ultralightui

Render HTML/CSS/JS interfaces as textures in Ebitengine using Ultralight 1.4.

• Full HTML5/CSS3/JS engine rendered to an off-screen bitmap
• Transparent backgrounds (HTML layers on top of your game)
• Bidirectional Go <-> JavaScript communication via native JavaScriptCore bindings
• Multiple independent views with per-view input routing
• Mouse, scroll, and keyboard forwarding from Ebiten to Ultralight
• Embedded assets via VFS: bundle HTML/CSS/JS/images inside the binary with go:embed
• No CGo required (uses purego + a small C bridge DLL)

Architecture

Go (Ebitengine) <--purego--> ul_bridge shared lib (GCC) <--dlopen/LoadLibrary--> Ultralight 1.4 libs

The C bridge shared library (ul_bridge.c) runs all Ultralight API calls on a dedicated worker thread (Ultralight requires single-thread affinity). Go communicates with it through simple exported functions via purego. On Windows it uses Win32 APIs (LoadLibrary, CreateThread, CreateEvent); on Linux/macOS it uses POSIX equivalents (dlopen, pthread, pthread_cond).

Setup

Prerequisites

Tool	Version	What for
Go	1.21+	Build your application
Ebitengine	v2.8+	Game engine (pulled automatically by go mod)
GCC	Any	Compile the bridge shared library from source (w64devkit on Windows, system GCC on Linux/macOS)
Ultralight SDK	1.4	HTML rendering engine (download separately)

Step 1: Download the Ultralight 1.4 SDK

Go to ultralig.ht/download (requires a free account) and download the SDK for your platform (Windows x64, Linux x64, or macOS). Extract the archive. You'll get a folder like this:

ultralight-sdk/
  bin/
    Ultralight.dll / libUltralight.so / libUltralight.dylib
    UltralightCore.dll / libUltralightCore.so / libUltralightCore.dylib
    WebCore.dll / libWebCore.so / libWebCore.dylib
    AppCore.dll / libAppCore.so / libAppCore.dylib
  resources/
    cacert.pem
    icudt67l.dat        <-- ICU Unicode data (required for all text/JS)
  include/              <-- C headers (not needed at runtime)
  lib/                  <-- .lib/.a files (not needed, we load libs at runtime)
  ...

Step 2: Copy SDK files to your project

Copy the SDK libraries and data files into the working directory of your application (typically the folder where your main.go is, or wherever you run the executable from):

# From the SDK archive (use the files matching your platform):
cp ultralight-sdk/bin/*Ultralight*      your-project/
cp ultralight-sdk/bin/*UltralightCore*  your-project/
cp ultralight-sdk/bin/*WebCore*         your-project/
cp ultralight-sdk/bin/*AppCore*         your-project/
cp ultralight-sdk/resources/icudt67l.dat  your-project/

SDK library names per platform:

Library	Windows	Linux	macOS
Ultralight	Ultralight.dll	libUltralight.so	libUltralight.dylib
UltralightCore	UltralightCore.dll	libUltralightCore.so	libUltralightCore.dylib
WebCore	WebCore.dll	libWebCore.so	libWebCore.dylib
AppCore	AppCore.dll	libAppCore.so	libAppCore.dylib

What each library does:

Library	Size	Purpose
Ultralight	~1 MB	Core Ultralight API (renderer, views, surfaces)
UltralightCore	~4 MB	Low-level rendering backend
WebCore	~35 MB	HTML/CSS/JS engine (WebKit fork + JavaScriptCore)
AppCore	~0.5 MB	Platform helpers (font loader, file system, logger)
icudt67l.dat	~25 MB	ICU Unicode data tables. Required for all text rendering, JavaScript string operations, CSS text layout, regex, and locale-aware formatting. Without this file, Ultralight cannot process any text.

Step 3: Compile the bridge DLL

The bridge is a thin C layer between Go and Ultralight. The source is included in this repository at bridge/ul_bridge.c. Compile it with GCC:

# Windows:
gcc -shared -o ul_bridge.dll bridge/ul_bridge.c -O2 -lkernel32

# Linux:
gcc -shared -fPIC -o libul_bridge.so bridge/ul_bridge.c -O2 -lpthread -ldl

# macOS:
gcc -shared -fPIC -o libul_bridge.dylib bridge/ul_bridge.c -O2 -lpthread -ldl

Windows tip: If you don't have GCC in your PATH, download w64devkit, extract it, and run from its shell. It's a single portable zip, no installer needed.

Place the resulting shared library in the same directory as the Ultralight SDK libraries.

Step 4: Add the Go module

go get github.com/YindSoft/ultralight-ebitengine-port@latest

Final directory layout

Your project directory should look like this:

your-project/
  main.go
  go.mod
  go.sum
  Ultralight.dll / libUltralight.so / libUltralight.dylib       \
  UltralightCore.dll / libUltralightCore.so / ...                 |  from Ultralight SDK
  WebCore.dll / libWebCore.so / ...                               |
  AppCore.dll / libAppCore.so / ...                               |
  icudt67l.dat                                                   /
  ul_bridge.dll / libul_bridge.so / libul_bridge.dylib   <-- compiled from bridge/ul_bridge.c
  ui/
    index.html            <-- your HTML interface

You can also place the libraries in a separate directory and pass it via Options.BaseDir.

