video-setup/planning.md

# Planning

## Approach

Build the system module by module in C11. Each module is a translation unit (`.h` + `.c`) with a clearly defined API. Modules are exercised by small driver programs in `dev/` before anything depends on them. This keeps each unit independently testable and prevents architectural decisions from being made prematurely in code.

The final binary is a single configurable node program. That integration work comes after the modules are solid.

---

## Directory Structure

```
video-setup/
  src/
    modules/
      common/        - shared definitions (error types, base types)
      media_ctrl/    - Linux Media Controller API (topology, pad formats, links)
      v4l2_ctrl/     - V4L2 camera controls (enumerate, get, set)
      serial/        - little-endian binary serialization primitives
      transport/     - framed TCP stream, single-write send
      protocol/      - typed write_*/read_* message functions
      test_image/    - test pattern generator (colour bars, ramp, grid; YUV420/BGRA)
      xorg/          - GLFW+OpenGL viewer sink; stub for headless builds
    node/            - video node entry point and top-level integration (later)
  include/           - public headers
  dev/
    cli/             - exploratory CLI drivers, one per module
    web/             - development web UI (Node.js/Express); browser-side equivalent
                       of the CLI tools; depends on protocol being finalised
    experiments/     - freeform experiments
  tools/
    gen_font_atlas/  - build-time bitmap font atlas generator (Python/Pillow);
                       outputs build/gen/font_atlas.h consumed by xorg module
  tests/             - automated tests (later)
  Makefile
  architecture.md
  planning.md
  conventions.md
```

---

## Module Order

Modules are listed in intended build order. Each depends only on modules above it.

| # | Module | Status | Notes |
|---|---|---|---|
| 1 | `common` | done | Error types, base definitions — no dependencies |
| — | `config` | done | INI file loader with schema-driven defaults, typed getters, FLAGS type for bitmask values |
| 2 | `media_ctrl` | done | Media Controller API — device and topology enumeration, pad format config |
| 3 | `v4l2_ctrl` | done | V4L2 controls — enumerate, get, set camera parameters |
| 4 | `serial` | done | `put`/`get` primitives for little-endian binary serialization into byte buffers |
| 5 | `transport` | done | Encapsulated transport — frame header, TCP stream abstraction, single-write send |
| 6 | `discovery` | done | UDP multicast announcements, peer table, found/lost callbacks |
| 7 | `protocol` | done | Typed `write_*`/`read_*` functions for all message types; builds on serial + transport |
| — | `node` | done | Video node binary — config, discovery, transport server, V4L2/media control request handlers |
| 8 | `test_image` | done | Test pattern generator — colour bars, luminance ramp, grid crosshatch; YUV420/BGRA output |
| 9 | `xorg` | done | GLFW+OpenGL viewer sink — YUV420/BGRA/MJPEG display, all scale/anchor modes, bitmap font atlas text overlays; XRandR queries and screen grab not yet implemented |
| 10 | `frame_alloc` | not started | Per-frame allocation with bookkeeping (byte budget, ref counting) |
| 11 | `relay` | not started | Input dispatch to output queues (low-latency and completeness modes) |
| 12 | `ingest` | not started | V4L2 capture loop — dequeue buffers, emit one encapsulated frame per buffer |
| 13 | `archive` | not started | Write frames to disk, control messages to binary log |
| 14 | `codec` | not started | Per-frame encode/decode — MJPEG (libjpeg-turbo), QOI, ZSTD-raw, VA-API H.264 intra; used by screen grab source and archive |
| 15 | `web node` | not started | Node.js/Express peer — speaks binary protocol on socket side, HTTP/WebSocket to browser; `protocol.mjs` mirrors C protocol module |
| — | `mjpeg_scan` | future | EOI marker scanner for misbehaving hardware that does not deliver clean per-buffer frames; not part of the primary pipeline |

---

## Dev Tools

### CLI (`dev/cli/`)

Each module gets a corresponding CLI driver that exercises its API and serves as both an integration check and a useful development tool.

| Driver | Exercises | Notes |
|---|---|---|
| `media_ctrl_cli` | `media_ctrl` | List media devices, show topology, configure pad formats |
| `v4l2_ctrl_cli` | `v4l2_ctrl` | List controls, get/set values — lightweight `v4l2-ctl` equivalent |
| `transport_cli` | `transport` | Send/receive framed messages, inspect headers |
| `test_image_cli` | `test_image` | Generate test patterns, write PPM for visual inspection |
| `xorg_cli` | `xorg` | Display test pattern in viewer window; exercises scale/anchor modes and text overlays |
| `v4l2_view_cli` | V4L2 + `xorg` | Live camera viewer — auto-selects highest-FPS format, FPS/format overlay; bypasses node system |
| `stream_send_cli` | V4L2 + `transport` + `protocol` | Capture MJPEG from V4L2, connect to receiver, send VIDEO_FRAME messages; prints fps/Mbps stats |
| `stream_recv_cli` | `transport` + `protocol` + `xorg` | Listen for incoming VIDEO_FRAME stream, display in viewer; fps/Mbps overlay; threaded transport→GL handoff |

### Web UI (`dev/web/`)

A Node.js/Express development web UI that connects to running video nodes as a binary protocol peer. Exposes V4L2 control inspection and adjustment, media topology view, and stream state through a browser interface. Supports live peer discovery via SSE — discovered nodes appear automatically in the UI.

This is a development aid, not the production dashboard. The production dashboard (full stream configuration UI) is a later, separate project.

---

## Future: Protocol Preprocessor

The C `protocol` module and JavaScript `protocol.mjs` will eventually be generated from a single schema by a future preprocessor. This eliminates drift between the two implementations. The preprocessor also handles error location codes (see `common/error`). Neither the schema format nor the preprocessor tool exists yet — the hand-written implementations are the interim state.

---

## Deferred Decisions

These are open questions tracked in `architecture.md` that do not need to be resolved before module work begins:

- Graph representation format
- Connection establishment model (push vs pull)
- Completeness queue drop policy (oldest vs newest, per-output config)
- Stream ID remapping across relay hops
- Transport for relay edges (TCP / UDP / shared memory)
- Node discovery mechanism
- Hard vs soft byte budget limits