Make spline EG drags audible live: held and new notes follow a dragged knot

Contours ride their own seqlock block into a double-buffered engine copy, and the swap step is smoothed. Each drag move publishes contours without a model write; release still commits and reloads.

src/core/instrument/CLAUDE.md

@@ -203,6 +203,14 @@ automatable parameters."* It rejects the precedent, not one instance of it.
 - **Two genuine level steps are smoothed, not ruled away**: a sustain level moved while the
   voice holds it, and a stage duration dialled to exactly zero mid-stage. Both are absorbed by
   the envelope's own bounded offset smoother.
+- **A drawn contour is live too, through a second block.** `LiveContours` (all three contours
+  as fixed-capacity arrays, ~9 KB) is published beside `LiveValues` rather than inside it, so a
+  knob publish, the automation merge and the block compare never carry it. Each `VoiceEngine`
+  double-buffers it, and `applyContoursToActive` (`voice_engine.h`) owns the rebind invariant
+  that makes the double buffer safe. A cursor `start()` left cleared (a Staged EG) stays
+  cleared: Staged|Spline is a note-on fact. A contour is positional, like the Trigger AHD,
+  so a swap is a level step at the read position, absorbed by a per-EG `StepSmoother` — one more
+  predicted branch per sample on a drawn EG, none on a Staged one.
 
 ### Non-goals / guardrails (instrument-specific; repo-wide invariants live in root CLAUDE.md)
 
@@ -255,8 +263,9 @@ consequences, each with one home:
 - **Normalized is what makes a contour length-independent.** There are no stored seconds to
   rescale, so a different-length capture replays the same shape proportionally.
 - **The contours sit on `PlayParams`/`PlaySeconds` directly, not inside the three envelope
-  structs.** Those are copied whole into the live block, which must stay trivially copyable
-  (`live_params.h`) — and a contour is not a live control anyway: like the velocity curves it
+  structs.** Those are copied whole into the live block, which must stay small and trivially
+  copyable (`live_params.h`). A contour still reaches the note already sounding — through its
+  OWN block (`live_contours.h`), see "Live parameter delivery". Only the Staged|Spline mode
   travels by reload.
 
