# Phase 3 — Render Backend: Progress

**Status (2026-07-03): Phase 3a complete — the render command stream is fully
captured and decoded, and a captured DIVRGB frame has been reconstructed to
pixels. First image ever produced from the Rel 4.10 VPX protocol without a
real board.**

![decoded DIVRGB calibration screen](divrgb-decoded.png)

That is `flyk divrgb.scn` — Division's SMPTE color-bar calibration scene —
rendered *entirely from the FIFO command stream captured by the emulated VPX
board*: geometry, connectivity, materials, camera, viewport and background all
come from the wire, none from the scene files. `divrgb-frame0.png` is the same
capture drawn with the actual frame-0 spline camera the app sent.

## 3a. Full FIFO capture

`vpxlog.cpp` now records every FIFO burst when `VPX_FIFODUMP=<path>` is set
(records: `'VPXM'` magic, u32 length, raw bytes; one record per burst between
`outputData` tag writes). The capture used here: `divrgb.fifodump`
(1297 messages) from `divrgb.conf`.

Tools:

- `decode_fifodump.py <dump> [--hex N] [--action A]` — action census + payload
  hexdumps.
- `render_capture.py <dump> [-o out.png] [--frame N] [--eye x,y,z]` —
  reconstructs the scene graph and software-renders the frame each
  `draw_scene` commits.

## The Rel 4.10 wire protocol (established from this capture)

One burst per message: `[action:4][payload]`, packetized at 508 bytes.
The i860 image download itself rides the FIFO in this build (actions 18–20,
`VREND.MNG`). The DPL3 `vr_action` enum holds for 0–23 but Rel 4.10 extends
and re-purposes the tail:

| action | meaning | payload |
|---|---|---|
| 0 | init | args string (`/device~0x150~/video~svga~…`) |
| 1 | create | `[type][name]` — **the host assigns node names** (1,2,3…); the board's create reply node value is ignored |
| 3 | flush | `[name][type][node struct]` (see node types below) |
| 7 | dcs_link | `[parent][child]` |
| 9 | draw_scene | `[view?][0][0][1][1.0f]` — commits the frame |
| 11 | list_add | `[parent][child]` |
| 18–21 | 860code/data/bss/args | i860 download (FIFO in this build) |
| 23 | **set_geom_verts** (Rel4.10; DPL3 had 22) | hdr `[name][0][n_verts][3][n_blocks][1][5][n_verts][1.0f]`, then float32 x,y,z per vertex |
| 25 | **set_geom_conns** (new) | hdr `[name][n_polys][loop_len][0]`, then indices; each poly a closed loop (last=first) |
| 31 | **camera** (new) | `[?][view][3×3 rotation, row-major][eye x,y,z]` per frame |
| 45 (0x2D) | sync | token ping (see PHASE2) |

Node types (from create/flush pairs in this capture):

| type | node | flush payload highlights |
|---|---|---|
| 2 | texture/ramp? | one word, `0xFFFFFFFF` / `0x0FFFFFFF` |
| 3 | view | window l,b,r,t (±1 × ±0.6154), window-plane distance 1.3, viewport 832×512, near 2, far 12000, background RGB |
| 4 | light | |
| 5 | dcs | 4×4 matrix |
| 6 | material (old-style) | RGB floats |
| 7 | object | |
| 8 | lod | 2×4 bounds + lists |
| 9 | geogroup | bounds; **payload int 14 = material node name** |
| 10 | geometry | `[geo_type][n_polys][n_verts]…[radius]` |
| 11 | material | **floats 10–12 (payload) = diffuse RGB** |

Graph: `list_add` links object→lod→geogroup→geometry (and zone-level nodes to
node 0); geogroups bind materials by name. The 13 DIVRGB bars decode to the
textbook SMPTE pattern — 7 color bars, the reverse strip, and the −I/white/+Q
PLUGE row — which validates vertices, connectivity, materials, camera and
window mapping in one image. (Division screen x runs opposite to a GL-style
eye space; without negating x the pattern comes out mirrored.)

## Next: 3b — live OpenGL backend

Feed the same decode path a live stream inside DOSBox-X instead of a dump:
maintain the node store in `vpxlog.cpp` (or a sibling `vpxrender.cpp`), upload
geometry on set_geom_verts/conns, draw on draw_scene into an OpenGL window
(or the DOSBox surface). The `divrgb.fifodump` fixture allows developing the
backend offline; `flyk yip.scn` (CYCLE, Red Planet geometry) is the next
fixture up once the DOS/4GW sync variant is handled, then the game itself
(`buttee.bgf` etc. via the production content path).