# Primitive blend mode — narrowed to one element, and it does not matter

**Classification: undecodable, with reach.** Looked in the bundle, in the colour
census, in the occlusion constraint, and at the oracle. The question survives; its
*consequences* do not.

## The question

[`ui-prm-primitives.md`](ui-prm-primitives.md) leaves blend mode open: "a dim quad
at `0x7f000000` is presumably straight alpha over what is beneath, but the flash
(`0xf0ffffff`) and the coloured ones (`0x60ff0000`) may well be additive. Nothing
measured." [`ui-forced-backdrop.md`](ui-forced-backdrop.md) then assumed
alpha-over, so the assumption became load-bearing.

## Where I looked

**1. The bundle — nothing.** `ui-prm-primitives.md` already refuted a bundle-side
key for a primitive's *layer*, on two grounds that apply identically to blend: the
declaration entry's unread words are constant across every element of three
measured screens, and the bundle carries **no RATC child at all** for a primitive.
There is no field to read.

**2. The colour census — the population is overwhelmingly black.** Every
full-screen `*eff00*` primitive on the disc carries **pure black** at its various
alphas: `ff000000`, `7f000000`, `40000000`, `b2000000`, `cc000000`, `d4000000`,
`00000000`. The `*base*` elements are `ffffffff`. **The only non-black primitive on
the disc is `pbafc.prm`**, RGB `00e8e0` — cyan.

**3. The occlusion constraint — inapplicable to the one candidate.** `pbafc.prm`
looked alarming at a declared 844×600 and alpha `ff`. It is not what it looks like:

```
t=10  a=255  xy=(178,60)  s=2/3        t=22  a=255  xy=(186,60)  s=2/3
t=12  a=124  xy=(178,60)  s=2/3        t=24  a=124  xy=(194,60)  s=2/3
t=14  a=255  xy=(178,60)  s=2/3        …travelling to x=291…
```

It **strobes** between alpha 255 and 124 every 2 units, it **travels** across the
screen, and its **scale is 2 % × 3 %** — so it draws about **17×18 pixels**, not
844×600. It is a small moving glint. At that size it occludes essentially nothing,
so the constraint that settles the backdrops cannot reach it.

**4. The oracle — not reachable.** `pbafc.prm` lives in `GP_READY_ROOM`, which
[`ready-room-probe.md`](../ready-room-probe.md) recorded as a no-go, and reaching
gameplay needs button input, which
[`ui-clock-freezes-at-settle.md`](ui-clock-freezes-at-settle.md) records as
faulting the guest in this container.

## ✅ Why it does not matter for the rule that depended on it

For a **black** quad — which is every primitive `forced_backdrop` touches — the
two hypotheses differ only in whether it hides what is beneath:

| | drawn **first** | drawn **last** |
|---|---|---|
| **alpha-over** black at α=255 | paints black, content over it — **correct** | blanks the screen — **wrong** |
| **additive** black at α=255 | adds nothing — **correct** | adds nothing — correct |

**"First" is right under both hypotheses; "last" is right only under one.** So the
forced-backdrop rule's *verdict* is robust to the blend question even though the
question is open — and the port's original "layerless sorts last", which produced
solid black on `build_12`/`build_15`, is wrong under alpha-over and merely
pointless under additive.

⚠️ **This is not evidence that the blend is alpha-over.** It is the reason the
open question stops being a risk. `pbafc.prm` remains genuinely unknown, and it is
the only element on the disc where the answer could change a pixel.

## 🔴 A guard the investigation added, which no current verdict needed

`forced_backdrop` tested coverage from the **pivot alone**. `pbafc.prm` is the
disc's own proof that scale can shrink a nominally full-screen element to 2 %.

Checked before changing anything: **all 80 forced instances are at scale 100 % on
every opaque instant**, so no verdict moved. The guard now requires
`scale_x >= 100 && scale_y >= 100` at the instants it counts as opaque — a guard
against data not yet met, not a fix. 4 + 13 disc tests green either side.

## Reach

⚠️ The colour census covers keyless elements. A **sprite**'s blend is a separate
question, and `T8aD +0x04` bit `0x02` was already refuted as an additive flag.

❔ `pbafc.prm` is unsettled and would need either a `GP_READY_ROOM` capture or a
blend-state read from Canary, neither of which is available here.