Model head anchors and independent pose timing

docs/timing-model.md

@@ -0,0 +1,86 @@
+# Timing model
+
+The source footage, authored drawings, generated face motion, and stage placement
+have different frame decisions. They share a clock but do not share one kept-frame
+list. `timing-handoff.md` records earlier implementation notes.
+
+## Frame spaces
+
+- A source frame addresses a decoded image and its measured face data. Keep the
+  source cadence and, for variable-rate video, its presentation timestamp.
+- A timeline frame addresses authored keys in the clip or symbol's local space.
+- A stage frame is mapped through the symbol instance's offset and rate before
+  local frame decisions are read. Moving a placement does not rewrite its keys.
+
+The analyzed source poses remain dense. A lower picture rate or a skipped pose
+never removes source data or shortens audio.
+
+## Head placement
+
+Analysis fits each source frame's rigid landmarks into one common head-local
+space. Its inverse is the measured head transform, stored densely on `:head`.
+The head node has one optional anchor map:
+
+```clojure
+;; no :anchors                 free movement: read measured frame f at f
+:anchors {0 12}                ; one lock: use frame 12's transform throughout
+:anchors {0 12, 40 42}         ; keyed locks: switch to frame 42 at local frame 40
+```
+
+A key is `(local change frame -> measured source frame)`. Its value holds to the
+next key. The map chooses position, rotation and scale together. Frame zero must
+have a key when the map exists. The dense transform blocks remain intact, so
+editing anchors is a small document change and re-freezing can replace the
+measurements without losing the anchor choices.
+
+A source image used for tracing should be registered with that image's measured
+stabilizing transform, then the selected head transform, then the authored
+`:face` placement. This makes the photo and head-local vectors share the same
+orientation and position. Tracing-photo selection is a separate editor address;
+it does not choose the head anchor.
+
+The prototype stabilizes into the shot's mean rigid pose and uses an early
+closed-mouth frame for raster framing. Those are internal analysis and framing
+choices. The authored head-anchor map above controls which measured head pose is
+shown over each range. It is independent of plate drawing starts.
+
+## Performance poses
+
+Generated mouth, eye, and brow channels can be sampled at a lower picture rate
+without retiming authored keys. The normal rule picks the latest available pose
+at or before a picture-grid time. A future performance policy may add important
+closed-mouth poses and store manual keeps/drops separately from the rate's
+proposal. Related parts should share a selected pose by default: a mouth outline,
+interior, teeth and generated visibility must not disagree about its frame.
+
+## Stage placement
+
+A symbol placement has optional pose-cut tracks, separate from head anchors:
+
+```clojure
+:playback {:tracks {:mouth {0 12, 8 27}
+                    :eye-r {0 0, 4 6}}}
+```
+
+These maps are also `(local change frame -> source pose frame)`. They select which
+baked/generated shape pose appears on that placement. Before the first explicit
+cut, normal generated motion continues. Cuts hold, without interpolation, until
+the next cut. A `[:node id]` track can override one shape in a shared group.
+Authored cels, transforms, and audio remain on their normal local time.
+
+The current implementation reads retained frozen channels. A separate resolved
+geometry bake is not implemented; when added, it must preserve addressable
+candidate poses so stage cuts can still select any of them.
+
+## Ownership
+
+| Choice | Owner | Current state |
+| --- | --- | --- |
+| Source frames and timestamps | Footage/analysis | Constant-rate frame indexing exists; variable timestamps remain future work |
+| Head anchor map | `:head` node | Implemented, stored with the node |
+| Tracing cel starts and photo address | Authored cel | Separate future work |
+| Generated picture-rate proposal and closure protection | Roto clip/symbol | Generated-only picture sampling exists; closure protection remains future work |
+| Stage pose cuts | Symbol instance | Implemented, stored with the instance |
+
+Preview and export use the same resolver for generated picture sampling and stage
+cuts. Export still emits every timeline frame at the clip's audio rate.