run-ebitengine-app-headless

run-ebitengine-app-headless skill

Targets ebiten commit a884b6732 (2026-07-14), verified against it. exp/vmhost is experimental, so if its API has moved since, the driver and the snippets here may need updating.

Drive an Ebitengine app from a small host program that runs it as a guest of the exp/vmhost package. The host controls the guest's ticks however it likes — one at a time, many per host frame, or free-running — injects input, reads back rendered frames, and observes the audio the app plays, all from a hidden window. The app under test is unmodified.

How it works (mental model)

Only the host touches a GPU or display; the guest is fully headless.

• Guest = the app under test. Its ebiten.RunGame connects to the host over a socket instead of opening a window, forwarding its graphics commands rather than rendering.
• Host = the small driver you run. It is itself an ebiten.Game with ebiten.SetWindowVisible(false), so it has no visible window. It replays the guest's draw commands on the real GPU, and exposes a vmhost.GuestSession to step the guest (AdvanceTicks/AdvanceFrame/ WaitFrame/CompositeFrame), inject input, read pixels back, and observe the audio it plays.

The endpoint

The guest reaches the host through an endpoint — the host listener's address as a URL, e.g. unix:///path/to/socket or tcp://127.0.0.1:PORT (the driver builds one from its listener with vmhost.EndpointURLFromAddr). The endpoint reaches the guest one of two ways:

• Built with -tags ebitenginevm, the guest reads the EBITENGINE_VM_ENDPOINT environment variable (no source change — this is what the driver uses).
• With no build tag, the app's code sets RunGameOptions.VMGuestEndpoint.

The driver

A host driver lives at _driver/main.go. Treat it as a starting-point template, not a finished tool: copy it and adapt the input script, assertions, and screen size to your case. It runs as-is only for the trivial screenshot (recipe step 1); it is a skill asset, not a stable command, so don't depend on its flags.

When to use

• You need to inject input (keys, mouse, wheel, runes, touches, gamepads) and observe how the app responds — bugs that only reproduce after a click/drag/keystroke sequence.
• You need to fast-forward to a state and assert it — advance N ticks deterministically (compressed, so a long run is near-instant, not real-time), then check the resulting frame. Interleave to step frame by frame when you need every one.
• You want a golden-image / pixel assertion without a human running the app and pasting a screenshot.
• You just want a screenshot of the app — run the driver with no input script and dump the frame after N ticks (recipe step 1).
• You want to check audio — verify the app plays the expected sound, at the expected sample rate and volume, as raw PCM per player (un-mixed), without playing anything.

When NOT to use

• You're testing pure logic with no rendering — write an ordinary Go test.
• Target platform is android / ios / js — vmhost is desktop-only.

Recipe

Everything here is built on the driver at _driver/main.go: step 1 runs it as-is, steps 2–3 copy and adapt it. The Go snippets below are excerpts of its Update, so d.guest (the *vmhost.GuestSession), d.screen, and *ticks (the -ticks flag value) are the driver's own identifiers.

Run these examples from the ebiten repo root — the -pkg ./examples/... paths below are repo-relative. To test your own app in another module, copy or invoke this driver from that module's root and point -pkg at the guest package there. The driver imports github.com/hajimehoshi/ebiten/v2/exp/vmhost, so the host and guest must resolve to the same Ebitengine version; use your module's go.mod or replace directives to keep them aligned.

1. No input — just capture a frame

Run the shipped driver directly against any guest package:

go run ./skills/run-ebitengine-app-headless/_driver \
  -pkg ./examples/rotate -ticks 60 -out /tmp/frame.png

Flags: -pkg (guest package), -ticks (how many ticks to run before dumping + exiting), -out (PNG path), -w/-h (logical screen size, default 320×240). In a restricted sandbox, go run may need a writable GOCACHE or approval to use the normal Go build cache.

2. With input — script the ticks

Copy the driver to a scratch dir, edit the INPUT SCRIPT block in Update, then run the copy (a .-prefixed dir stays out of go build ./...; remove it when you are done):

mkdir -p .vmdriver && cp skills/run-ebitengine-app-headless/_driver/main.go .vmdriver/
# edit .vmdriver/main.go — the INPUT SCRIPT block in Update
go run ./.vmdriver -pkg ./examples/paint -ticks 120 -out /tmp/frame.png

The injectors on *vmhost.GuestSession. Injected state is observed at the next AdvanceTicks and persists until changed, so timing comes from where you inject in the run — split the run into AdvanceTicks segments and inject between them:

// keyboard
d.guest.PressKey(ebiten.KeyArrowRight)
d.guest.ReleaseKey(ebiten.KeyArrowRight)
d.guest.TypeRune('a')                 // a typed character (input chars)
// mouse — coordinates are outside-screen device-independent pixels
d.guest.MoveCursor(160, 120)
d.guest.PressMouseButton(ebiten.MouseButtonLeft)
d.guest.ReleaseMouseButton(ebiten.MouseButtonLeft)
d.guest.ScrollWheel(0, -1)
// touch — id, then device-independent pixels
d.guest.PressTouch(0, 100, 100)
d.guest.MoveTouch(0, 120, 100)
d.guest.ReleaseTouch(0)
// gamepads — full snapshot each call; see vmhost.GamepadState
d.guest.UpdateGamepads([]vmhost.GamepadState{ /* ... */ })

Tick counts are in the guest's own time units — see Ticks and TPS to convert seconds of app time into AdvanceTicks counts.

