Native Chromecast display mirroring.
Low latency. No Chrome.
CastMirror is a native C++20 sender for Google Cast display mirroring. It discovers devices on the LAN, captures the Linux desktop and system audio, encodes H.264 (libx264) and Opus, and streams Cast RTP/RTCP over UDP — the same media path Chrome uses for “Cast screen,” without embedding Chrome.
The shipping UI is a GTK 4 + libadwaita desktop app. A CLI is included.
- Not a Chrome / CEF wrapper
- Not Sunshine or Moonlight (those are ~20–50 ms game streams; Cast keeps a playout delay, about 200 ms here)
- Not an HLS/DASH “fling” to the Default Media Receiver
- Not affiliated with Google. Chromecast, Google Cast, and Google TV are Google trademarks
app/winui/is a blueprint, not the shipping Windows product
Official Cast sender SDKs cannot mirror a desktop. Chrome’s mirroring path is private as a product and open as a protocol. CastMirror implements that protocol as a LAN sender.
- One-click Cast to Chromecast, Google TV, and Cast TVs (or add a device by IP) with dynamic hardware model glyphs
- Screen or window sharing — cast an entire monitor or pick a single application window with application icon resolution. On Wayland the system portal picker handles selection; on X11 CastMirror enumerates and captures windows directly with XComposite redirection
- Quality presets Auto / High / Balanced / Smooth with an inline bitrate slider (1–25 Mbps) synchronized between Cast and Settings
- Live studio controls — Freeze display and Mute TV audio on the fly with clean silence-frame injection
- Real-time vector sparklines — Hardware-accelerated Cairo mini-charts for live FPS, bitrate, RTT, and packet loss
- Host speakers mute while audio is mirrored; previous mute state is restored on Stop
- Adaptive bitrate is always on: it holds your selected bitrate target, drops on congestion, and ramps back up aggressively once the link recovers
- Appearance & Diagnostics — System / Light / Dark theme switcher and built-in hardware/network self-test wizard
- AES-128-CTR per-frame media crypto as required by Cast Streaming
- TLS control plane on port 8009
- Stop budget under 500 ms (capture does not run except during a live session)
sudo apt update
sudo apt install -y build-essential cmake ninja-build pkg-config protobuf-compiler libprotobuf-dev \
libssl-dev libopus-dev libpulse-dev libx11-dev libxext-dev libxrandr-dev libxfixes-dev \
libxcomposite-dev libxdamage-dev \
libva-dev libavcodec-dev libswscale-dev libavutil-dev nlohmann-json3-dev libgtest-dev \
libgtk-4-dev libadwaita-1-dev
cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Release
cmake --build build -j"$(nproc)"
./build/app/castmirror-gui
./build/app/castmirrorFull package notes, firewall, and audio behavior: docs/building.md.
The GUI is split into four dedicated tabs:
Discover LAN receivers, select a monitor display or open application window, tune target bitrate, and start mirroring.
Real-time streaming pipeline visualization, hardware-accelerated Cairo vector sparkline charts (FPS, Bitrate, RTT, Packet Loss), dynamic adaptive ladder rungs, and live studio controls (Freeze display, Mute TV audio).
Configure video encoding presets, target playout buffer delay, host audio mute behavior, color scheme theme (System default / Light / Dark), and run hardware/network self-test diagnostics.
Live searchable diagnostic event logs with filter levels, quick copy, and open log directory actions.
CLI:
# Cast a full display
./build/app/castmirror --device 192.168.1.150 --display 0 --preset High
# List available windows, then cast one
./build/app/castmirror --list-windows
./build/app/castmirror --device 192.168.1.150 --window 12345
# Audio-only
./build/app/castmirror --device 192.168.1.150 --no-audioDesktop launcher: app/io.github.vindeckyy.CastMirror.desktop.
flowchart LR
X11[X11 screen/window capture] --> x264[libx264]
Portal[Wayland portal+PipeWire] --> x264
Pulse[Pulse/PipeWire monitor] --> Opus[Opus]
x264 --> AES[AES-128-CTR]
Opus --> AES
AES --> RTP[Cast RTP]
RTP --> UDP[UDP to device]
Device[Cast device] -->|RTCP CAST NACK PLI| Adapt[Adaptive controller]
Adapt --> x264
TLS[Cast V2 TLS 8009] --- Device
Details: docs/ARCHITECTURE.md, docs/protocol.md.
Chromecast 3rd gen, Ultra, Google TV / Streamer, and built-in Cast TVs. Nest Hub is 720p-class. Matrix and preset table: docs/COMPATIBILITY.md.
CastMirror/
├── app/gui/ # Shipping GTK 4 + libadwaita UI
├── app/cli/ # Interactive / flag CLI
├── app/winui/ # Windows UI blueprint (not v1 shipping)
├── core/ # castcore C++20 library
├── tests/ # Google Test
├── tools/ # poc-control, poc-streaming, poc-encode, poc-join, fake-receiver
├── docs/ # Pages site + architecture / protocol / building
└── receiver-fallback/ # CAF research fallback, not the primary path
cd build && ctest --output-on-failure
./tests/castmirror_tests
./tools/poc-encode
./tools/fake-receiver 28009 53533See CONTRIBUTING.md. Historical lab notes: docs/TEST_REPORT.md.
LAN-only. TLS to :8009. Media AES as the protocol requires. No cloud. Report privately: SECURITY.md.
Source in this repository is Apache License 2.0 — see LICENSE.
Binaries linked against GPL libx264 generally must be treated as GPL. Read NOTICE before you distribute builds.
CastMirror is an independent project. It is not affiliated with, endorsed by, or sponsored by Google LLC. It speaks a Chromium-compatible Cast Streaming protocol; firmware app IDs can change.