Quick Start

package main

import (
    "fmt"
    ultralightui "github.com/YindSoft/ultralight-ebitengine-port"
    "github.com/hajimehoshi/ebiten/v2"
)

type Game struct {
    ui *ultralightui.UltralightUI
}

func (g *Game) Update() error { return g.ui.Update() }

func (g *Game) Draw(screen *ebiten.Image) {
    screen.DrawImage(g.ui.GetTexture(), nil)
}

func (g *Game) Layout(w, h int) (int, int) { return 800, 600 }

func main() {
    ui, err := ultralightui.NewFromFile(800, 600, "ui/index.html", nil)
    if err != nil {
        panic(err)
    }
    defer ui.Close()

    ui.OnMessage = func(msg string) {
        fmt.Println("from JS:", msg)
    }

    ebiten.SetWindowSize(800, 600)
    ebiten.RunGame(&Game{ui: ui})
}

API

Creating views

// From a local HTML file (path relative to working directory):
ui, err := ultralightui.NewFromFile(width, height, "ui/index.html", nil)

// From raw HTML bytes:
ui, err := ultralightui.NewFromHTML(width, height, []byte("<h1>Hello</h1>"), nil)

// From a URL:
ui, err := ultralightui.NewFromURL(width, height, "https://example.com", nil)

All sync constructors (NewFromFile, NewFromHTML, NewFromURL, NewFromFS) return a fully loaded view in a single call (~3-7 ms). For non-blocking creation, use the async variant:

// Async: returns immediately, view loads in the background (~5 ticks / ~83ms)
ui, err := ultralightui.NewFromFSAsync(800, 600, "ui/index.html", uiFiles, nil)
// ui.IsReady() returns false until loading completes
// ui.Update() is safe to call immediately (renders transparent until ready)

Options

opts := &ultralightui.Options{
    BaseDir: "/path/to/libs",  // Where to find the bridge and SDK libraries (default: working dir)
    Debug:   true,             // Create bridge.log and ultralight.log for troubleshooting
}
ui, err := ultralightui.NewFromFile(800, 600, "ui/index.html", opts)

JS -> Go (messages)

JavaScript sends messages to Go using go.send():

// Send a string
go.send("attack");

// Send JSON (receives as a JSON string on the Go side)
go.send(JSON.stringify({ action: "move", x: 10, y: 20 }));

Go receives messages via the OnMessage callback:

ui.OnMessage = func(msg string) {
    // msg is "attack" or '{"action":"move","x":10,"y":20}'
    data, err := ultralightui.ParseMessage(msg) // auto-parses JSON
    fmt.Println(data)
}

This uses native JavaScriptCore bindings under the hood (no console.log hacks).

Go -> JS (eval and send)

Run arbitrary JavaScript:

ui.Eval("document.getElementById('score').textContent = '100'")
ui.Eval("updateHP(75, 100)") // call a JS function you defined

Send structured data (serialized as JSON):

ui.Send(map[string]any{"hp": 80, "maxHp": 100, "items": []string{"sword", "shield"}})

JavaScript receives it via go.receive:

go.receive = function(data) {
    console.log(data.hp, data.maxHp);   // 80 100
    console.log(data.items);            // ["sword", "shield"]
};

Input

Mouse and scroll events are forwarded when the cursor is inside the view's bounds. Keyboard events go to whichever view has focus.

// Restrict this view to a screen region (for multi-view layouts):
ui.SetBounds(x, y, width, height)

// Give keyboard focus to this view:
ui.SetFocus()

Clicking inside a view automatically gives it focus.

Multiple views

You can create multiple independent views, each with its own HTML page:

mainUI, _ := ultralightui.NewFromFile(600, 400, "ui/main.html", nil)
sidebar, _ := ultralightui.NewFromFile(200, 400, "ui/sidebar.html", nil)

mainUI.SetBounds(0, 0, 600, 400)
sidebar.SetBounds(600, 0, 200, 400)

Transparency

HTML views have transparent backgrounds by default. This lets you layer HTML on top of your game rendering. Use CSS background: rgba(...) for semi-transparent panels.