Example — use -ticks as the total run length, settle 30 ticks, hold a click for one tick, then run the remaining ticks — as the INPUT SCRIPT block. For shorter total runs, reduce the settle count first.

d.guest.AdvanceTicks(30) // let the app settle
d.guest.MoveCursor(160, 120)
d.guest.PressMouseButton(ebiten.MouseButtonLeft)
d.guest.AdvanceTicks(1)  // one tick with the button held
d.guest.ReleaseMouseButton(ebiten.MouseButtonLeft)
if rest := *ticks - 31; rest > 0 {
	d.guest.AdvanceTicks(rest)
}

3. Many states — snapshot each one

The driver's snapshot(i) method renders the guest's current state and writes it to a numbered PNG derived from -out (frame.png → frame_00.png, frame_01.png, …). Call it between input segments in the INPUT SCRIPT block to walk the app through N states and capture every one — in practice the highest-value pattern (for example, clicking through each page of a multi-page app and golden-comparing all of them):

d.guest.AdvanceTicks(30) // let the app settle
for page := 0; page < pageCount; page++ {
	if page > 0 {
		d.guest.MoveCursor(nextButtonX, nextButtonY) // click "next page"
		d.guest.PressMouseButton(ebiten.MouseButtonLeft)
		d.guest.AdvanceTicks(1)
		d.guest.ReleaseMouseButton(ebiten.MouseButtonLeft)
		d.guest.AdvanceTicks(30)
	}
	if err := d.snapshot(page); err != nil {
		return err
	}
}

Each snapshot runs the AdvanceFrame / WaitFrame / CompositeFrame sequence itself, so the numbered PNGs are exact frames; the final -out frame at the end of Update is still written as usual.

4. Inspect the output

Inspect the PNG with the available image-viewing tool. For a programmatic assertion, read pixels in Update after CompositeFrame and compare — d.screen.ReadPixels(buf) gives premultiplied-alpha RGBA, 4 bytes per pixel. Input coordinates and -w/-h are logical device-independent pixels, but d.screen and the PNG are physical pixels (logical × DeviceScaleFactor; for example, a 320×240 logical screen can produce a 640×480 PNG on a 2x display). Use b := d.screen.Bounds() for assertion dimensions; the center pixel of that physical w*h image is at 4*((h/2)*w + w/2). For a golden check, dump once, eyeball it, then compare bytes against the saved PNG on later runs — see Verifying a run for the pitfalls.

Verifying a run

Judge success by the presence of the output PNG (or the driver's final wrote frame log line) — never by the exit status observed through a pipe. Don't pipe the driver's output through tail -1 or send it to /dev/null: when the driver fails, its actual one-line error (the fmt.Fprintln in main) is easily discarded, leaving only go run's generic exit status 1 with nothing to diagnose. Save the full output to a log file and read it on failure:

go run ./.vmdriver -pkg ./examples/paint -ticks 120 -out /tmp/frame.png \
  > /tmp/run.log 2>&1 || cat /tmp/run.log

When byte-comparing captures with cmp -s, check that both files exist first: a missing file (from a capture that silently failed) also reads as a difference, masquerading as a pixel regression.

Golden baselines across a code change

To golden-compare before/after a code change, capture the baseline from the committed state via git show <commit>:<file> extraction or a temporary commit — not git stash round-trips: the stash list is repo-global and shared across worktrees, so a stray git stash pop can grab an unrelated stash entry belonging to the user.

How the driver drives the guest

The whole run happens in one host Update:

d.guest.AdvanceTicks(*ticks) // run every tick back-to-back, no real-time pacing
d.guest.AdvanceFrame()       // request the final frame
d.guest.WaitFrame()          // block until all queued ticks have run and it is rendered
d.guest.CompositeFrame()     // composite it into the host-owned screen image; check the bool in assertions

AdvanceTicks(*ticks) queues every tick at once; WaitFrame then blocks until they have all run and the final frame is rendered, so one wait covers the lot. That order keeps capture deterministic — the screen reflects exactly the run's end. Only the one frame is rendered, because a tick forwards no rendering; only a frame does.