 **Which shape a STAGED envelope takes is decided by the play mode, not by what it modulates:**
@@ -294,7 +303,7 @@ anything for a trigger shape.
 - The engine is the `sampler_core` CMake target over FOUR headers and TWO TUs, split on its own responsibility seam — cold note routing vs the hot per-sample render:
   - `play_params.h` — the value layer: `PlayParams`/`AdsrParams`/`TriggerParams`/`PitchEnvParams`/`FilterParams`, the per-instance mode enums (`ChannelMode`/`VoiceMode`/`MonoTrigger`), and `SampleData` (the ONE loaded capture: decoded PCM + root + loop + start + keyTrack + velocity curve + play params). Shared by the engine, the codec, and the editor, so a UI/codec TU reading a param struct doesn't recompile when a `Voice` member changes. `FilterParams` stores the filter module's own `FilterSettings` by value rather than a parallel copy of its normalized positions. Also the ONE home of the drawn-EG rule family — `splineActive`, `effectivePlayMode`, `enforceGateUnavailableWhileDrawn` and `effectiveLengthFraction` — all templated over the frames and seconds representations, so no consumer of either can re-read the raw fields instead.
   - `envelopes.h` — the three per-frame evaluators (`AdsrEnvelope` AHDSR, `AhdEnvelope` the sustain-less Attack/Hold/Decay, `PitchEnvelope` the AHD pitch offset), CONCRETE and fully header-inline. Never give them a common base or a virtual `tick()`: they are called per-voice-per-sample. Also home to `fitAhd`/`ahdLevelAt`, THE span split and shape every sustain-less envelope shares. A voice carries two of each shape — the amp's and the filter's — and its play mode picks which pair it reads. `AdsrEnvelope`/`PitchEnvelope` own `applyLive` (the φ-holding mid-stage rule), its fresh-note peer `snapLive`, and `StepSmoother`, the bounded offset that absorbs the level steps φ cannot cover; `AhdEnvelope` is POSITIONAL (evaluated at a source offset, not ticked), so it has no phase to hold and smooths a live reshape instead.
-  - `live_params.h` / `live_params.cpp` — the live-parameter block: `LiveValues` (the plain, trivially-copyable bundle the audio thread observes), the single-writer `LiveParams` seqlock that publishes it without a lock or a torn read, `foldLive` (the ONE derivation from `PlayParams` — every publisher goes through it so the two representations cannot drift), the block's FIELD-wise `operator==` (never a memcmp — the header owns why the padding makes a byte compare report differences that do not exist), and `ValueRamp`, the per-frame glide whose EXACT termination is what lets the filter's equality-compare cutoff skip re-engage. Links no engine: the block is a value the voice observes, not a thing the engine owns.
+  - `live_params.h` / `live_params.cpp` — the live-parameter block: `LiveValues` (the plain, trivially-copyable bundle the audio thread observes), the single-writer `LiveParams` seqlock that publishes it without a lock or a torn read, `foldLive` (the ONE derivation from `PlayParams` — every publisher goes through it so the two representations cannot drift), the block's FIELD-wise `operator==` (never a memcmp — the header owns why the padding makes a byte compare report differences that do not exist), and `ValueRamp`, the per-frame glide whose EXACT termination is what lets the filter's equality-compare cutoff skip re-engage. Links no engine: the block is a value the voice observes, not a thing the engine owns. The same library carries `live_contours.h`/`.cpp` (the drawn-contour block beside it, and `foldContours`, templated over both parameter representations like `splineActive`) and the header-only `seqlock.h`, the ONE seqlock both blocks publish through.
   - `voice.h` / `voice.cpp` — one voice. The per-SAMPLE render half (`advanceFrame` and everything it calls) is INLINE IN THE HEADER by RT constraint; the per-NOTE half (note-on setup incl. the Preserve ring prime, legato retune, gate-off, the off-thread shifter presize) is out of line in the TU. The voice owns its own `VoiceFilter` and filter envelope, run between the pitch stage and the amp multiply — see `engine/filter/CLAUDE.md`. **Documented ~600-line-ceiling exception** (root `CLAUDE.md` structural heuristic 1): `voice.h` sits over the ceiling because `advanceFrame`'s RT-inline constraint forbids the seam a split would need — a documented exception, not silent overshoot.
   - `voice_engine.h` / `voice_engine.cpp` — `VoiceEngine`: note routing, bounded-stealing allocation, user-parameterized voice count (1–32, default 16), `VoiceMode` Poly/Mono (last-note held-note stack, `MonoTrigger` Retrigger/Legato), two-tier panic (CC 123 = all-notes-off release, CC 120 = immediate hard-stop including Trigger one-shots), and the block render loops. Preview injects a synthetic note-on at the loaded capture's root note into the main `VoiceEngine` — no dedicated `PreviewCard`; preview obeys polyphony/mono/voice-stealing/envelopes.
 - `engine/loop/` — the sustain loop's ONE validity/clamp fold (`resolveLoop`) plus its pre-seam crossfade geometry and the editor's default handle span; see `engine/loop/CLAUDE.md`. The voice folds it once at note-on; the crossfade weight is header-inline because it rides the per-sample read.
@@ -396,7 +405,7 @@ anything for a trigger shape.
   envelope that was never running.
 - **A live edit leaves the snapshot's own `sample.play` stale, on purpose.** The block, not the
   snapshot, is the audio thread's source; a new voice latches the stale copy and is corrected by
-  `snapLive` before its first frame.
+  `snapLive` (and a drawn EG by `applyContours`' snap) before its first frame.
 - **`keyboard_strip`'s width-uniformity guarantee is client-pixel only.** Its test sweep
   covers client-pixel widths (including multiples standing in for larger client areas);
   nothing in the instrument implements `IPlugViewContentScaleSupport`, so host-side DPI

src/core/instrument/engine/CMakeLists.txt

@@ -26,10 +26,11 @@ add_subdirectory(filter)
 # marker layer. After filter: it links play_params' dependency set, which includes it.
 add_subdirectory(loop)
 
-# The live-parameter block: the value layer plus its publication, deliberately linking no
-# engine — the block is a plain value the voice observes, not a thing the engine owns.
+# The live-parameter block and the drawn-contour block beside it: value layers plus their
+# publication, deliberately linking no engine — a block is a plain value the voice observes, not
+# a thing the engine owns.
 reasampler_pure_library(live_params
-    SOURCES live_params.cpp
+    SOURCES live_params.cpp live_contours.cpp
     LINK PUBLIC peaks velocity_curve filter curve_law)
 reasampler_test(live_params LINK live_params)
 
@@ -52,6 +53,10 @@ reasampler_test(sampler_filter LINK sampler_core)
 # does to a voice that is already sounding, and what it must leave alone.
 reasampler_test(live_delivery LINK sampler_core)
 
