re: the transition between screens is a fade through black, and most of

its timing is on the disc

Q7. Every title-side screen carries a full-screen black .prm quad that
paints last, and its keyframe group IS the transition: black at T0, clear
by T1, clear until T2, then back to black on exit. Read with the corpus's
start-of-a-ramp rule and Q1's time unit that gives 0.87s for EXTRAS,
0.97s for the main menu, 4.08s for the title -- from the file, not from a
stopwatch.

The disc-wide check is per-pak all-or-nothing rather than the 41% the
headline count suggests, and GP_TITLE's 6 of 12 is the useful row: the
six builds carrying a fade quad are exactly the six SCREENS, and the six
without are exactly the six overlays. GP_DIALOG is 0 of 133. That is
independent corroboration of the overlay finding from two iterations ago.

One piece is NOT on the disc and says so: the fade-OUT length. The fourth
keyframe has no time slot, because a group's last block stops four bytes
short. Measured instead, at 30fps, ~0.4s and the same both directions.

And a warning I earned: the luminance rise after a transition is NOT the
quad's ramp. The incoming screen's own elements animate in after the quad
has cleared -- 1.47s observed against a declared 0.97s. Time the fade
from where the frame is pure black.

Rig: screenshot samples at 0.5 Hz and cannot see a 0.4s fade at all,
which is why an earlier burst called this an instant cut. ffmpeg x11grab
at 30fps instead; both go in METHOD.

