Sylpheed/docs/port/blend-decoded-adoption.md

The blend map is deleted — and adopting the decoded field found a counter-example

Status: ✅ adopted. ❌ My counter-example failed — the bit is right and the regression is a metric artefact. See the last two sections. Port at 7dd754f + this commit; formats pinned at formats-pin-2026-09-01; HANDOFF on this branch answers 9ca1eb5.

PORT-MISSION §3: "When the RE agent later decodes something you had authored, delete the authored entry and let the exporter emit it. That deletion is the measure of progress." This is that deletion.

What changed

	before	after
source	authored/rendering.json → additive_elements, keyed by screen name	blend_additive per element, emitted by the exporter
origin	transcribed from the Decoder's per-draw RB_BLENDCONTROL0 log	decoded — T8aD +0x04 bit 0x02
reach	three screens somebody drove the game to	every screen on the disc

The pin bump is its own commit (7dd754f). The exporter emits blend_additive on Element and on nested focus/leaf elements — both spellings of the accessor are needed, because a button's focused variant is reached through focus_link and ptbtn00f.t32 is in build.sprites while no element carries it as sprite. ptbtn00f is exactly the sharp case: the plate is alpha-over and its own glow is additive, on one screen in adjacent draws.

The check before the swap — the map was a subset, not the answer

Over main_menu, extras, press_start and title:

	count
map says additive and the disc agrees	15
map says additive and the disc does not	0 — no contradictions
disc says additive and the map did not	17

Nothing transcribed was wrong. It was incomplete and was being read as complete. The 17 include:

• pteff03 / pteff03a — the sweep leaves. draw_leaf_for means those are what actually reach the screen while the map listed their parents ptloop01/ptloop02. (Both parent and leaf carry the bit, so this one turned out to change nothing — established below, not assumed.)
• twelve on title, where the map was deliberately empty. The port has been drawing every title effect alpha-over.

And it answers BLOCKED.md H6 with no capture at all. The JP asymmetry — the port drawing main_menu additive and main_menu_jp alpha-over, asserting by omission that the JP build differs — was an artefact of a name-keyed map. The bit is on the disc for every screen at once.

🔴 The regression, which is one element

Scored against the oracle captures, on the GPU, before and after:

screen	before	after	Δ
main_menu	10.88	13.02	+2.14
main_menu_options	11.56	13.57	+2.01
extras	13.10	13.10	—
title	14.11	14.11	—
title_plate, title_band, both splashes	unchanged	unchanged	—

The scores are deterministic — two further runs gave 13.02 / 13.10 / 13.57 to the digit — so this is a real change, not sampling noise.

It is pteff10, isolated

• main_menu's only newly-additive top-level element is pteff10.
• extras has no newly-additive top-level element, and its score did not move. That is the control: the same change applied to a screen with nothing new moves nothing.
• The leaf rule was tested separately by disabling it — main_menu stayed at 13.02, so pteff03/pteff03a are not the cause. That prediction of mine failed and the rule was restored, being provably neutral here.

title did not move despite twelve newly-additive elements, which is consistent: verify-capture poses at settle t=198, and the title's effect quads — ptlogo_back2eff1…5, ptlogoall_eff, pteff01 — are transparent there.

🔴 WHY I THOUGHT THIS WAS A COUNTER-EXAMPLE — and it was not. Kept because the premise-check is the lesson

Their own map lists pteff10 as additive on extras and not on main_menu, and they logged both screens. So either their per-draw log shows main_menu's pteff10 drawn alpha-over — a direct contradiction between a capture and the disc bit, on one element — or it was not drawn during that capture. The oracle comparison independently prefers alpha-over there.

❌ Wrong, and the premise was the failure. The oracle does measure it additive on main_menu — three sessions, every frame. What I read was a stale coverage table of theirs sitting upstream of its own correction. I inferred "their log does not cover this" from a table, and called it a contradiction with a capture. The lesson is not that the map was stale; it is that I treated a summary as the log. See the resolution at the foot of this page.

Why the change ships anyway, stated rather than assumed

1. main_menu carries a ±3.78 capture-phase term in the harness's own note — the capture caught the free-running sweep at an unknown phase. +2.14 is inside that stated uncertainty and cannot adjudicate a disc fact. main_menu_options is a sub-region of the same screen and inherits the same sweep.
2. The decoded source is far better evidenced than the comparison that moved, and it fixes two known defects — twelve title effects drawn with the wrong blend, and a JP/EN asymmetry the port was asserting by omission.
3. Fitting an exception for pteff10 would put an authored entry back to make one number smaller. That is the move this project keeps having to undo.

🔴 This was a known regression shipped deliberately, not an unnoticed one — and the decision was right for a stronger reason than the one I used. Not only is +2.14 inside the ±3.78 phase term: the oracle had already adjudicated this element, so the metric is the thing disagreeing, not the render.

What this does not claim

• That the bit is wrong. One element, inside a stated uncertainty, against a disc-wide check with an out-of-sample prediction.
• That pteff10 on main_menu and on extras are the same sprite. Not checked.
• That the leaf rule is right — only that it is neutral here, so nothing in this page rests on it.

---

❌ The counter-example failed, and the regression is RMSE's area-weighting

The oracle had already adjudicated pteff10

blend-bit-vs-oracle.txt carries it on both screens — entry 5 (main menu) and entry 6 (extras), +0x04 = 0x8832, bit set, both labels read out of the guest command stream — and HANDOFF records it "additive, in all three menu sessions, every frame."

My premise was a stale coverage table, not a reading of the log. The correction existed; the wrong table was still visible upstream of it. So the adversarial attempt lands as survived: the claim is stronger for having been challenged, and the challenge cost one message.

⚠️ And the regression was flagged on this exact element before I adopted it — 🟡 in HANDOFF: nearly exact under alpha-over in our render, additive in the game, "the one row here your renderer does not independently corroborate."

But their explanation makes a prediction, so I checked it

If additive and alpha-over nearly coincide on a dim glow (max alpha 130) over a dark background, the score should barely move. Mine moved 20 %. That is a real tension and it was worth one measurement.

Diffing the two port renders — the only change between them is pteff10's blend:

pixels identical	373 588 (40.5 %)
pixels differing by > 3	330 794 (35.9 %)
maximum difference, anywhere	32 levels
pixels differing by ≥ 60	0
bounding box	x 288–993, full height

Their explanation holds and my scepticism does not. No pixel moves by more than 32/255 — 12.5 % — which is "nearly coincide" measured rather than asserted. What moved the RMSE is area: a third of the frame shifting slightly.

The thing worth keeping: RMSE cannot tell broad-and-shallow from narrow-and-deep

raw-rmse is area-weighted, so a shallow change over a third of the frame moves it far more than a deep change over a few hundred pixels — and the number alone does not say which you have. 10.88 → 13.02 reads like a serious regression and is a difference invisible to an eye.

This bears on every verify-capture number in the corpus, not just this one. A row that moves should be asked "broad or deep?" before it is called a regression, and that costs one histogram.

What I did NOT do: exclude the element

The Decoder suggested considering pteff10 excluded from the metric with the reason stated, rather than letting 13.02 sit as debt. I have not, and the measurement above is why: max-32 over a wide area is a characterised difference, not an unscoreable one. Excluding it would remove the only signal that would fire if the bit were ever wrong on some other screen — and the reason to exclude ("the metric can't distinguish these") is now known to be false; it distinguishes them fine, it just weights them by area.

The 13.02 stands, with its explanation attached. That is cheaper than an exclusion nobody will revisit.