+# The drawn-contour half of live delivery. Its own executable because it replaces the global
+# allocator to prove the audio-side swap allocates nothing, which must not leak into other tests.
+reasampler_test(live_contours LINK sampler_core)
+
 # The staged-envelope system across the same engine: per-segment curves, the sustain-less AHD
 # both mode shapes share, and the Trigger tail's terminal behaviour.
 reasampler_test(staged_envelopes LINK sampler_core)

src/core/instrument/engine/live_contours.cpp

@@ -0,0 +1,16 @@
+// live_contours.cpp — the VelocityCurve -> fixed-capacity copy the contour block carries.
+
+#include "core/instrument/engine/live_contours.h"
+
+namespace reasampler::instrument::engine {
+
+ContourPoints contourPoints(const VelocityCurve& curve) {
+    ContourPoints out{};
+    const std::vector<VelocityPoint>& pts = curve.points();
+    const std::size_t n = pts.size() < kMaxCurvePoints ? pts.size() : kMaxCurvePoints;
+    for (std::size_t i = 0; i < n; ++i) out.points[i] = pts[i];
+    out.count = static_cast<std::uint32_t>(n);
+    return out;
+}
+
+}  // namespace reasampler::instrument::engine

src/core/instrument/engine/live_contours.h

@@ -0,0 +1,50 @@
+#pragma once
+// live_contours.h — the three drawn EG contours as one fixed-capacity plain value, and the block
+// that publishes them to the audio thread so a sounding voice follows a contour edit. Separate
+// from LiveValues on purpose: at kMaxCurvePoints x 3 this is ~9 KB, and riding the live block
+// would put it on every knob publish, automation merge and field compare.
+
+#include <cstdint>
+#include <type_traits>
+
+#include "core/instrument/engine/seqlock.h"
+#include "core/instrument/engine/velocity_curve.h"
+
+namespace reasampler::instrument::engine {
+
+// One contour's points, copied out of its VelocityCurve so the audio thread can hold it without
+// a vector. `count` never exceeds kMaxCurvePoints — the curve's own ceiling — and the points
+// past it are ignored.
+struct ContourPoints {
+    std::uint32_t count = 0;
+    VelocityPoint points[kMaxCurvePoints];
+};
+
+// All three are carried whatever each envelope's mode: Staged|Spline is a note-on fact that
+// travels by reload, so the voice decides which cursors to rebind and the block stays one shape.
+struct LiveContours {
+    ContourPoints amp;
+    ContourPoints pitch;
+    ContourPoints filter;
+};
+
+static_assert(std::is_trivially_copyable_v<LiveContours>,
+              "the contour block is copied under a seqlock — it must stay a plain value");
+
+ContourPoints contourPoints(const VelocityCurve& curve);
+
+// Templated over the frames and seconds parameter representations for the same reason
+// splineActive is (play_params.h): both spell the three fields identically. Off the audio thread.
+template <class Play>
+LiveContours foldContours(const Play& p) {
+    LiveContours c{};
+    c.amp = contourPoints(p.ampSpline.contour);
+    c.pitch = contourPoints(p.pitchSpline.contour);
+    c.filter = contourPoints(p.filterSpline.contour);
+    return c;
+}
+
+// A class rather than an alias so play_params.h can forward-declare it.
+class LiveContourBlock : public Seqlock<LiveContours> {};
+
+}  // namespace reasampler::instrument::engine

src/core/instrument/engine/live_params.h

@@ -1,15 +1,15 @@
 #pragma once
 // live_params.h — the live playback-parameter block: the plain value bundle the audio thread
-// observes once per BLOCK, the single-writer seqlock that publishes it without a lock or a
-// torn read, the ONE fold from PlayParams that keeps the two representations in step, and the
+// observes once per BLOCK, its publication (the seqlock.h protocol) without a lock or a torn
+// read, the ONE fold from PlayParams that keeps the two representations in step, and the
 // per-frame ramp that keeps a block-rate step inaudible. Ownership belongs above every
 // instrument snapshot (see SampleData::live).
 
-#include <atomic>
 #include <cstdint>
 #include <type_traits>
 
 #include "core/instrument/engine/play_params.h"
+#include "core/instrument/engine/seqlock.h"
 
 namespace reasampler::instrument::engine {
 
@@ -96,64 +96,9 @@ inline bool operator!=(const LiveValues& a, const LiveValues& b) { return !(a ==
 // PlayParams — SampleData::keyTrack at the reload, InstrumentParams::keyTrack at a live commit.
 LiveValues foldLive(const PlayParams& params, double keyTrack);
 
