Note 13 — Smoke simulation at the leading edge (Stephen's directive, captured) (Incident Viewer Taos Engine App)

**Note** from Bead: Incident Viewer Taos Engine App · [canonical source](https://redfish.acequia.io/guerin/.agents/cbf8b027-e80c-4b9f-8eb3-91d40af072d5/2026-06-11/notes/13-smoke-simulation-directive.md) · session 2026-06-11 · discussion: Talk: Incident Viewer Taos Engine App

Verbatim intent (2026-06-12): *"we will have an option to simulate smoke at the leading edge that is intense and then trails off. in that case the smoke height can be frozen if we scrub and then reanimated when we land on a point."*

## Unpacking - **Source field:** the ToA raster gives the leading edge for free — the isochrone at `masterMs` is the fire front; the arrival-time **gradient** (∂arrival/∂xy) gives front speed and spread direction per pixel. - **Emission model:** smoke seeds at the current front, **intense at the edge, trailing off** behind it (intensity ∝ recency of arrival — the same `rat` the shader uses; young burn smokes hard, old burn smolders). - **The scrub-freeze semantics (the key interaction design):** smoke has its OWN simulation clock, decoupled from the master playhead: - **Scrubbing** (playhead moving discretely / fast): the smoke field — including plume **height** — FREEZES (no re-simulation per scrub frame; columns hold their shape, only the emission footprint snaps to the new front). - **Landing on a point** (playhead settles, or play resumes): the smoke sim **reanimates** from the frozen state — plumes grow/advect again. - This is the master-clock architecture extended: the video chase-syncs instantly, the smoke *relaxes* — different slaving disciplines per item kind. Worth generalizing later: every dynamic layer declares its scrub policy (snap / freeze-and-relax / interpolate). - **Likely first implementation:** billboard/particle columns seeded on front pixels (downsampled), height/opacity driven by sim age + local spread rate; wind direction could come from the ToA gradient (fronts run downwind) until real weather data joins the catalog.

## Status Captured only — queued behind the rotation fix (note 12) and Epic 2 proper. Prereqs in place: ToA decode + front extraction (shipped 827d259), master clock, per-frame render features.