VaCuus
Your game's interface in HTML and CSS, rendered entirely off the game thread.
You author screens as .rml / .rcss documents — the web languages, not a UMG graph — and the plugin runs them on its own UI thread: layout, styling and draw-command recording all happen away from the game thread, which only enqueues input and reads back a snapshot. Full-screen HUD, menu, or a panel on a quad in the world.
Off the game thread, measured
docs/perf-guideOne UI thread owns every document: layout, styling and draw-command recording. The render thread replays the resulting command buffer into a persistent render target, and Slate composites that target every engine frame. What is left on the game thread is enqueueing input and reading one published snapshot.
Publication is withheld when a frame's content hash is unchanged and no resource traffic occurred, so a static UI stops costing anything to draw: the static M1 HUD published 1 frame in 6,910, and the glass demo 0 of 3,363. The reference HUD below publishes 100% of frames by design — it animates every one of them, which is why it is the worst case.
Every figure here is measured and re-runnable in your own project: vacuus.RefHud to load the scene, vacuus.M1HUD.PerfLog 1 to print the timings and the publish ratio.
| Game thread / frame budget 0.10 ms · ~8–11× | 0.012 ms dev 0.008–0.009 ms shipping |
|---|---|
| Composite only, idle UI budget 0.05 ms · ~12–25× | 0.002 ms shipping 0.003 ms dev |
| UI thread, update + record budget 1.2 ms steady avg | 1.050 ms shipping 1.077 ms dev |
| Added RAM budget 32 MB · A/B median | +14.3 MB |
| Added disk, shipping budget 10 MB · Linux proxy | +3.22 MiB |
Plain text, live reload
docs/setup.rml is the markup, .rcss the styles, and both are plain files on disk. There is no editor asset to open and no graph to wire — your edit is complete when the file is saved, and it diffs, reviews and merges like the rest of your source.
Screens live under Content/DevUI: the plugin's own tree ships the demos, and your project's tree is an extension point beside it, not an override.
- Save the file. Any editor. No import step, no reimport prompt, no asset registry entry.
- The watcher reloads the document in place while PIE keeps running — the editor watches
Content/DevUI. - At runtime,
vacuus.ReloadUIdoes the same thing from the console. - No recompile. C++ is what hosts the document; it is not what the document is made of.
Built for AI agents
docs/ai-guideThe entire UI surface is text an agent can read and write. That is the easy half. The hard half is that most authoring mistakes here produce no error and a screen that renders — just not the one you asked for. A human notices immediately; an agent, which cannot see the screen, does not.
So the package ships the other half too: a front door the agent finds on its own, a generated ground truth to check itself against, and a verification loop that works without eyes.
UI is VaCuus (HTML/CSS off the game thread). Before touching anything under Content/DevUI, read Plugins/VaCuus/docs/buyer/ai-guide.md and Plugins/VaCuus/docs/buyer/rcss-matrix.md.
AGENTS.md
At the package root, where an agent looks first. Short by design: the five things that would otherwise cost the session, and links to the real pages.
ai-guide.md
The full brief. Which mistakes warn, which are drawn wrong in silence, and how to tell the two apart.
rcss-matrix.md
99 properties, 20 shorthands, 16 decorators — parsed out of the exact RmlUi vendored in the package, not out of a model's training data. If a property is not in that file, it does not exist.
The verification loop
A declaration RmlUi cannot parse logs a warning naming file:line, in every configuration including Shipping. Plus a headless 1920×1080 render recipe and the 227-test suite, so an agent can check its own work.
Engine-native data
docs/setup Data models bind straight from your UPROPERTY fields. The plugin reads them through the engine's own reflection — no mirror struct to keep in sync, no serialisation step between your game state and the document.
JavaScript is optional, not assumed. QuickJS-ng is vendored in-tree for the projects that want scripting, and a TypeScript/Preact workflow sits on top of it when you want that too.
- UVaCuusWidget — "VaCuus View" in the UMG palette. Screen space: HUD, menu, overlay. Drop it into any UMG tree and point it at a document.
- UVaCuusWorldComponent — "VaCuus World Panel". A pixel panel on a quad in the world.
- Both work from Blueprint and from C++. Either one hands you a view, and the view is what you feed and drive.
Rendered by the plugin
headless, 1920×1080
:hover/:active/:focus, a wheel-scroll list, a text field with tab order, and a region that lets clicks reach the gameWhat it is not
docs/rcss-matrixNot a browser engine
No <a href> navigation, no CSS Grid, no fetch, no DOM library you remember. Navigation is the host calling LoadDocument; layout is flexbox and absolute positioning.
RCSS is not CSS
It is RmlUi's style language: a deliberate subset with its own additions. The generated matrix is the exact difference, keyed to the engine in your package — read it before you write a property.
Not every property is drawn
A few parse and are then not rendered — box-shadow is the headline case, and it warns per view and names a substitute. The matrix marks them; nothing here is a surprise you find at ship time.
Source only, so C++ only
There is no Binaries/ directory and that is deliberate. A Blueprint-only project has no toolchain to compile the plugin with — converting takes two minutes: Tools → New C++ Class → None.