-// Single-writer / single-reader seqlock. The writer publishes a whole block between an odd
-// and an even generation; the reader copies the block and re-checks the generation, retrying
-// a bounded number of times, so it can never act on a half-applied edit. Wait-free for the
-// reader: after the retry budget it reports "nothing new" and the caller keeps its last good
-// snapshot rather than spinning on the audio thread.
-//
-// SINGLE-WRITER IS THE CALLER'S JOB and is load-bearing: two concurrent writers can leave the
-// generation EVEN mid-write (A stores gen+1, B reads odd and stores gen+2) while both copy the
-// block, and a reader then accepts a torn block as coherent. Every publisher must serialize.
-//
-// The plain (non-atomic) block copied across the fences is the standard pragmatic seqlock:
-// the fences give correct ordering, but the concurrent read of a non-atomic object is a data
-// race under the C++ object model, so TSan/UBSan will report it. That report is expected, not
-// a defect — there is no clean lock-free standard-C++ alternative that keeps the block a plain
-// value the audio thread can copy in one shot.
-//
-// The writer interface deliberately assumes NO particular thread beyond single-writer, so a
-// host's own parameter-change queue (delivered on the audio thread with sample offsets) can
-// drive it later without a redesign.
-class LiveParams {
-public:
-    // A generation of 0 means "never published"; the first publish lands on 2.
-    void publish(const LiveValues& values) {
-        const std::uint32_t gen = seq_.load(std::memory_order_relaxed);
-        seq_.store(gen + 1, std::memory_order_relaxed);   // odd: a write is in progress
-        std::atomic_thread_fence(std::memory_order_release);
-        values_ = values;
-        std::atomic_thread_fence(std::memory_order_release);
-        // Skip 0 on wrap (~2^31 publishes): landing there would read as "never published" and
-        // stall every reader until the NEXT publish — a silent mode, unlike a loud one.
-        const std::uint32_t next = (gen + 2 == 0u) ? 2u : gen + 2;
-        seq_.store(next, std::memory_order_release);      // even: complete and coherent
-    }
-
-    // The last generation published, without copying the block — one relaxed load, so a reader
-    // that only needs "has anything moved" pays nothing for asking on a block where nothing has.
-    std::uint32_t generation() const { return seq_.load(std::memory_order_relaxed); }
-
-    // Copies the block into `out` and returns the generation actually observed, or 0 when
-    // nothing has been published yet or the retry budget ran out (in which case `out` may hold
-    // a torn copy and MUST be discarded — compare the return against 0 before using it).
-    std::uint32_t read(LiveValues& out, int maxAttempts = 4) const {
-        for (int attempt = 0; attempt < maxAttempts; ++attempt) {
-            const std::uint32_t before = seq_.load(std::memory_order_acquire);
-            if (before == 0) return 0;        // never published
-            if ((before & 1u) != 0u) continue;  // writer mid-update
-            std::atomic_thread_fence(std::memory_order_acquire);
-            out = values_;
-            std::atomic_thread_fence(std::memory_order_acquire);
-            if (seq_.load(std::memory_order_relaxed) == before) return before;
-        }
-        return 0;
-    }
-
-private:
-    std::atomic<std::uint32_t> seq_{0};
-    LiveValues values_{};
-};
+// The live-parameter block's publication — seqlock.h owns the protocol and its single-writer
+// contract. A class rather than an alias so play_params.h can forward-declare it.
+class LiveParams : public Seqlock<LiveValues> {};
 
 // Linear per-frame glide with EXACT termination: once the target is within one step the value
 // becomes the target itself. An asymptotic smoother would leave the value forever a hair off,

src/core/instrument/engine/play_params.h

@@ -15,7 +15,8 @@
 
 namespace reasampler {
 
-namespace instrument::engine { class LiveParams; }  // live_params.h; SampleData holds a pointer
+// SampleData holds a pointer to each (live_params.h, live_contours.h).
+namespace instrument::engine { class LiveParams; class LiveContourBlock; }
 
 using audio::AudioSample;
 using instrument::engine::VelocityCurve;
@@ -181,8 +182,8 @@ struct PlayParams {
     // The three drawn contours: the alternative to adsr/trigAhd, to pitchEnv.shape, and to
     // filter.env/trigEnv respectively. They sit HERE rather than inside the three envelope
     // structs because those are copied whole into the live block, which must stay trivially
-    // copyable (live_params.h) — and a contour is not a live control anyway: like the velocity
-    // curves it travels by reload.
+    // copyable (live_params.h). A contour reaches a sounding voice through its own block
+    // instead (live_contours.h); only its Staged|Spline mode travels by reload.
     SplineEnv      ampSpline;
     SplineEnv      pitchSpline;
     SplineEnv      filterSpline;
@@ -295,6 +296,9 @@ struct SampleData {
     // same knob as a live one. That is the desired behaviour — it is the note the user is
     // hearing. Do not "fix" it by moving ownership into the snapshot.
     const instrument::engine::LiveParams* live = nullptr;
+    // The drawn-contour peer of `live`, under the same ownership and null contract: null plays
+    // the contours in `play` for the note's whole life.
+    const instrument::engine::LiveContourBlock* liveContours = nullptr;
 
