MultiMon

MultiMon takes any set of monitors and turns them into one seamless video wall. Load your media onto monitor slots, pick a mode, then go fullscreen across every screen you have chosen. It is free and open source under EUPL-1.2, and version 1.0.0 is available for Windows and macOS (Apple Silicon).

Two platforms, one wall

Four modes

The mode dropdown has four entries:

• Span — one source sampled across every display MultiMon detects, as a single continuous canvas. There is no monitor picker: the control machine’s own screen is one of the outputs, and each screen gets an equal share of the picture per screen, not per pixel, so a wall of mixed resolutions will not line up geometrically.
• Individual — one source per monitor. Free-running by default; tick Sync to put every source on the one shared clock.
• Hap — one HAP clip per monitor, all of them on the one shared clock. HAP’s compressed-texture decode is fast and even enough to hold several streams tightly frame-synced, which is why this is its own mode rather than a flag on Individual.
• Split — one source cut into a rows-by-columns grid, 2×2 by default, with each output mapped to a grid cell by row,col. A monitor left with no cell in the grid stays black.

Hap or Individual + Sync? Choose Individual + Sync when your clips are H.264 or HEVC, or a mix of formats. Choose Hap when every clip is a HAP file and the wall has to stay tight — Individual takes the Media Foundation path even for a .mov that holds HAP, so it never gets HAP’s frame-accurate decode. Either way, frame-locking needs clips of equal duration.

Codecs and decode

• HAP and HAP Q, written from scratch. Each HAP variant maps to its own block-compressed texture format: plain HAP is RGB DXT1 (BC1); HAP Alpha is RGBA DXT5 (BC3); HAP Q is scaled YCoCg DXT5 — also BC3 — decoded to RGB by a YCoCg pixel shader; the RGBA BPTC variant is BC7; and the alpha plane of HAP Q Alpha is A RGTC1 (BC4). No third-party HAP library is involved.
• If the GPU does not support a clip’s texture format, MultiMon hands the file to Media Foundation instead. If Media Foundation cannot open it either, that output stays black and the failure is logged — nothing crashes.
• H.264 and HEVC. On Windows these decode through Media Foundation, using hardware decode with an automatic fallback to software decode when a hardware decoder is unavailable.
• Containers accepted: mp4, mov, mkv, avi.

Audio mixer

Each track has its own volume, pan, mute and solo, and a remove button on tracks you added yourself. Above the track list sit a MASTER fader and a Mute box, and an Add audio… button for standalone stems (mp3, wav, flac, aac, m4a, and the audio of an mp4 or mov).

There is one AUDIO DEVICE dropdown for the whole show, next to the mode dropdown. Every track is sent to that one device — there is no per-track output device. The mixer is inline in the main window; there is no separate routing dialog. On Windows, output runs through WASAPI in shared mode, clocked to the same master timeline that drives the video.

If the audio device is removed mid-show, MultiMon rebuilds the WASAPI endpoint and retries. After three failed attempts that output stops for the rest of the show — the audio ends, and the application and the video carry on.

Sync

There is one master clock. Every source shows the nearest frame at or before master time, dropping or repeating a frame as needed. There is no speed adjustment and no seek correction. Span and Split are inherently in sync because they are one source, and Hap mode is always on the shared clock. Individual mode has a checkbox labelled Sync (frame-lock same-length clips) — the name is the condition: frame-locking only holds for clips of equal duration.

The window and the keys

MultiMon is a single main window. Along the top sit the mode dropdown, the show-wide AUDIO DEVICE dropdown, the Sync (frame-lock same-length clips) checkbox in Individual mode, and Identify screens, which puts a numbered overlay on every display so you can confirm the assignment before you perform.

Below that are the monitor slots (or the single source field in Span and Split), and in Split mode an Auto (by screens) tickbox with rows and columns fields. Leave Auto ticked and MultiMon derives a near-square grid from the number of screens — four screens give 2×2, six give 2×3 — or untick it and set the grid yourself.

The audio mixer is inline on the right. The transport row carries PERFORM and Stop, then the project buttons New, Open project, Save project and Convert to HAP.

Key	What it does
F11	Toggle perform mode
Space	Pause / resume while performing
Esc	Exit perform mode

Projects and tools

• .mmproj project files — JSON, schema version 2. Version 1 files are migrated automatically on open.
• Convert to HAP — transcodes a source to HAP into a mov container, taking mp4, mov, mkv, avi or wmv as input. It calls FFmpeg, which must be on your PATH; FFmpeg is not bundled, and no playback feature needs it.
• Device-removed recovery — if the graphics device is lost, MultiMon rebuilds it and resumes from the master clock.

Known limitations

Stated plainly, because a live tool is worth less than nothing if it surprises you:

• FFmpeg is resolved from your PATH. Install it only from a source you trust.
• Only open .mmproj files you created. Media paths inside a project file are used as written.
• Per-link mixer overrides are not saved into the project file yet.
• The Windows build is unsigned, so SmartScreen will warn on first run — choose More info, then Run anyway. Code signing is planned.

System requirements

Requirement	Detail
Windows	Windows 10 version 1809 or later, or Windows 11 (64-bit). Self-contained — .NET 9 is bundled
macOS	macOS 14 (Sonoma) or later on Apple Silicon. Intel Macs are planned for 1.1
GPU	Windows: a Direct3D 11 Feature Level 11.0 GPU or better — a GPU that does not expose Feature Level 11.0 fails at startup with a logged error. macOS: the Apple Silicon GPU, through Metal
Outputs	At least one monitor. No hard upper limit, and MultiMon has been tested with up to four simultaneous outputs
Optional	FFmpeg on your PATH, only if you want Convert to HAP

Documentation

• Getting started — download MultiMon and get your first clip on screen
• Standard playback — H.264 and HEVC, containers and decode fallback
• Video wall setup — Split and Span across several screens
• Video synchronisation — the master clock and the Sync checkbox
• HAP codec — HAP and HAP Q playback, and converting your media
• Audio mixer — volume, pan, mute, solo and output devices
• What you can configure — every setting MultiMon actually has

Download MultiMon 1.0.0-rc.1 for Windows → (x64, self-contained zip, ~115 MB)

Download MultiMon 1.0.0-rc.1 for macOS → (Apple Silicon DMG, ~110 MB — right-click → Open on first launch)

Full changelog → · Releases on GitHub →

Licence

MultiMon is free and open source under the European Union Public Licence v1.2 (EUPL-1.2), with the full source and history on GitHub.