Embedded assets (VFS)

You can bundle all your HTML/CSS/JS/images inside the Go binary using go:embed and the built-in virtual file system (VFS). No files need to exist on disk at runtime (except the Ultralight SDK libraries and icudt67l.dat).

package main

import (
    "embed"
    ultralightui "github.com/YindSoft/ultralight-ebitengine-port"
    "github.com/hajimehoshi/ebiten/v2"
)

//go:embed ui
var uiFiles embed.FS

type Game struct{ ui *ultralightui.UltralightUI }
func (g *Game) Update() error              { return g.ui.Update() }
func (g *Game) Draw(screen *ebiten.Image)  { screen.DrawImage(g.ui.GetTexture(), nil) }
func (g *Game) Layout(w, h int) (int, int) { return 800, 600 }

func main() {
    ui, err := ultralightui.NewFromFS(800, 600, "ui/index.html", uiFiles, nil)
    if err != nil { panic(err) }
    defer ui.Close()

    ebiten.SetWindowSize(800, 600)
    ebiten.RunGame(&Game{ui: ui})
}

How it works

1. go:embed ui compiles the entire ui/ folder into the binary
2. NewFromFS walks the embedded FS and registers every file in a C-level VFS via RegisterFile()
3. The VFS is checked first on every Ultralight file request; disk is used as fallback
4. The main page is loaded via file:///ui/index.html which resolves from the VFS
5. All relative references (<link href="style.css">, <script src="app.js">, etc.) resolve from the VFS too

VFS API

You can also register individual files manually (useful for dynamic content):

// Register files before creating views that reference them
ultralightui.RegisterFile("ui/config.json", configBytes)

// Query the number of registered files
count := ultralightui.VFSFileCount()

// Clear all registered files
ultralightui.ClearFiles()

Examples

Multi-view example

See the example/ directory for a complete working demo with two views, animated Ebiten shapes behind HTML, bidirectional communication, and keyboard input.

cd example
go run .

Embedded assets example

See the example_embed/ directory for a demo that loads all HTML/CSS/JS from go:embed with no files on disk. It demonstrates:

• Separate .html, .css, and .js files all served from the VFS
• Go -> JS communication (counter updates every second)
• JS -> Go communication (button clicks via go.send())

cd example_embed
go run .

Both examples expect the SDK libraries to be in the parent directory (the repo root).

Performance

View creation is optimized for minimal latency. The bridge uses a fast sync path that combines view creation and content loading into a single worker thread roundtrip with no sleep loops:

Scenario	Time
Simple HTML	~3 ms
Complex HTML (commerce UI)	~7 ms
URL (file:/// with VFS)	~2.5 ms
6 views (simple)	~13 ms
6 views (complex)	~35 ms
First view (cold start, includes renderer init)	~32 ms

The pixel pipeline uses an async goroutine for BGRA-to-RGBA conversion, keeping the main game loop free from blocking work. Dirty tracking ensures pixel copies only happen when the surface has actually changed.

How it works (internals)

1. The bridge shared library loads the Ultralight SDK at runtime via LoadLibrary/GetProcAddress (Windows) or dlopen/dlsym (Linux/macOS)
2. A dedicated worker thread handles all Ultralight API calls (renderer, views, JS eval)
3. Go sends commands to the worker thread via a simple command queue with platform-native synchronization (Win32 events on Windows, pthread mutex+cond on POSIX)
4. View creation and content loading are combined into a single worker command (CMD_CREATE_WITH_HTML / CMD_CREATE_WITH_URL) for minimum latency
5. Pixel data is read from Ultralight's surface bitmap (BGRA), converted to RGBA in an async goroutine, and written to an Ebiten image
6. JS -> Go communication uses JavaScriptCore's native C API (JSObjectMakeFunctionWithCallback) to register window.go.send() as a native function that pushes messages to a queue
7. Go -> JS communication calls window.go.receive(data) via ulViewEvaluateScript

Troubleshooting

Problem	Solution
failed to load ul_bridge	Make sure the bridge shared library (ul_bridge.dll / libul_bridge.so / libul_bridge.dylib) is in your working directory or in Options.BaseDir. Recompile it if needed.
FAIL: Ultralight / FAIL: WebCore in bridge.log	One of the SDK libraries is missing. Copy all 4 libraries from the SDK bin/ folder.
All pixels are zero / blank screen	Make sure icudt67l.dat is present. Enable Debug: true and check ultralight.log.
Buttons don't respond to clicks	Verify SetBounds() matches where you draw the texture. Input is only forwarded inside bounds.
Keyboard doesn't work	Call SetFocus() on the view, or click inside it first.

Platform support

Platform	Bridge library	SDK libraries	Status
Windows x64	ul_bridge.dll	*.dll	Tested
Linux x64	libul_bridge.so	lib*.so	Supported (requires Ultralight Linux SDK)
macOS x64/arm64	libul_bridge.dylib	lib*.dylib	Supported (requires Ultralight macOS SDK)

The C bridge uses #ifdef _WIN32 to select between Win32 APIs and POSIX equivalents (dlopen/dlsym, pthread_create, pthread_mutex/pthread_cond). The Go side uses build tags (bridge_windows.go, bridge_unix.go) to select the library loading mechanism.

License

MIT - Copyright (c) 2026 Javier Podavini (YindSoft)

Ultralight is a separate product with its own license. The SDK DLLs are not included in this repository; you must download them from the Ultralight website.