     // A framesR of a different length than frames is treated as absent — a malformed pair
     // never half-plays.

src/core/instrument/engine/seqlock.h

@@ -0,0 +1,76 @@
+#pragma once
+// seqlock.h — the lock-free publication every live block rides from its off-thread writer to
+// the audio thread: a single-writer seqlock over one plain value. Generic over that value so the
+// live-parameter block and the drawn-contour block share one implementation of the protocol.
+
+#include <atomic>
+#include <cstdint>
+#include <type_traits>
+
+namespace reasampler::instrument::engine {
+
+// Single-writer seqlock. The writer publishes a whole block between an odd and an even
+// generation; a reader copies the block and re-checks the generation, retrying a bounded number
+// of times, so it can never act on a half-applied edit. Wait-free for the reader: after the
+// retry budget it reports "nothing new" and the caller keeps its last good snapshot rather than
+// spinning on the audio thread. Reading does not mutate, so any number of readers may share one
+// block (the live and the draining engine both do).
+//
+// SINGLE-WRITER IS THE CALLER'S JOB and is load-bearing: two concurrent writers can leave the
+// generation EVEN mid-write (A stores gen+1, B reads odd and stores gen+2) while both copy the
+// block, and a reader then accepts a torn block as coherent. Every publisher must serialize.
+//
+// The plain (non-atomic) block copied across the fences is the standard pragmatic seqlock:
+// the fences give correct ordering, but the concurrent read of a non-atomic object is a data
+// race under the C++ object model, so TSan/UBSan will report it. That report is expected, not
+// a defect — there is no clean lock-free standard-C++ alternative that keeps the block a plain
+// value the audio thread can copy in one shot.
+//
+// The writer interface deliberately assumes NO particular thread beyond single-writer, so a
+// host's own parameter-change queue (delivered on the audio thread with sample offsets) can
+// drive it later without a redesign.
+template <class T>
+class Seqlock {
+    static_assert(std::is_trivially_copyable_v<T>,
+                  "the block is copied under a seqlock — it must stay a plain value");
+
+public:
+    // A generation of 0 means "never published"; the first publish lands on 2.
+    void publish(const T& values) {
+        const std::uint32_t gen = seq_.load(std::memory_order_relaxed);
+        seq_.store(gen + 1, std::memory_order_relaxed);   // odd: a write is in progress
+        std::atomic_thread_fence(std::memory_order_release);
+        values_ = values;
+        std::atomic_thread_fence(std::memory_order_release);
+        // Skip 0 on wrap (~2^31 publishes): landing there would read as "never published" and
+        // stall every reader until the NEXT publish — a silent mode, unlike a loud one.
+        const std::uint32_t next = (gen + 2 == 0u) ? 2u : gen + 2;
+        seq_.store(next, std::memory_order_release);      // even: complete and coherent
+    }
+
+    // The last generation published, without copying the block — one relaxed load, so a reader
+    // that only needs "has anything moved" pays nothing for asking on a block where nothing has.
+    std::uint32_t generation() const { return seq_.load(std::memory_order_relaxed); }
+
+    // Copies the block into `out` and returns the generation actually observed, or 0 when
+    // nothing has been published yet or the retry budget ran out (in which case `out` may hold
+    // a torn copy and MUST be discarded — compare the return against 0 before using it).
+    std::uint32_t read(T& out, int maxAttempts = 4) const {
+        for (int attempt = 0; attempt < maxAttempts; ++attempt) {
+            const std::uint32_t before = seq_.load(std::memory_order_acquire);
+            if (before == 0) return 0;        // never published
+            if ((before & 1u) != 0u) continue;  // writer mid-update
+            std::atomic_thread_fence(std::memory_order_acquire);
+            out = values_;
+            std::atomic_thread_fence(std::memory_order_acquire);
+            if (seq_.load(std::memory_order_relaxed) == before) return before;
+        }
+        return 0;
+    }
+
+private:
+    std::atomic<std::uint32_t> seq_{0};
+    T values_{};
+};
+
+}  // namespace reasampler::instrument::engine

