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105 lines
3.2 KiB
Markdown
105 lines
3.2 KiB
Markdown
# USB-C / HDMI and USB-tether cast (roadmap D & E)
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| Field | Value |
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|---|---|
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| Author | Anton Afanasyeu |
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| Revision | R1 |
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| Creation date | 2026-05-22 |
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| Last modification date | 2026-05-22 |
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| Co-authored | |
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| Severity | medium |
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| State | postponed |
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| Document type | draft |
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\newpage
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\newpage
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---
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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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---
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---
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---
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---
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---
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---
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---
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---
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---
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## Table of contents
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<!-- toc -->
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- [Layer model](#layer-model)
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- [Phase D — external display awareness & future capture](#phase-d-external-display-awareness-future-capture)
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- [Phase E — USB-tether transport](#phase-e-usb-tether-transport)
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- [Tablet + adapter checklist (e.g. Doogee Tab G6 Max → HY300)](#tablet-adapter-checklist-eg-doogee-tab-g6-max-hy300)
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<!-- /toc -->
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**Documentation index:** [README.md](README.md)
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---
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## Layer model
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| Layer | Mechanism | This app today |
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|-------|-----------|----------------|
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| Wired mirror | `DisplayManager` + HDMI/DP alt mode | Not encoded by us; OS drives the sink |
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| Wi‑Fi cast | MediaProjection + `CastTransport` | Primary product path |
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| USB-tether cast (E) | IP over USB (RNDIS/ADB reverse) + same framing | **Stub** (`usb_tether` transport) |
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## Phase D — external display awareness & future capture
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**Implemented (foundation):**
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- `WiredDisplayMonitor` — tracks non-default displays (HDMI, presentation, etc.).
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- `ExternalDisplayCapturePolicy` — developer preference for future capture routing.
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- Developer settings: live wired-display status + capture policy selector.
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- `ScreenCastService` registers extended display listener (rotation + hot-plug).
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**Not implemented yet (post-alpha):**
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- Capture **secondary display** via `MediaProjection` / `createVirtualDisplay` on non-default `Display`.
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- Android 14+ per-app capture rules and extend-vs-mirror modes.
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- Automatic switch to external display when passive/active USB-C adapter works.
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**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.
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## Phase E — USB-tether transport
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**Implemented (foundation):**
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- `UsbTetherCastTransport` stub implementing `CastTransport`.
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- `CastConfig.TRANSPORT_USB_TETHER` — selectable only when **Developer settings → USB-tether transport** is on.
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- Factory routes to stub; connect/listen throw until RNDIS/ADB tunnel + discovery are wired.
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**Future work:**
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- Detect USB tether / ADB TCP forward (`adb reverse`).
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- Optional discovery on `127.0.0.1` or tether interface IP.
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- Same `CastProtocol` framing as UDP/TCP.
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## Tablet + adapter checklist (e.g. Doogee Tab G6 Max → HY300)
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1. **Settings → Display** — does a second screen appear when the dongle is plugged in?
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2. **Passive** adapter: requires USB-C **DisplayPort Alt Mode** on the tablet port.
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3. **Active** adapter: needs a chip/firmware pair the tablet supports (often still DP alt mode internally).
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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.
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See [ROADMAP.md](ROADMAP.md) for alpha and commercial tracks.
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