# USB-C / HDMI and USB-tether cast (roadmap D & E)

<!-- doc-meta:start -->
| Field | Value |
|---|---|
| Author | Anton Afanasyeu |
| Revision | R1 |
| Creation date | 2026-05-22 |
| Last modification date | 2026-05-22 |
| Co-authored |  |
| Severity | medium |
| State | postponed |
| Document type | draft |
<!-- doc-meta:end -->

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Android Cast streams over **Wi‑Fi** (UDP/TCP/QUIC). USB-C→HDMI adapters use Android’s **wired external display** stack, which is separate from the cast protocol.

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## Table of contents

<!-- toc -->
- [Layer model](#layer-model)
- [Phase D — external display awareness & future capture](#phase-d-external-display-awareness-future-capture)
- [Phase E — USB-tether transport](#phase-e-usb-tether-transport)
- [Tablet + adapter checklist (e.g. Doogee Tab G6 Max → HY300)](#tablet-adapter-checklist-eg-doogee-tab-g6-max-hy300)
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**Documentation index:** [README.md](README.md)

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## Layer model

| Layer | Mechanism | This app today |
|-------|-----------|----------------|
| Wired mirror | `DisplayManager` + HDMI/DP alt mode | Not encoded by us; OS drives the sink |
| Wi‑Fi cast | MediaProjection + `CastTransport` | Primary product path |
| USB-tether cast (E) | IP over USB (RNDIS/ADB reverse) + same framing | **Stub** (`usb_tether` transport) |

## Phase D — external display awareness & future capture

**Implemented (foundation):**

- `WiredDisplayMonitor` — tracks non-default displays (HDMI, presentation, etc.).
- `ExternalDisplayCapturePolicy` — developer preference for future capture routing.
- Developer settings: live wired-display status + capture policy selector.
- `ScreenCastService` registers extended display listener (rotation + hot-plug).

**Not implemented yet (post-alpha):**

- Capture **secondary display** via `MediaProjection` / `createVirtualDisplay` on non-default `Display`.
- Android 14+ per-app capture rules and extend-vs-mirror modes.
- Automatic switch to external display when passive/active USB-C adapter works.

**User workaround when HDMI dongle fails:** LAN cast to a receiver (phone/box) connected to the projector by HDMI or built-in Android on the projector.

## Phase E — USB-tether transport

**Implemented (foundation):**

- `UsbTetherCastTransport` stub implementing `CastTransport`.
- `CastConfig.TRANSPORT_USB_TETHER` — selectable only when **Developer settings → USB-tether transport** is on.
- Factory routes to stub; connect/listen throw until RNDIS/ADB tunnel + discovery are wired.

**Future work:**

- Detect USB tether / ADB TCP forward (`adb reverse`).
- Optional discovery on `127.0.0.1` or tether interface IP.
- Same `CastProtocol` framing as UDP/TCP.

## Tablet + adapter checklist (e.g. Doogee Tab G6 Max → HY300)

1. **Settings → Display** — does a second screen appear when the dongle is plugged in?
2. **Passive** adapter: requires USB-C **DisplayPort Alt Mode** on the tablet port.
3. **Active** adapter: needs a chip/firmware pair the tablet supports (often still DP alt mode internally).
4. If the OS never sees an external display, **no app feature** can fix the cable path; use Wi‑Fi cast or receiver-on-HDMI.

See [ROADMAP.md](ROADMAP.md) for alpha and commercial tracks.