This compresses wall-clock time — the point of the VM. The guest is not paced to the host's ~60 TPS, so a 1000-tick run finishes near-instantly instead of taking ~16 real seconds. To capture intermediate frames or pace input by hand, interleave instead: AdvanceTicks(n), AdvanceFrame, WaitFrame, CompositeFrame, repeat. PendingTicks and WaitTicks track the backlog; ticks coalesce, so queuing many is cheap.

guest.Err() becomes non-nil when the guest terminates, crashes, or times out; a panicking guest prints its stack to stderr and the driver stops. The driver also puts a deadline on accepting the guest connection so a guest that never dials the host fails instead of hanging indefinitely.

Ticks and TPS

A tick is one guest Update call, and AdvanceTicks(n) runs exactly n of them. The guest's TPS never throttles or scales this — how many ticks to run, and at what real-time rate if any, is entirely the host's choice.

TPS matters as a unit conversion. The guest's game logic is written assuming Update runs TPS times per second of real time, so its timers, animations, and cooldowns count in those units. d.guest.RequestedTPS() reports what the game requested via ebiten.SetTPS (the standard 60 until changed). To simulate a duration of app time, multiply seconds by that value: one second in a 30-TPS game is AdvanceTicks(30), not 60.

RequestedTPS can also return ebiten.SyncWithFPS (-1), meaning the game ties its ticks to rendered frames rather than a fixed rate. There is no seconds-to-ticks conversion then; treat a tick as a frame and choose counts by observing the app.

Observing audio

The guest plays no local audio; instead each audio stream it starts is handed to the host through the OnAudioStream handler registered when the session is created, as a separate, un-mixed *vmhost.GuestAudioStream. The driver already wires this up: d.onAudioStream collects the streams and d.appendAudioStreams(nil) hands back the still-open ones (dropping any the guest has closed). The audio methods are goroutine-safe and not frame-bound (unlike CompositeFrame/Close), so inspect them anywhere; sample data is pulled from the guest on demand. Drop this into Update once the ticks have run (e.g. after WaitFrame, so the streams the run started have been delivered):

rate := d.guest.AudioSampleRate()             // per-channel samples/sec; 0 until audio plays
for i, s := range d.appendAudioStreams(nil) { // one entry per still-open guest player
    slog.Info("guest audio player", "i", i,
        "playing", s.IsPlaying(), "volume", s.Volume(), "position", s.Position())
    // Raw PCM: 32-bit little-endian floats, two interleaved channels, at `rate`.
    // Read pulls on demand; volume is reported but NOT applied to these samples.
    buf := make([]byte, 4096)
    n, _ := s.Read(buf) // 0 bytes while paused; io.EOF once the guest closes/ends the source
    slog.Info("read PCM", "bytes", n, "rate", rate)
}

OnAudioStream fires once per stream, during AdvanceTicks and WaitTicks on the goroutine calling them, when the guest starts it. The driver's handler only stashes the handle; read the samples from Update, as above. A stream stays valid after io.EOF (a seek-and-replay yields more samples); once the guest closes its player, IsClosed() returns true and Read reports io.EOF for good, so appendAudioStreams drops it — returning only the guest's still-open streams. To actually hear it rather than just inspect it, feed the stream to a host audio.Player as its source — see examples/vm/main.go.

Gotchas

• Host and guest must be the same ebiten version. The host/guest protocol is version-locked. The driver builds the guest in your current module, so they match automatically — but a guest built against a different ebiten version (such as a prebuilt binary) fails the handshake.
• The host still needs a display/graphics context. It is windowless, not displayless: on macOS it works from a normal shell; on bare headless Linux CI you still need Xvfb or an EGL setup to create the hidden window. The guest needs neither.
• SetOutsideScreen before the first AdvanceTicks, and the image is in physical pixels (logical size × DeviceScaleFactor). The driver handles this; keep it if you change the screen size.
• Call GuestSession.Close from the host frame. The driver closes the guest inside Update, after dumping the frame. Keep that shape if you copy the template; cmd.Wait can run after ebiten.RunGame returns. Losing the host ends the closed guest's RunGame without an error, so it exits 0 on its own — the driver waits for it rather than killing it, and treats a non-zero exit as a genuine guest crash.
• Launches are occasionally flaky. Back-to-back runs sometimes fail on the first attempt while an identical rerun succeeds (root cause not identified; possibly socket/handshake timing). Retry once before debugging.
• Wall-clock isn't deterministic. Tick count is exact, but code using time.Now() still sees real time between ticks.
• exp/vmhost is experimental — its API may change.
• Unix socket path length is limited (~104 bytes on macOS), so the driver keeps its socket in a short temp dir. Unix sockets work on every desktop target, Windows included; a tcp loopback listener (127.0.0.1:0) is an equivalent alternative if a filesystem socket is inconvenient.

In-repo alternative (Go test)

To exercise an app as a guest from a go test instead of a standalone driver, mirror the package's own end-to-end tests: exp/vmhost/guest_test.go (the startGuest / tickAndFrame helpers) and exp/vmhost/readpixels_test.go. These use internal/testing's MainWithRunLoop, which is importable only inside the ebiten module.