docs/re/screen-transitions.md

@@ -0,0 +1,117 @@
+# What happens between two screens — a fade through black, and where its timing lives
+
+**Status:** ✅ `CONFIRMED`. The quad and its ramp are **decoded** (a keyframe
+group on the disc, with a disc-wide check); the wall-clock timings are
+**measured** off the running game at 30 fps. One piece is **undecodable from this
+field** and is called out below.
+
+Answers [MISSION Q7](../port/MISSION.md).
+
+## The mechanism — decoded
+
+Every title-side screen carries a full-screen untextured primitive that is
+**black, and paints last**: `pteff00.prm` in `GP_TITLE`, `pfeff00.prm` in
+`GP_SAVE_LOAD`. That it sorts last was already established
+([`structures/ui-paint-order-key.md`](structures/ui-paint-order-key.md)); what is
+new here is that its **keyframe group is the transition**.
+
+The group is always four blocks, and always this shape:
+
+| block | alpha | meaning |
+|---|---|---|
+| 1 | `0xff` at `t = T0` | the screen starts **black** |
+| 2 | `0x00` at `t = T1` | ramp to fully clear — the screen **fades in** |
+| 3 | `0x00` at `t = T2` | clear; this is the resting pose, the quad is invisible |
+| 4 | `0xff`, **no time** | ramp back to black — the screen **fades out** on exit |
+
+Read with the corpus's rule that a keyframe is the *start* of a ramp
+([`structures/ui-resting-pose.md`](structures/ui-resting-pose.md)).
+
+```
+$ tools/re-capture/fade_quads.py 4 5 6          # GP_TITLE
+build 4 (title)      pteff00.prm   t=16 α=255   t=261 α=0   t=269 α=0   t=— α=255
+build 5 (main menu)  pteff00.prm   t=12 α=255   t= 70 α=0   t= 80 α=0   t=— α=255
+build 6 (EXTRAS)     pteff00.prm   t=12 α=255   t= 64 α=0   t= 74 α=0   t=— α=255
+```
+
+Under [Q1](ui-keyframe-time-unit.md)'s `1 unit = 1/60 s`: the screen holds black
+for **0.20 s**, then fades in over **0.87 s** (`EXTRAS`), **0.97 s** (main menu)
+or **4.08 s** (the title).
+
+### ❔ The fade-OUT duration is not in this field
+
+The fourth block has **no time** — a group's last block stops 4 bytes short and
+that word is already the next group's element index
+([`ui_layout.rs`](../../crates/sylpheed-formats/src/ui_layout.rs) documents the
+packing). So the disc gives the ramp's *target* (black) and not its length. That
+duration is **measured** below, and the port is authoring it.
+
+### The disc-wide check, and what it shows about overlays
+
+Over every `GP_*.pak`, counting bundles that have a `.prm` element **and** ≥8
+elements (screen-sized rather than a two-element fragment):
+
+> **40 of 97** carry a 255 → 0 → 255 quad; 56 of the 60 found disc-wide have
+> exactly 4 keyframes.
+
+41 % sounds weak until it is read per pak, where it is nearly all-or-nothing:
+
+| pak | with quad / screen-sized |
+|---|---|
+| `GP_MISSION_SELECT`, `GP_MOVIE_THEATER`, `GP_OPTIONS`, `GP_TUTORIAL`, `GP_SYSTEM` | 2/2 each |
+| `GP_BUNK`, `GP_CHALLENGE` | 6/6 |
+| `GP_STAGE_CLEAR` | 4/4 |
+| `GP_SAVE_LOAD` | 10/12 |
+| **`GP_TITLE`** | **6/12** |
+| `GP_DIALOG` | 0/133 |
+| `GP_DEBRIEFING_PILOTLOG` | 4/102 |
+
+**`GP_TITLE`'s 6 of 12 is the interesting row, and it is not a gap.** The six
+that carry the quad are exactly the six *screen* builds — title, main menu and
+`EXTRAS`, English and Japanese. The six that do not are exactly the six
+**overlays**: the `PRESS Ⓐ BUTTON` plate and the two `DELTASABER` plates
+([`ui-title-build-map.md`](ui-title-build-map.md)). An overlay composited onto a
+screen has no transition of its own, so it has no fade quad — which is
+independent corroboration that those builds are overlays rather than screens.
+`GP_DIALOG`'s 0/133 says the same thing about dialog boxes.
+
+## The timing — measured
+
+Recorded with `ffmpeg -f x11grab -framerate 30` over the game surface, mean
+frame luminance per frame; raw data in
+[`captures/transitions/transition-luminance.csv`](captures/transitions/transition-luminance.csv),
+filmstrip in
+[`transition-filmstrip.png`](captures/transitions/transition-filmstrip.png).
+
+| | main menu → `EXTRAS` (Ⓐ) | `EXTRAS` → main menu (Ⓑ) |
+|---|---|---|
+| press → first visible change | 0.07 s | 0.37 s |
+| **fade-out to black** | **0.367 s** | **0.400 s** |
+| **pure black** (luminance 0.02) | 0.233 s | 0.167 s |
+| luminance rise until settled | 0.567 s | 1.467 s |
+
+The fade-out is the number the file cannot give, and it comes out the same both
+ways: **~0.4 s**, i.e. ~24 units under Q1's rule.
+
+The black hold is *consistent with* the file's 12 units (0.20 s) but does not
+confirm it — the plateau spans the tail of the outgoing screen's fade-out and the
+head of the incoming screen's black, and this measurement cannot separate them.
+
+### ⚠️ The luminance rise is **not** the quad's ramp
+
+The two columns differ by 2.6× where the quad's declared ramps differ by only
+1.12× (58 units vs 52). The filmstrip says why: the background reappears *first*
+and the labels arrive after it, so what the luminance curve is timing is the
+incoming screen's **own element animations**, not the fade quad. Quoting 1.47 s
+as "the main menu's fade" would be wrong. The quad's ramp is decoded; the
+screen's build-in is a separate, longer thing.
+
+## For the port
+
+* a screen change is: **fade the outgoing screen to black over ~0.4 s**, hold
+  black briefly, then **fade the incoming screen in over its own declared ramp**
+  while its elements play their own keyframes;
+* the fade-in ramp is **read from the file** (`T1 − T0` on the screen's fade
+  quad);
+* the ~0.4 s fade-out and the black hold are **authored from this page** — the
+  disc does not carry them.