src/core/instrument/engine/velocity_curve.h

@@ -224,17 +224,20 @@ private:
 // sample. A jump (a loop wrap, a fresh note) falls back to a binary search, <= 7 steps at the
 // 128-point ceiling.
 //
-// Holds a RAW POINTER into the bound curve's point array: the caller guarantees the curve
-// outlives the cursor. The voice binds against its SampleData, which has exactly that lifetime.
+// Holds a RAW POINTER into the bound point array: the caller guarantees the array outlives the
+// cursor, or rebinds it first. The voice binds against its SampleData, which has exactly that
+// lifetime, or against its engine's live contour copy, which the engine rebinds before reusing.
 class SplineCursor {
 public:
     // Binds `c` if it has an evaluable segment; a shorter curve leaves the cursor inactive so
     // the caller's `if (active())` skips the whole spline path.
-    void bind(const VelocityCurve& c) {
-        const std::vector<VelocityPoint>& pts = c.points();
-        if (pts.size() < 2) { clear(); return; }
-        pts_ = pts.data();
-        n_ = pts.size();
+    void bind(const VelocityCurve& c) { bind(c.points().data(), c.points().size()); }
+    // The same over a raw X-ordered array (live_contours.h's fixed-capacity copy). Rebinding a
+    // cursor mid-read is legal: it restarts the segment cache, and the next eval re-locates.
+    void bind(const VelocityPoint* pts, std::size_t n) {
+        if (pts == nullptr || n < 2) { clear(); return; }
+        pts_ = pts;
+        n_ = n;
         select(0);
     }
     void clear() { pts_ = nullptr; n_ = 0; }

src/core/instrument/engine/voice.cpp

@@ -109,6 +109,9 @@ void Voice::start(int note, int velocity, const SampleData& sample, bool declick
     } else {
         filterSplineCur_.clear();
     }
+    ampSplineSmooth_.clear();
+    pitchSplineSmooth_.clear();
+    filterSplineSmooth_.clear();
 
     // Amplitude envelope: Gate = AHDSR (all five fields read from play.adsr, resolved to
     // frames from stored seconds at load time); Trigger = the staged AHD over the % play span.
@@ -330,6 +333,29 @@ void Voice::applyLive(const instrument::engine::LiveValues& live, bool snap) {
                      rMorph_.moving() || rDrive_.moving();
 }
 
+namespace {
+
+// One EG's half of Voice::applyContours. `before` is read through the OLD binding, which is
+// still intact: the engine only overwrites a buffer no active voice is bound to.
+void rebindContour(SplineCursor& cursor, StepSmoother& smooth,
+                   const instrument::engine::ContourPoints& pts, double phase, bool snap) {
+    if (!cursor.active()) return;
+    const double before = snap ? 0.0 : cursor.eval(phase);
+    cursor.bind(pts.points, pts.count);
+    if (snap || !cursor.active()) return;
+    const double after = cursor.eval(phase);
+    if (after != before) smooth.absorb(before - after);
+}
+
+}  // namespace
+
+void Voice::applyContours(const instrument::engine::LiveContours& contours, bool snap) {
+    const double phase = splinePhase();
+    rebindContour(ampSplineCur_, ampSplineSmooth_, contours.amp, phase, snap);
+    rebindContour(pitchSplineCur_, pitchSplineSmooth_, contours.pitch, phase, snap);
+    rebindContour(filterSplineCur_, filterSplineSmooth_, contours.filter, phase, snap);
+}
+
 void Voice::retune(int note) {
     // Mono legato takeover: move the pitch, touch NOTHING else — the amplitude envelope keeps
     // running (no re-attack), the read head keeps its position, the shifter keeps its ring

src/core/instrument/engine/voice.h

@@ -15,6 +15,7 @@
 #include "core/instrument/engine/envelopes.h"
 #include "core/instrument/engine/filter/filter_params.h"
 #include "core/instrument/engine/filter/voice_filter.h"
+#include "core/instrument/engine/live_contours.h"
 #include "core/instrument/engine/live_params.h"
 #include "core/instrument/engine/loop/loop_span.h"
 #include "core/instrument/engine/pitch_shift.h"
@@ -179,6 +180,14 @@ public:
     // pitch ratio, and the decoded PCM stay latched at note-on.
     void applyLive(const instrument::engine::LiveValues& live, bool snap);
 
+    // Re-points every drawn EG this note is ALREADY reading at `contours` — a cursor start()
+    // left cleared stays cleared, because Staged|Spline is a note-on fact. Block-boundary only,
+    // like applyLive. Without `snap` the level step at the current read position is absorbed
+    // by that EG's StepSmoother, the same way a live reshape of a positional AHD is; with it
+    // (a voice that has rendered nothing) the new contour is taken outright. `contours` must
+    // outlive the binding or be rebound first — VoiceEngine's double buffer owns that.
+    void applyContours(const instrument::engine::LiveContours& contours, bool snap);
+
     // Pre-sizes this voice's Preserve pitch shifters (both channels) to `windowFrames`, off
     // the audio thread (allocates; also sizes the prime scratch buffer), so start() — which
     // runs inside process() — never allocates. <= 1 leaves the shifters pass-through.
@@ -268,6 +277,14 @@ private:
     // its opening value.
     double splinePhase() const { return readPos_ * splineScale_; }
 
