ENGINE=passthrough · src/engine/passthrough/passthrough_engine.h (header-only) · class PassthroughEngine

The smallest possible engine: stereo in → stereo out, tracking the last-block peak to drive a level meter on the rings. Its purpose is documentation, not music — it shows how little a non-granular engine needs, and is the template to copy when starting a new engine. It is the minimal IEngine reference (a code example), so it is developer-facing by nature and is not split into a separate impl file.

What it does

• process(in, out, size) — copies input to output, updating a decayed peak per channel.

• capabilities() = CapOwnDisplay — it fills its own DisplayModel (render() draws a level meter on both rings and lights the play indicators); silence collapses the meter.

• Everything else stays at the IEngine no-op defaults: no params, no MIDI, no pads, no CV, no storage, no transport use.

Why it exists

• It proves the IEngine contract holds for a non-looper with zero granular concepts.

• It is header-only and depends only on the contract + nocopy.h + <cmath> — the floor for engine size and coupling.

• Copy it (into src/engine/<name>/) to start a new engine, then override the methods you need and add an engine_select.h + Makefile branch (see README and docs/engine-layout.md).

Files

src/engine/passthrough/passthrough_engine.h; build via engine_select.h (SPK_ENGINE_PASSTHROUGH)

• Makefile (ENGINE=passthrough, make engine-passthrough). No .cpp.