Render trace

Pipeline-level kinds emitted by the renderer itself: a Perfetto-importable Chrome trace, a phase breakdown, the device clip mask, and the device background.

At a glance


Kinds	`render/composeAiTrace`, `render/trace`, `render/deviceClip`, `render/deviceBackground`
Schema version	1
Modules	`:data-render-core` (published) · `:data-render-connector` · `:data-render-compose`
Render mode	default · live
Cost	low
Token usage	~50 tok inline per kind; +~1.5 k per `render/deviceClip` / `render/deviceBackground` PNG read. See token usage.
Transport	inline (JSON) · path (PNG for clip / background)
Platforms	Android · Desktop · shared

What it answers

render/composeAiTrace — a full Perfetto / Chrome-trace JSON of the render pipeline (discovery → setup → compose → capture → encode). Drop it into ui.perfetto.dev to see exactly where time went.
render/trace — a flatter, summary-shaped phase breakdown (totalMs, ordered phases, free-form metrics map). Cheap to read in a CI log.
render/deviceClip — the device-shape clip mask used for round Wear watches and other non-rectangular previews. Pure-image.
render/deviceBackground — the device chrome / wallpaper layer composited under the preview. Pure-image.

What it does NOT answer

render/trace is a renderer pipeline trace, not a Compose composition trace — for that, use compose/recomposition.
It does not contain on-device frame timing (Choreographer, jank stats) — the renderer runs under Robolectric / ImageComposeScene, not on a real Choreographer.
The Perfetto trace is sampled at function boundaries, not at the JVM-method granularity of an async-profiler dump.

Use cases

Diagnose a slow render that nudges past daemon.dataFetchRerenderBudgetMs — open the Perfetto trace and find the phase that ran long.
Build a CI dashboard of totalMs per preview to catch gradual regression.
Verify a Wear round-device clip mask matches the watch face’s intended bezel.
Composite a custom device background under a preview for marketing screenshots.

Payload shape

PerfettoTraceDataProducer, RenderTraceDataProduct in :data-render-core.

// render/trace
{
  "totalMs": 412,
  "phases": [
    { "name": "render", "startMs": 0, "durationMs": 412 }
  ],
  "metrics": { "tookMs": 412, "previewWidthPx": 1080, "previewHeightPx": 1920 }
}

// render/composeAiTrace — Chrome trace JSON, importable into ui.perfetto.dev
{ "traceEvents": [ { "ph": "X", "name": "compose", "ts": 1234, "dur": 89, … } ] }

Enabling

render/composeAiTrace is gated by the JVM property composeai.daemon.perfettoTrace=true. The other three kinds run whenever their extension is publicly enabled. CLI / Gradle output:

build/compose-previews/data/<id>/render-perfetto-trace.json
build/compose-previews/data/<id>/render-trace.json
build/compose-previews/data/<id>/render-deviceClip.png
build/compose-previews/data/<id>/render-deviceBackground.png

Companion products

Test failure — test/failure for postmortems when a render fails before producing a trace.
Recomposition — compose/recomposition for the per-node side of the wall-clock story.