A lot of times I wanted to demonstrate as a TA some code walkthroughs, or I had a presentation in PDF, and I couldn't easily have my presenter notes open without exposing them to the audience when sharing my entire screen.
When presenting via HDMI, screen share (Zoom, Teams, Meet), or a projector, you usually have two less-than-ideal options: share everything (and let the audience see your private notes) or use complex setups like OBS and manually switch sources mid-presentation. Neither is acceptable for a natural teaching or presenting workflow.
So, I built PresenterShield.
PresenterShield handles two main scenarios:
- Basic Shielding (Start Shield Session): Best for remote meetings (Zoom, Teams, Slack). It applies the privacy overlay directly to your windows. Since these apps capture the "Window" or "Desktop" pipelines, the shellcode-injected affinity flags keep your private windows hidden.
- Hardware Mirroring (Start Secure Mirroring): Required when connecting to physical outputs like HDMI, DisplayPort, or Projectors. Direct hardware outputs often bypass standard window capture flags. In this mode, PresenterShield mirrors your primary display into a dedicated window. Note: For this to work, your Windows Projection settings must be set to "Extend", not "Duplicate".
Private windows are automatically highlighted with a subtle red border. This gives you immediate visual feedback on what is being shielded from the audience, even if you have the opacity set to 100%.
The app remembers your choices. Once you mark a window title as private, PresenterShield will automatically apply the shield and border the next time you open that application.
The project is built using C# and WPF on .NET 10.0, leveraging CommunityToolkit.Mvvm for clean separation.
These mechanisms work together to ensure privacy without breaking the user experience:
- Windows must be invisible to all capture pipelines (HDMI out, screen share, OBS).
- Windows must remain fully interactive regardless of where the user clicks.
- Windows must never drop behind other windows or flash on the public view during focus changes.
A naive approach would solve one and break the others. PresenterShield keeps all three intact by combining the following mechanisms:
1. Hiding from Capture via Shellcode Injection
SetWindowDisplayAffinity with the WDA_EXCLUDEFROMCAPTURE flag makes a window invisible to any capture mechanism. However, Windows strictly requires this API to be called from within the process that owns the window. To bypass this, ShellcodeInjector.cs injects x64 assembler shellcode directly into the target process's memory via VirtualAllocEx and CreateRemoteThread, forcing the target application to call the API on itself.
2. Hiding from the Taskbar and Alt-Tab
By swapping the target window's extended styles (GWL_EXSTYLE), removing WS_EX_APPWINDOW and applying WS_EX_TOOLWINDOW, the window is physically removed from the taskbar and Alt-Tab switcher. This prevents it from accidentally surfacing on the public view during rapid task switching.
3. The Always-On-Top Interactive Overlay
WS_EX_LAYERED + SetLayeredWindowAttributes controls the window's opacity. SetWindowPos with HWND_TOPMOST ensures it stays above all other windows, so interacting with background apps never pushes the overlay behind.
4. DXGI Desktop Duplication (Hardware Mirroring)
For hardware outputs, standard window affinity flags are bypassed. PresenterShield uses SharpDX to interface with the DXGI Desktop Duplication API. It captures the primary display at 30+ FPS, filters any "Private" windows out of the texture, and renders the result to a dedicated projection window on the secondary display.
(All changes are fully reversible when you stop the session.)
Beyond my initial pain point of TA sessions, PresenterShield is incredibly versatile for:
- Live Coding & Teaching: Keep your reference implementations, terminal cheatsheets, or lesson plans open right next to your code editor.
- Corporate Presentations: Read from a hidden script or keep an eye on incoming Slack messages while projecting financial reports.
- Content Creators & Streamers: Keep your stream chat, OBS controls, or sensitive dashboard information visible to you on a single-monitor setup without it ever broadcasting.
- Remote Meetings: Cross-reference confidential internal documents while screen-sharing collaborative whiteboards or Jira boards to clients.
- Windows 10/11.
- .NET 10 SDK.
- x64 Architecture: The shellcode handles 64-bit processes specifically.
- Clone the repo.
- Open in Visual Studio.
- Set platform to x64.
- Run as Administrator (required for memory injection/process manipulation).
Mirroring brings up a black screen?
Mirroring mode requires a secondary output (HDMI, Projector, or a Virtual Display Driver). If you're on a single monitor, the app will warn you before starting.