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KB: decode the pod's RGB SPLIT -- one VGA port drives THREE mono MFDs

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Answering 'how do the panels split RGB into 3 monitors' from primary sources
rather than inference:

- content/GAUGE/L4GAUGE.CFG (the authentic 1996 pod config) configures each
  gauge port with a bit-plane mask AND A COLOUR CHANNEL: Comm=red,
  Mfd2=green, Heat=blue on clut2 (the upper row); Mfd1=red, Mfd3=green on
  clut1 (the lower row, blue spare); sec/radar = full rgb, rotation 270 (the
  portrait CRT).  Eng1/2/3 are the engineering-page twins on the same
  monitors, swapped in/out via reconfigure() with 'blank'.
- L4GraphicsPort::BuildSecondaryColor (L4VB16.cpp) proves the mechanism at
  T0: it walks the palette entries owned by the port's bit group and writes
  exactly ONE component (RedChannel->Red, GreenChannel->Green,
  BlueChannel->Blue, AllChannels->whole triplet); BlankColor blanks the
  group.  So one palettized framebuffer emits three independent pictures on
  the R/G/B analog lines, and the splitter feeds each line to its own mono
  monitor -- which is also what the '1280x480 horizontally spanned' MFD
  surface actually is: two VGA outputs x three channels.

Port consequence recorded: the per-panel window path (BT_POD_SURFACES) is
right for per-panel outputs but WRONG for splitter-wired glass, which needs a
channel-composite mode (three planes -> one RGB image, pure primary tints).
ExpandPlaneToBGRA already does the per-plane half.  Open: how Nick's cart is
actually wired.  Also lands the pod bring-up scratch (ssh helper, layout cfg,
launcher, firestorm repo browser).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
This commit is contained in:
Joe DiPrima
2026-08-06 12:35:39 -05:00
co-authored by Claude Fable 5
parent 99956d54e3
commit e0e9dcc292
4 changed files with 110 additions and 0 deletions
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@@ -184,6 +184,46 @@ All cockpit surfaces are bit-plane MASKS over ONE shared `SVGA16` pixelBuffer: `
byte; `Heat`=0x4000, `Mfd2`=0x0400, `Comm`=0x8000, `Mfd1`=0x0100, `Mfd3`=0x1000; `Eng1-3` =
engineering-mode alt planes; `overlay`=0x00C0 (shares the sec surface). See [[gauges-hud]]. [T2]
## ⭐ THE RGB SPLIT — how ONE VGA port drives THREE mono MFDs (decoded 2026-08-06) [T0 engine source + T1 authentic pod config]
The five monochrome MFDs are NOT five video outputs. Each VGA port's **R, G and B analog lines
are split to three separate monochrome monitors**, and the software puts a different MFD in each
colour channel of one shared palettized framebuffer. Mechanism, end to end:
1. **Every surface is a bit-plane + a CHANNEL.** `content/GAUGE/L4GAUGE.CFG` (the authentic 1996
pod config) configures each port as `configure(idx, port, rotation, bitMask, clut, COLOUR, palette)`:
| port | panel | mask | clut | channel |
|---|---|---|---|---|
| `Comm` | upper right | 0x8000 | clut2 | **red** |
| `Mfd2` (Engineering) | upper centre | 0x0400 | clut2 | **green** |
| `Heat` | upper left | 0x4000 | clut2 | **blue** |
| `Mfd1` | lower left | 0x0100 | clut1 | **red** |
| `Mfd3` | lower right | 0x1000 | clut1 | **green** |
| `sec` (+`overlay` 0x00C0) | secondary/radar | 0x003F | clut0 | **rgb** (full colour, rotation 270 — the physically ROTATED portrait CRT) |
`Eng1/2/3` are the engineering-page twins of Mfd1/2/3: same monitor, second bit-plane, switched
by `reconfigure(...)` giving one plane the channel and the other `blank`.
2. **The channel assignment is literally a palette write.** `L4GraphicsPort::BuildSecondaryColor`
(L4VB16.cpp) walks the palette entries owned by the port's bit group (`BitWrangler(byteMask,8)`)
and writes ONE component: `RedChannel -> triplet->Red`, `GreenChannel -> ->Green`,
`BlueChannel -> ->Blue`, `AllChannels -> the whole triplet`. `BlankColor` blanks the group
(`BlankPalette()`), which is how a page swap silences the plane it replaces. The
`*TransparentZero` variants skip colour 0 so zero reads as transparent for that group.
3. **So the DAC output carries three independent pictures**, one per analog line, and the splitter
hands each line to its own mono monitor. Three MFDs per VGA port; the pod's two MFD ports are
the **1280x480 "horizontally spanned" surface** (2 x 640x480 halves) the Displays section
describes — clut2 = the upper row (Comm/Mfd2/Heat), clut1 = the lower row (Mfd1/Mfd3, blue
spare). The radar rides its own port in real colour.
**Why this matters for the port [T2]:** our modern path renders each surface as its own
mono-tinted window on its own Windows display (see §MFD PANELS ON REAL HARDWARE), which is right
when every panel has its own output. **On splitter-wired glass it is wrong** — three monitors
would share one Windows display and each would show only its channel's share of a single tinted
image. Driving original splitter hardware needs a CHANNEL-COMPOSITE mode: extract three planes
into ONE 640x480 RGB image with pure (255,0,0)/(0,255,0)/(0,0,255) tints, one window per VGA
output. `SVGA16::ExpandPlaneToBGRA` already does the per-plane extraction with a tint, so the
composite is a small addition. OPEN: which way Nick's crash cart is wired (Windows shows three
separate 640x480 displays there, which suggests per-panel outputs via the Trigger 6 USB adapter,
not a splitter) — settle it by eye before building.
## MFD PANELS ON REAL HARDWARE — the bring-up path (2026-08-06) [T2 local / T4 on-pod]
Nick's crash cart (pod hardware + Chrome Remote Desktop on a burner account) is the first chance
to drive the real panels. **The 1995 display path is NOT the way in.** That rig spanned the five