ac-docs/USB_HDMI_CAST.md

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

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

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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

• Layer model
• Phase D — external display awareness & future capture
• Phase E — USB-tether transport
• Tablet + adapter checklist (e.g. Doogee Tab G6 Max → HY300)

Documentation index: 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 for alpha and commercial tracks.