+    // One drawn EG's value this frame: the contour at the read position plus whatever step a
+    // live contour swap left to absorb (applyContours). The smoother is inactive at rest, so a
+    // contour nobody is editing costs one predicted branch over the bare cursor read.
+    double readSpline(SplineCursor& cursor, StepSmoother& smooth) {
+        const double v = cursor.eval(splinePhase());
+        return smooth.active() ? v + smooth.advance() : v;
+    }
+
     // This frame's amplitude in [0,1] from the active envelope. Spline: the drawn contour read
     // at the normalized position (one cached-segment compare per frame). Gate: AHDSR ticks once
     // per output frame (envelope time is wall-clock, independent of read rate). Trigger: the AHD
@@ -284,7 +301,7 @@ private:
         if (ampSplineCur_.active() && playMode_ == PlayMode::Trigger) {
             // Early-free at a genuine permanent terminus (the spline analogue of a staged AHD's
             // finished()) — onFinalSegment()/segmentEndValue()'s own doc comments own the why.
-            amp = ampSplineCur_.eval(splinePhase());
+            amp = readSpline(ampSplineCur_, ampSplineSmooth_);
             if (amp == 0.0 && ampSplineCur_.onFinalSegment() &&
                 ampSplineCur_.segmentEndValue() == 0.0) {
                 amplitudeDone_ = true;
@@ -321,7 +338,7 @@ private:
         // the source-offset AHD in Trigger. playMode_ is fixed for the note's lifetime, so the
         // branch is perfectly predicted.
         const double envOut = filterSplineCur_.active()
-                                  ? filterSplineCur_.eval(splinePhase())
+                                  ? readSpline(filterSplineCur_, filterSplineSmooth_)
                                   : ((playMode_ == PlayMode::Gate)
                                          ? filterEnv_.tick()
                                          : filterAhd_.amplitudeAt(sourceOffset()));
@@ -514,9 +531,10 @@ private:
             seedTerminalDeclick();
         }
         const double gain = amp * velocityGain_;
-        const double pitchEnvSemis = pitchSplineCur_.active()
-                                         ? pitchSplineDepth_ * pitchSplineCur_.eval(splinePhase())
-                                         : pitchEnv_.tick();
+        const double pitchEnvSemis =
+            pitchSplineCur_.active()
+                ? pitchSplineDepth_ * readSpline(pitchSplineCur_, pitchSplineSmooth_)
+                : pitchEnv_.tick();
 
         // 2^(semis/12); when the envelope is off (semis exactly 0) this is 1.0 and skips the
         // pow entirely — no per-frame transcendental on the common path.
@@ -748,13 +766,18 @@ private:
     bool amplitudeDone_ = false; // set when the active amplitude envelope finished
 
     // The three drawn contours, bound at note-on to the loaded capture's own point arrays (the
-    // SampleData outlives the voice — same contract as sample_). A Staged EG leaves its cursor
-    // inactive, so a purely staged instrument's per-sample path gains three predicted branches
-    // and nothing else. splineScale_ is 1/frameCount, the readPos -> [0,1] map every contour
-    // shares; pitchSplineDepth_ is the pitch envelope's peak, zero while it is disabled.
+    // SampleData outlives the voice — same contract as sample_) and, when the engine observes a
+    // contour block, re-pointed at its engine's live copy (applyContours). A Staged EG leaves its
+    // cursor inactive, so a purely staged instrument's per-sample path gains three predicted
+    // branches and nothing else. splineScale_ is 1/frameCount, the readPos -> [0,1] map every
+    // contour shares; pitchSplineDepth_ is the pitch envelope's peak, zero while it is disabled.
+    // Each smoother holds the step a live contour swap left, in the contour's own y domain.
     SplineCursor ampSplineCur_;
     SplineCursor pitchSplineCur_;
     SplineCursor filterSplineCur_;
+    StepSmoother ampSplineSmooth_;
+    StepSmoother pitchSplineSmooth_;
+    StepSmoother filterSplineSmooth_;
     double splineScale_ = 0.0;
     double pitchSplineDepth_ = 0.0;
 

