# The render roadmap — from headless to visible (researched 2026-07-24)

Four-dossier workflow synthesis (card protocol / video-TU delta / emulator
side / gauge path). The headline: **the render seam is far closer than
expected** — the engine half of BOTH render paths already compiles and links
into our btl4opt.exe from the CODE originals, and the emulator's board-side
device is EXE-agnostic.

## The two independent render paths (either can run without the other)

1. **The 3D main view (division card)**: the game speaks dPL — a
   RETAINED-mode scene graph API (never pixels, never transformed polys) —
   through the statically linked libDPL, which frames every call into
   vr_action messages on the INMOS C012 link adapter at port 0x150
   (FIFO fast path: body as REP OUTSW words to 0x154, trailing length word
   to 0x151 commits). Gate: `DPLARG` env (set by SETENV.BAT when
   L4VIDEO != OFF); NULL DPLARG = the headless shape we run today
   (`MakeVideoRenderer()` returns NULL, tolerated).
   - Per-frame submission: **engine code** — `DPLRenderer::
     ExecuteImplementation` (CODE/RP/MUNGA_L4/L4VIDEO.CPP:5065) walks the
     dplRenderableSocket; renderables (L4VIDRND.CPP, ~30 classes) set DCS
     matrices; `dpl_FlushDCSArticulations` = wire action 0x1f (batched
     poses, absolute 3x3+T or [sin,cos] joint records); `vr_draw_scene`
     (9) kicks the frame; `velocirender_sync` (0x2d, exe @0x48D220) is the
     ack-less bulk-op fence.
   - **Our build ALREADY COMPILES this whole engine layer** (mungal4.lib:
     l4video/l4vidrnd/l4dplmem + prebuilt 1995 STARTDPL.obj + LIBDPL.LIB;
     see build410 obj logs). The missing piece is GAME code: **btl4vid**
     (BTL4VideoRenderer — MakeEntityRenderables/MakeMechRenderables/
     effects/wreck swap/material substitution; BT411 donor 3039/848 lines
     with per-method @ADDR evidence, binary extent @4cc40c..@4d2bbc; our
     stub Fails at LoadMissionImplementation).
2. **The gauge path (cockpit instruments)**: completely separate hardware —
   the PC's own SVGA (VESA 0x111, 640x480x16, L4SVGA16.ASM banked blits),
   split across VGA heads by the VDB board (port 0x300, palette groups; the
   octopus-cable 5-mono-MFDs + color-radar decode). Gate: `L4GAUGE` env.
   The plasma readout is a gauge-renderer graphics port (PlasmaDisplay on
   COM2), not a separate renderer.
   - Engine half (GaugeRenderer/L4GaugeRenderer/the 5171-line L4GAUGE
     widget interpreter/GAUGMAP/L4VB16) already compiles + links in
     build410; one documented deviation (interpreter table 46864 -> 262144
     for BT's larger CFG).
   - Missing: the 6 BT gauge TUs — **btl4gaug (130 fn) / btl4gau2 (77) /
     btl4gau3 (46) / btl4rdr (36) / btl4grnd (13, ours is a 4-fn stub) /
     btl4galm** — ~302 functions/~43KB, binary extent 0x4c19fc-0x4cc40c,
     the largest function-count block left in btl4.lib. BT411 donors are
     COMPLETE (semantic .cpp/.hpp with @ADDR evidence, ~5900 donor lines);
     the work is RE-HOSTING onto 1995 engine APIs, not fresh decompilation.
     Class inventory per TU in the dossier (widget primitives / composite
     clusters / MP-HUD / radar MapDisplay / the renderer / alarm manager).

## The emulator side is READY and EXE-agnostic

The DOSBox-X fork's VPX device (emulator/vpx-device/vpxlog.cpp, ports
0x150-0x161 + VDB 0x300-0x31A) implements the BOARD side purely at the wire
level: blind firmware absorb, iserver handshake (VPX_RESPOND=1), generic
acks, the 0x2d sync echo, draw_scene frame-acks with a REAL raycast reticle
pick against its own decoded scene, and the GL renderer (VPX_RENDER=1) that
draws the main view + cockpit windows. It never reads game memory and keys
only on wire contents — **our reconstructed exe drives it unmodified once it
emits the same traffic**. The AWE32 pair is mandatory for any timed run
(SOS FAST clock); gauges also render live on the emulated S3 with the
shipped exe (GAUGE-NOTES.md: HEAPSIZE=15000000 + granularityInKB=64).

## The plan (order of value)

1. **Gauges-only first** (decisively cheaper): set `L4GAUGE=640x480x16` (+
   HEAPSIZE), reconstruct the 6 gauge TUs from the BT411 donors. Built-in
   oracle: the shipped exe renders reference output from the SAME GAUGE
   content — per-head diffable. Live instruments need the attribute
   sources, which the 5.3.x waves have largely built (weapon states, heat,
   power, damage, radar).
2. **Then btl4vid** (the 3D world renderer) + DPLARG on: the engine
   DPLRenderer + libDPL do the wire; btl4vid builds the renderables per
   entity; verify against the VPX device's GL window and the dpl3-revive
   bridge.
3. The review/playback pair (btl4pb + the BTL4.CPP review branch) closes
   the last absent TUs.

Message-id / protocol quick refs: vr_create=1 (host-assigned handles),
vr_flush=3 (node structs; VIEW proves 832x512), geometry 0x17/0x19/0x1a
(texel word [pad,B,G,R]), artics 0x1f, draw 9, sync 0x2d, fire 0x23,
reticle 0x26. DPLARG reference value in emulator/baseline.conf:21.