SCUMM v5 — screen effects (roomOps screenEffect, 0x33 sub-op 0x0A)

How room-transition fades are requested, what MI1 actually uses, and why only the effect state — not the transition animation — is modelled.

1. The opcode

roomOps (0x33) sub-op 0x0A is SO_ROOM_FADE. It reads one var-or-word operand a and splits it:

• switchRoomEffect = a & 0xFF — the fade-IN effect, played when the next room is revealed.
• switchRoomEffect2 = a >> 8 — the fade-OUT effect, played when leaving the current room.

Special case: a == 0 is not "effect 0" — it is the "fade the current room in NOW" trigger. It reveals the current room with the pending effect and leaves the two effect numbers unchanged.

This is all derivable from the opcode shape, so the engine records the two effect numbers plus the "fade in now" request as state.

2. The MI1 idiom

Every room change is bracketed by two fades:

cutScene []
roomOps screenEffect 0x8180   ; in=0x80(128) out=0x81(129)
loadRoom room=86              ; leave → fadeOut(switchRoomEffect2), then load
roomOps screenEffect 0x8181   ; in=0x81(129) out=0x81(129)
...
endCutScene
roomOps screenEffect 0x8181

(global #103, room 25 — the smallest clean example.)

3. The effect vocabulary MI1 uses

Surveying every global script, only three operand values ever appear:

The 0x80-high-bit values (128/129) dominate; effect 1 (a genuine transition wipe) appears only twice, in rooms not on the intro path.

4. State modelled, animation not

The engine records the requested effect numbers faithfully but renders every transition as an instant cut — because the effect-number → animation mapping (which of 1 / 128 / 129 is an instant cut vs. a dissolve vs. a scroll, and the exact pattern) isn't available from any public source.

On the intro-reachable path — Mêlée title (room 10) → "Le Tre Prove" (room 96) → first room (33) — only effect 129 appears, alongside a bare loadRoomWithEgo (room 96's entry has no fade at all). So every transition that can currently be watched is an instant cut in the original too, and the instant-cut rendering matches. Validating a real wipe animation would need a non-instant effect (1 or 128) to reach the screen.