SecondMonitor

SecondMonitor is a native Windows display-streaming project with:

• a WinUI 3 desktop app for creating a virtual display, selecting a monitor, and starting the stream
• a native C++ bridge/core that handles monitor enumeration, virtual-display creation, H.264 encode/decode, and WebRTC transport
• an Android client that receives the WebRTC H.264 track and renders it with MediaCodec

Repository layout

• windows-ui/SecondMonitor.Desktop: WinUI 3 desktop app
• src: native Windows core and bridge sources
• android-client: Android tablet client
• cmake: CMake helper modules
• docs: notes and planning docs
• third_party: locally provisioned native dependencies, not committed

Windows prerequisites

Install these first:

• Visual Studio 2022 with MSVC C++ build tools
• CMake 3.21+
• Windows 10/11 SDK
• .NET 8 SDK with Windows desktop workload support
• OpenSSL for Windows
• FFmpeg shared build unpacked at third_party/ffmpeg/pkg/ffmpeg-8.1-full_build-shared
• libdatachannel checked out at third_party/libdatachannel
• mbedtls checked out at third_party/mbedtls
• an MttVDD virtual display driver payload, or another compatible indirect display driver package

Notes:

• The native Windows build links against OpenSSL through libdatachannel.
• FFmpeg is not discovered from the system; the current CMake expects the vendored path above.
• Creating a virtual monitor requires elevation.
• The virtual-display flow depends on the MttVDD driver and writes config under %ProgramData%\MttVDD.

Native dependencies

libdatachannel

git clone --recursive https://github.com/paullouisageneau/libdatachannel.git third_party/libdatachannel

mbedtls

git clone https://github.com/Mbed-TLS/mbedtls.git third_party/mbedtls

FFmpeg

Download a shared Windows build of FFmpeg 8.x and unpack it so these paths exist:

• third_party/ffmpeg/pkg/ffmpeg-8.1-full_build-shared/include
• third_party/ffmpeg/pkg/ffmpeg-8.1-full_build-shared/lib
• third_party/ffmpeg/pkg/ffmpeg-8.1-full_build-shared/bin

The current CMake helper expects DLL/import library names matching:

• avcodec-62.dll
• avformat-62.dll
• avutil-60.dll
• swscale-9.dll
• swresample-6.dll

Virtual display driver

This repo does not include the actual virtual display driver package.

The current code looks for MttVDD.inf in:

1. C:\Windows\System32\DriverStore\FileRepository\mttvdd.inf_amd64_*\MttVDD.inf
2. a hardcoded developer fallback inside src/VirtualDisplay.cpp

In practice:

• if the driver is already installed or staged in the Windows Driver Store, the app can usually find it automatically
• if not, you need to stage/install the driver yourself or update the fallback path in src/VirtualDisplay.cpp

Build the desktop app

The desktop app depends on the native bridge DLL, so build in this order.

1. Build the native core and bridge

From the repo root:

build.bat

Or manually:

cmake -S . -B build -G "Visual Studio 17 2022" -A x64
cmake --build build --config Release

This produces:

• build/bin/Release/SecondMonitor.exe
• build/bin/Release/SecondMonitorBridge.dll

2. Build the WinUI desktop shell

From the repo root:

dotnet build windows-ui\SecondMonitor.Desktop\SecondMonitor.Desktop.csproj -c Release -p:Platform=x64

Desktop app output:

• windows-ui/SecondMonitor.Desktop/bin/x64/Release/net8.0-windows10.0.19041.0/SecondMonitor.Desktop.exe

The desktop project copies SecondMonitorBridge.dll, FFmpeg DLLs, and Windows App SDK bootstrap/runtime DLLs into its output directory during build.

3. Run the desktop app

windows-ui\SecondMonitor.Desktop\bin\x64\Release\net8.0-windows10.0.19041.0\SecondMonitor.Desktop.exe

What the desktop app does:

• lists detected displays
• can create a second virtual display
• starts/stops the native streaming server
• can connect the built-in native client for local testing
• stores UI settings under %LocalAppData%\SecondMonitor\desktop-settings.json

4. Build and install an MSIX locally

For local sideloading, the package must be signed with a certificate trusted by the machine that installs it.

Build a local MSIX:

dotnet publish windows-ui\SecondMonitor.Desktop\SecondMonitor.Desktop.csproj -c Release -p:Platform=x64 -p:GenerateAppxPackageOnBuild=true

Package output:

• windows-ui/SecondMonitor.Desktop/bin/x64/Release/net8.0-windows10.0.19041.0/AppPackages/SecondMonitor.Desktop_1.0.0.0_x64_Test/SecondMonitor.Desktop_1.0.0.0_x64.msix

Important notes:

• an unsigned .msix will not install, even on the same development machine
• the package identity publisher in windows-ui/SecondMonitor.Desktop/Package.appxmanifest must match the subject of the signing certificate
• for self-signed local builds, trust the signing certificate in Windows before running the generated install script
• Visual Studio and the Windows App SDK packaging targets can generate Add-AppDevPackage.ps1 and a .cer file next to the package; those are the easiest files to use for local sideload installs

Typical local install flow:

1. Create or select a code-signing certificate whose subject matches the package publisher.
2. Configure the desktop packaging profile to sign the package with that certificate.
3. Build the package.
4. Install the generated .cer into TrustedPeople. Some setups may also require trusting the certificate in Root.
5. Run Install.ps1 or Add-AppDevPackage.ps1 from the generated AppPackages folder.

Build the Android app

Android prerequisites:

• Android Studio or command-line SDK tools
• Android SDK Platform 34
• Android NDK 26.x
• Java 17

The Android native build also expects third_party/libdatachannel and third_party/mbedtls in the repo root.

From android-client:

gradlew.bat :app:assembleDebug

APK output:

• android-client/app/build/outputs/apk/debug/app-debug.apk

Install to a connected device:

adb install -r android-client\app\build\outputs\apk\debug\app-debug.apk

Native CLI usage

The native executable still exists for direct testing and scripting:

List monitors:

build\bin\Release\SecondMonitor.exe --list

Create a virtual display and stream it:

build\bin\Release\SecondMonitor.exe --width 1920 --height 1080 --listen-port 5004

Capture an existing monitor instead:

build\bin\Release\SecondMonitor.exe --stream-monitor 1 --listen-port 5004

Run the Windows receive client:

build\bin\Release\SecondMonitor.exe --client --stream-host 127.0.0.1 --stream-port 5004

Current assumptions

• local build outputs, screenshots, and machine-specific config are not committed
• third_party dependencies are provisioned locally
• the top-level README is the main setup/build reference