247), and there the disagreement is **not** the six-element bug alone. `rest`
picks a mid-timeline hold for several glows (`pteff01`, `ptlogoall_eff`,
`ptlogoall_eff2`, `ptlogo_back2eff5`) where the timeline runs on to a much
brighter pose.

I could not settle which is right, and did not try to make the numbers agree:

* No element's alpha ever reverses direction anywhere in this export, so the
  title's 4.48 s timeline is a slow one-way ramp, not a pulse — which removes the
  obvious reason to expect a loop, but does not prove there is none.
* The only live title capture composites the **`PRESS Ⓐ` plate (build 2) over
  the title (build 4)**, so it cannot be diffed against build 4 alone. Mean
  luminance is oracle 64.1, `rest` 62.8, timeline 80.0 — which looks like it
  favours `rest`, except that the plate *adds* brightness and `rest` is carrying
  a 25 % black dim quad (`pteff02`) that is itself one of the six misidentified
  elements. The comparison is confounded in both directions and settles nothing.
* **Both modes are visibly wrong anyway.** Side by side with the capture, the
  port draws a washed-out cyan glow slab across the logo that the running game
  does not have — in `rest` mode too. That is a third problem, independent of
  this one, and it is P3's.

So: the timeline is the default because it is derived from the disc's own
keyframes with one measured constant and no heuristic, and because it is proven
right on the screen this milestone gates. On the title it is **unverified**, and
P3 should not assume P2 settled it.

---

## P2, corrected — the pin moved, and the settle rule was wrong, 2026-08-28

### Answering the RE agent's question: which six, and on what screens

They asked, having found only two elements on the English main menu satisfying
the condition this port proposed. The six span the whole 12-screen export:

| element | screens | trailing run |
|---|---|---|
| `ptframe1`, `ptframe2` | `main_menu`, `main_menu_jp` | alpha `0xff` — **visible** |
| `pteff02` | `title`, `title_jp` | alpha `0x00` — **transparent** |

So four of the six are the pair they already found, once per language build, and
their alpha rule accepts exactly those. The other two are `pteff02`, whose
trailing run is transparent, so their rule **excludes** it and leaves `rest` at
`0x40`.

**That exclusion is right, and their own measurement proves it.** `pteff02` is
the 25 % dim quad; they measured the title render going from **+13.14 to +0.55**
against the plate-free capture once the dim is drawn. `rest` must therefore stay
at `0x40` and must *not* move to the transparent trailing run — which is what
their rule does. Two investigations converging from opposite directions.

The condition this port proposed was **too loose**; the alpha discriminator is
the correct rule and the port has no amendment to offer.

### The pin moved 8b6dbcf → 5414db3

Its own commit, and what I wanted from it is the fixed `ui_layout::rest()`.
Pinned at `5414db3` rather than `4bc9706` where the fix was written, because
`5414db3` is where it carries its disc-wide check — 30 of 13 991 elements move,
4 become visible, **0 become invisible**.

The re-export is the evidence the change was contained: **two files changed, and
within them exactly four `rest` blocks** — `ptframe1`/`ptframe2` on both main
menus moving from `(620,108)/(403,267)` at `t=16` and alpha `0x00` to
`(440,108)/(583,267)` at `t=62` and alpha `0xff`. Every diff line pairs; the
other ten screens are byte-identical, `pteff02` did not move, and no sprite
changed.

### The settle rule was wrong, and their title finding is what showed it

P2 shipped "hold the last **timed** keyframe", on the reasoning that the exit is
the final untimed frame. **That is wrong**, and the title is the counter-example:
`pteff02` holds at `t=46` with the dim at alpha `0x40` and then ramps to `0x00`
by `t=236`. The exit is not only the untimed frame — it can be a long run of
timed ones. Running to the end drops the dim and makes the whole screen ~13/255
too bright, which is exactly the luminance excess P2 recorded (oracle 64.1,
`rest` 62.8, timeline 80.0) and could not explain.

A group is `pre-roll → ramp in → hold → ramp out → post-roll`, and a screen that
has arrived sits on **the hold**. So the timeline now plays in and stops at
`rest`, which is the decoders' identification of that hold and carries its own
`t`. `settle_units()` is `rest.t`.

The check is that the disagreement vanishes: on **all twelve** screens the
settled timeline is now byte-identical to the `--pose=rest` render, where before
this change four of them differed by up to 247/255. The timeline's endpoint
*should* be `rest` — the animation is what the timeline adds, not a different
destination — so this is the property to want, and it now holds without a
special case.

That also retires P2's open question about looping, from the other side: the RE
agent measured that groups hold rather than loop (`ptloop01`/`ptloop02` park
off-screen at x=1521 and x=−839; 18 s of settled title sits at sd ≤ 0.01).

## The reference renderer was stale for three diff runs

Worth recording as a process failure, because it defeated the project's whole
verification method for a while and it failed *silently*.

After bumping the pin I rebuilt the reference CLI, and `build-reference-cli`
reported success at rev `5414db3`. `verify-screen` then showed `main_menu`
jumping from 3/255 to **72/255**. The natural reading — the port had regressed —
was wrong. The port was right and **the reference was a revision behind**: the
shared `CARGO_TARGET_DIR` still held a `sylpheed-cli` built from `8b6dbcf`, and
cargo reported `Finished in 0.13s` and left it in place. Building into a clean
target directory produced a binary that resolves `ptframe1` to `(440,108) t=62`;
the shared one still said `(620,108) t=16`.

The old check — "does `screen list` run?" — cannot catch this, because a stale
binary runs perfectly.

Two changes:

* `build-reference-cli` builds into `$CARGO_TARGET_DIR/reference-cli/$rev`, a
  tree **keyed by the pinned revision**, so a new pin has no artifacts to reuse.
  A stable copy is placed alongside for consumers.
* It then checks the binary **against `export/`**: both come from the same pin,
  so if the CLI resolves `ptframe1`'s rest differently from what the exporter
  wrote, the two halves of the verification are not the same revision and it
  fails loudly. It compares the two rather than asserting a literal, so it stays
  true when the pin moves again.

`docker/bin/` is baked into the image, so this takes effect on the next image
build; until then the repo copy has to be invoked by path. The RE agent hit the
same class of trap this session from the other side (`./target/debug` stale
against a redirected `CARGO_TARGET_DIR`). It is worth naming the general shape:
**a build system reporting success is not evidence that the artifact you are
about to trust is the code you pinned.**

### What this did not change

`title` (6/255), `extras` (4/255) and `title_jp` (154/255) are unchanged, and
their diagnoses stand — a paint-order tie, two pixels, and nearest-neighbour
sampling phase at 125 % scale. The title's swoosh defect the RE agent localised
(drawn thick and white where the game draws it thin and pink) is untouched by
any of this and remains P3's.