src/core/instrument/engine/voice_engine.cpp

@@ -52,7 +52,25 @@ void VoiceEngine::applyLiveToActive() {
     }
 }
 
+void VoiceEngine::applyContoursToActive() {
+    const instrument::engine::LiveContourBlock* block = sample_.liveContours;
+    if (block == nullptr) return;  // bare engine: every voice keeps its snapshot's contours
+    const std::uint32_t published = block->generation();
+    if (published == 0 || published == contourGeneration_) return;
+    const std::size_t back = contourFront_ ^ 1u;
+    const std::uint32_t observed = block->read(contours_[back]);
+    if (observed == 0) return;  // torn: the back buffer is discarded, retried next boundary
+    contourFront_ = back;
+    contourGeneration_ = observed;
+    haveContours_ = true;
+    for (Voice& voice : voices_) {
+        if (voice.active()) voice.applyContours(contours_[contourFront_], /*snap=*/false);
+    }
+}
+
 void VoiceEngine::startVoice(Voice& voice, int note, int velocity) {
+    // FIRST, so no sounding voice is left bound to a buffer the next read overwrites.
+    applyContoursToActive();
     refreshLive();
     // THE read of the note-on-latched commit class, and the only one: a published block outranks
     // the snapshot's own copy (a live edit deliberately leaves that stale), and applyLive below
@@ -65,6 +83,8 @@ void VoiceEngine::startVoice(Voice& voice, int note, int velocity) {
     voice.start(note, velocity, sample_, /*declickTakeover=*/takeoverDeclick_, rate, keyTrack,
                 lengthFraction);
     if (haveLive_) voice.applyLive(live_, /*snap=*/true);
+    // The snapshot's contours are stale after a live edit, exactly as its play params are.
+    if (haveContours_) voice.applyContours(contours_[contourFront_], /*snap=*/true);
     voice.setStartOrder(nextStartOrder_++);
 }
 
@@ -258,6 +278,7 @@ void VoiceEngine::render(AudioSample* out, std::size_t frameCount) {
     // audio thread never touches the heap.
     if (out == nullptr || frameCount == 0) return;
     applyLiveToActive();  // block boundary, once — never inside the frame loop
+    applyContoursToActive();
     for (Voice& voice : voices_) {
         if (!voice.active()) continue;
         for (std::size_t f = 0; f < frameCount; ++f) {
@@ -274,6 +295,7 @@ void VoiceEngine::render(AudioSample* left, AudioSample* right, std::size_t fram
     // discipline; only the per-frame call differs (renderFrameStereo vs renderFrame).
     if (left == nullptr || right == nullptr || frameCount == 0) return;
     applyLiveToActive();  // block boundary, once — never inside the frame loop
+    applyContoursToActive();
     for (Voice& voice : voices_) {
         if (!voice.active()) continue;
         for (std::size_t f = 0; f < frameCount; ++f) {

src/core/instrument/engine/voice_engine.h

@@ -10,6 +10,7 @@
 #include <vector>
 
 #include "core/audio/peaks.h"
+#include "core/instrument/engine/live_contours.h"
 #include "core/instrument/engine/live_params.h"
 #include "core/instrument/engine/play_params.h"
 #include "core/instrument/engine/voice.h"
@@ -57,6 +58,11 @@ public:
                 MonoTrigger monoTrigger = MonoTrigger::Retrigger,
                 bool takeoverDeclick = false);
 
+    // Voices may hold raw pointers into this engine's own contour buffers, so a copy would
+    // leave them reading the original's.
+    VoiceEngine(const VoiceEngine&) = delete;
+    VoiceEngine& operator=(const VoiceEngine&) = delete;
+
     // MIDI note-on. Allocates a free voice, or steals one per the policy above. Returns the
     // index of the voice used, or kNoVoice when nothing is playable (no decoded PCM, an
     // out-of-range note, or a Preserve note-on past the cap) — a defined no-play, not an error.
@@ -124,6 +130,22 @@ private:
     std::uint32_t liveGeneration_ = 0;  // last generation observed; 0 = none yet
     bool haveLive_ = false;
 
+    // --- Drawn-contour observation (live_contours.h) ---
+    // Reads a new contour generation, when one landed, into the buffer no voice is bound to and
+    // rebinds EVERY active voice to it before returning — which is what keeps the other buffer
+    // free for the next read. That invariant is why this, and never a bare read, is what both
+    // the block boundary and startVoice call.
+    void applyContoursToActive();
+
+    // Double-buffered because a voice's SplineCursor points INTO the buffer: reading over it
+    // would change a contour under a sounding voice without the rebind that absorbs the step,
+    // and a torn read (which the seqlock only reports after the fact) must land somewhere no
+    // voice reads. ~9 KB each, in the engine, so nothing on the audio thread allocates.
+    std::array<instrument::engine::LiveContours, 2> contours_{};
+    std::size_t contourFront_ = 0;
+    std::uint32_t contourGeneration_ = 0;  // last generation observed; 0 = none yet
+    bool haveContours_ = false;
+
     // Count of active Preserve-engine voices (for the Preserve cap). Rescanned per note-on
     // (cheap: bounded by maxVoices) rather than maintained as a running tally.
     std::size_t activePreserveVoices() const;