# The paint order comes from a layer key in the T8aD sprite header

**Status:** ✅ `CONFIRMED` on both screens whose paint order has been measured —
the key is **non-decreasing in paint order on every element that has a sprite**,
20 of 24 on one and 6 of 7 on the other. 🟡 ties are not explained. ❔ the field's
full meaning (it looks like flags, not a plain depth).

## The field

Every `T8aD` sprite begins with a 44-byte header. The word at **`+0x08`** —
untouched by this project's decoder, which reads width/height/tile-count at
`+0x14`/`+0x18`/`+0x1c` — sorts the screen.

Read in the order the game paints them (`ui-screen-runtime.md` has how that order
was measured — off the live child list, checked against a draw capture):

**`GP_TITLE` build 4, the title screen**

| paint slot | element | sprite | `+0x08` |
|---|---|---|---|
| 0 | 9 | `ptbase2` | `0x8000` |
| 3 | 10 | `pteff04` | `0x8020` |
| 5 | 6 | `pteff01` | `0x8040` |
| 6 | 20 | `ptlogo_back2eff` | `0x8081` |
| 7 | 19 | `ptlogo_back2` | `0x8082` |
| 8–12 | 14,15,18,16,17 | `back2eff1…5` | `0x8083` ×5 |
| 13–19 | 0,2,4,7,1,3,5 | `ptlogo1`/`tm`/`ptlogo2` | `0x80a0` ×7 |
| 20 | 22 | `ptlogoall_eff` | `0x80a8` |
| 21 | 23 | `ptlogoall_eff2` | `0x80a9` |
| 22 | 21 | `ptcopyright` | `0x8100` |

**`GP_TITLE` entries 11/14, the developer-logo splash**

| paint slot | sprite | `+0x08` |
|---|---|---|
| 1–3 | the three `_eff` glows | `0x a100` ×3 |
| 4–6 | the three base logos | `0x a110` ×3 |

Both are **ascending, with no inversion anywhere**. On the splash it explains the
whole permutation — the glows sort before their logos because `0xa100 < 0xa110`,
which is why the declaration table's interleaving (base, glow, base, glow) is not
what you see.

## Why this matters

It is the first **file-derivable** account of the paint order. Everything checked
before it failed: declaration order and its reverse, the placement region, the
RATC child order, keyframe start and rest times, resting Y, the runtime element
record's fields, and every other build's table. This is a per-sprite value the
port can read directly.

## What is not settled

* **Ties.** Two groups share a key (`0x8083` ×5 and `0x80a0` ×7) and the game
  paints them in an order that is *not* the declaration order —
  `14,15,18,16,17` and `0,2,4,7,1,3,5`. Something breaks those ties and it is not
  known. For the port it may not matter (tied elements are same-layer, and the
  three `ptlogo1`/`ptlogo2` instances are the ghosts that are not drawn at rest),
  but it is unmeasured, not proven harmless.
* **The field's meaning.** `0x8000`, `0x8020`, `0x8040`, `0x8081`… `0x8100` on
  one screen and `0xa100`/`0xa110` on another look like flag words with a layer
  in some of the bits rather than a plain integer depth. Sorting on the whole
  word works on both screens; which bits actually carry the layer is unknown.
* **Two screens is two screens.** A third measured permutation would either
  promote this to a rule or break it. The cheapest one available is any screen
  whose object is resident at the same time as the title's.


## Landed in the compositor (2026-08-19)

`ui_layout::compose` now paints in the **derived** order — a stable sort of the
elements by `sprite_layer_key` — for every build except the two whose measured
order is hard-coded, which stay as the ground truth they are. Elements with no
sprite (the `.prm` primitives) have no key; they keep their declaration position
among themselves and `compose` skips them anyway.

Verified three ways rather than by a green build:

* a disc-gated test asserts the measured orders are **non-decreasing** in the key
  and that the composite's key sequence comes out sorted — and it was checked
  both ways: reading the word from `+0x0c` instead of `+0x08` makes it fail;
* the title still composites identically (its measured order is used);
* a screen nobody has captured — `GP_MISSION_SELECT` — now composites cleanly
  ([capture](../captures/mission-select-derived-order.png)).

**Found on the way, and worth its own line:** the developer-logo splash bundle
has no `.rat` child, so `ui_layout::is_build` rejects it and the compositor never
sees it. Its measured order is therefore inert in practice, and the splash cannot
be rendered by `screen render` at all. That is a separate gap in what counts as a
"build", not a paint-order question.