Sylpheed/docs/re/structures/ui-blend-mode-measured.md

The UI blend mode — ✅ MEASURED: the frames are drawn ADDITIVE

Status: ✅ CONFIRMED, classification measured — nothing on the disc selects it, and this is what the running game tells the GPU, per draw, on two screens. 2026-08-31.

Closes the one route t32-blend-mode-not-on-disc.md left open: "the executable's draw path, which is where a mode selected in code rather than data would live. That is a route, and it is the one left."

The answer

The title-side UI uses two blend states, and one pixel shader:

RB_BLENDCONTROL0	src	op	dst	what it is	drawn this way
0x07010701	ONE	ADD	1−SRC_ALPHA	alpha-over, premultiplied	ptbase, pteff05, the fade quad, ptmsg, ptmsg2, pttitle, every button
0x01010101	ONE	ADD	ONE	ADDITIVE	ptframe1, ptframe2, ptframe3, pteff20, both rotated sweep strips
0x00010001	ONE	ADD	ZERO	opaque, blending off	the one non-UI blit that opens the frame

Reference data and both raw logs: data/ui-blend-mode-measured.txt · captures/ui-draws/blend-main-menu-2026-08-31.log · captures/ui-draws/blend-extras-2026-08-31.log

How it was measured

Canary's CaptureUiDrawForRE already dumped every UI draw in submission order with its shaders and bound texture. It was extended to log RB_BLENDCONTROL0, RB_COLORCONTROL and RB_COLOR_MASK as well — raw and decoded, so a decode bug here cannot quietly become the answer. One emulator, driven to the main menu and then into EXTRAS, F10 at each.

The log names no elements, so a draw is identified by the pixel size of its quad: NDC extents × the 1280×720 surface, matched against sprite dimensions read straight off the disc (tools/re-capture/ui_blend_map.py).

The controls, both run before the result was read

1. The size conversion reproduces two independently measured numbers. The title's two rotated sweep strips were measured at 1134 and 1303 px tall in data/title-sweep-drawn-at-rest.txt, by a different tool in a different session. This conversion produces 1134.0 and 1303.2 — on both screens. The tool prints PASS/FAIL and says outright that its sizes are worthless if the check fails.

2. It is the blend register, not a different shader. Pixel shader 0xE59B2B3DA4AA9008 is used with both states on the main menu — 12 draws additive, 18 alpha-over. ptframe1 and ptbase run the same shader; only the blend differs. Without this the result could have been "the frames use a different shader", which is a different finding.

The identification, from the artefact

draw	quad px	disc sprite	blend
menu 7	243.2 × 280.8	ptframe1.t32 247×281	ADDITIVE
menu 7	256.0 × 309.6	ptframe2.t32 257×311	ADDITIVE
menu 8	224.0 × 39.6	ptmsg.t32 223×38	alpha-over
menu 9	211.2 × 54.0	ptbtn01f.t32 216×56	alpha-over
extras 7	243.2 × 219.6	ptframe3.t32 246×220	ADDITIVE
extras 7	614.4 × 518.4	pteff20.t32 309×259 @2×	ADDITIVE
extras 8	352.0 × 39.6	ptmsg2.t32 354×38	alpha-over
extras 8	179.2 × 32.4	pttitle.t32 182×34	alpha-over

ptframe1 and ptframe2 are in one draw call; so are pteff20 and ptframe3. A draw call carries one blend state, so ptframe4 needs no separate argument: it is inside EXTRAS' 24-index additive draw with ptframe3.

🔴 The first version of this paragraph said "elements that share a mode are batched together". That is false, and it is refuted by the very log it was written from: in one main-menu frame, draws 5, 6 and 7 are three separate additive draws — consecutive, identical state, not merged. Only the one-way implication holds: elements inside one draw share a blend state; sharing a state does not put elements in one draw. The wrong version would have licensed inferring a mode for an element that was never observed, which is exactly what this page must not do.

What this settles, and what it does not

✅ The port's independent measurement is confirmed by the oracle. It solved the composite per pixel from two backgrounds and found additive halves alpha-over's error on both frames (34.305/28.948 against 65.046/71.299). Two methods with nothing in common — a solved composite against a capture, and the register the GPU was handed — give the same answer.

✅ This is no longer authored. The not-on-disc page concluded "any blend you choose is authored and must carry that label". That was true of the disc; it is not true any more of the game. Additive is measured, and the port can transcribe it.

⚠️ src = ONE, not SRC_ALPHA, in BOTH states. The fixed-function blend multiplies the pixel shader's output by 1, so whatever alpha weighting happens, the shader does it. This says nothing about whether the .t32 texels are stored premultiplied — that is a separate question, and the shader is unread.

❔ Where the mode is selected is still unknown. This measures what the GPU was told, not which field decided it. The disc-side search found nothing (reach here), and the batching means the selection happens before the draw, in whatever sorts the elements. So the port should read this table as a per-element fact it can transcribe, not as a rule it can extend to elements not in it.

❔ Two additive draws on the menu are unidentified — 819.2×720 and 691.2×720. The second is within 8 px of pteff12 @2×; the first matches nothing on the screen at either scale. Both are additive, so they do not change the answer, and neither is guessed at here.

⚠️ Reach. Two screens, one session, one emulator. Every element on the two screens the port ships is in the table except the two above and pteff10, which did not appear as an identifiable quad. The title screen (build 4) was not captured.