perf(render): take-4 slice 2 — IsOpaque memcpy, integer bilinear, pump scratch pool

Take 4: pacing held (sync perfect) but avg render stayed 58.9ms — 2M managed row-walk iterations + a fresh 8.3MB buffer every tick (LOH churn into GC stalls inside the render measurement). - VideoFrame.IsOpaque: producer-contract flag (screen capture + webcam — DWM/MF fill alpha 255; media/chat/web/static NOT flagged). Full-cover aligned opaque backdrop = ONE Buffer.BlockCopy; black pre-fill skipped when it covers. - General BlitContent: integer 8.8 fixed-point bilinear + blend, row invariants hoisted, no per-pixel division/Math.Round. Within ±1 of the float reference (pixel tests allow ±2). Research per derivative-work rule: libyuv row/scale kernels (chromium.googlesource.com/libyuv/libyuv). - FramePump scratch pool (max 4, length-keyed, owned-by-reference): release strictly AFTER SubmitFrameAsync returns (stdin write copies); Contains-guard makes the transition Cut alias safe. - Removed the dead per-tick fromScene render + fromSceneProvider seam — BlendFrame consumes TransitionService.FromFrame captured at Start; the pump's render fed nothing. MainViewModel call site updated (signature). Bugs caught by the pixel probes pre-ship (recorded MyMistakes): first Bilinear double-shifted both stages (solid-255 sampled to ~1 -> general path drew nothing); sentinel 0xAB collided with an x+y pixel. FakeEncoder snapshots submitted frames (mirrors real copy semantics under recycling). ONE integration test: Pump_Pools_ScratchBuffers_Across_Frames_Without_ Stale_Pixels (alternating backdrops + repeated backing identity). Direct pin: Composite_OpaqueFullCover_Backdrop_CopiesEveryPixel_Into_Scratch. Clean build 0 warnings; 59/59 per-class + RealApp boot-smoke. take 5 verdict: expect avg render <= ~10ms, ~300/300 frames. Docs same commit: ai.md pipeline section, TASKS.md TASK 18, HANDOFF rewritten (Unit B spec + settled decisions queued).
2026-09-04 09:35:59 -07:00
parent 716a77f61a
commit 432adfdaef
12 changed files with 370 additions and 102 deletions
@@ -2,67 +2,84 @@

 ## Branch / Commit State

-**`main`** — take-3 starvation fix commits land locally (see `git log`); push only when the user says so.
-The take-3 audit itself happened 2026-09-02/03: user recorded take 3 (`Downloads\recordings\`, manually
-renamed `llamacasty-recording-3.mp4`), ffprobe + `startup.log` + an extracted frame did the diagnosis.
+**`main`** — Unit A (render starvation, slice 2) committed locally this session; push on the user's word
+(his pattern: says "push" explicitly). Slice 1 (deadline pacing + row-blit, take-3 fix) is already on
+`origin/main` as `716a77f`.

-## What the take-3 audit found
+## What just happened (2026-09-04, Unit A slice 2)

- **37s recording → 2.1s file, 127 frames @60fps** = ~17x time-lapse, audio truncated to match
-  (ffmpeg's rawvideo demuxer was blocked on the starved video input, so only ~2.3s of audio was consumed).
-  The user's "webcam at 6x, garbled" = the same file-level time-compression, most visible on the webcam.
- **The stage-timing seam paid for itself in one line**: `17/300 frames per 5s, avg render 258.1ms,
-  avg submit 1.5ms` — the encoder/pipe was healthy; `SceneCompositor.Render` was the whole bottleneck.
- **Two defects, both fixed (2026-09-03)** — research-first (OBS `libobs/media-io/video-io.c` deadline
-  pacing + libyuv row-blit pattern; cited in the commit and `MyMistakes.md` recipe):
-  1. `FramePump` slept the FULL interval after each render → period = render+interval. Now an absolute
-     `nextTick += intervalTicks` deadline; on overrun rebase (no catch-up burst).
-  2. Per-pixel float sampling + `Math.Round` blends over all 2.07M master px; `BlitOverlay` scanned the
-     full destination per overlay. Now: 1:1 aligned row-walk fast path (alpha branch, integer fixed-point
-     blend), `BlitOverlay` clipped to the intersection, backdrop-cover skips the black pre-fill.
-  3. **Webcam-in-output VERIFIED** from the take-3 frame (bottom-right, border + social bar render too).
- Test: `Pump_Paces_To_The_Deadline_Compensating_Render_Cost` (records the requested wait via the pacing
-  seam). Per-class runs: FramePumpTests 10/10, SceneCompositor+SceneGraph+SocialBar+StretchMath 20/20.
-  Clean build 0 warnings. **Landmine found the hard way:** a pacing fake returning
-  `Task.CompletedTask` synchronously runs the whole pump loop on `StartAsync`'s continuation and HANGS
-  the vstest run — fakes must genuinely await (`Task.Delay(d, ct)`) or yield.
- Minor logged on take-3 stop: `FramePump: encoder stop failed: No process is associated` — cosmetic
-  teardown race (process already exited before StopAsync's kill); follow-up only if it grows teeth.
+Take 4 verdict: pacing HELD (no stall cliff, sync intact — user confirmed game+webcam in-sync) but render
+stayed **58.9ms** (budget 16.7) → file still ~3.4x time-lapse. Root: the 2M-iteration managed row walk +
+8.3MB fresh buffer every tick. Shipped:

-## OPEN — do next, in order
+1. **`VideoFrame.IsOpaque`** producer-contract flag — set ONLY by `ScreenCaptureFrameSource` +
+   `MediaCaptureFrameSource` (DWM/MF fill alpha 255). Full-canvas aligned blit of an opaque frame =
+   ONE `Buffer.BlockCopy`; black pre-fill skipped when the backdrop covers.
+2. **Integer fixed-point bilinear** in `BlitContent` general path (webcam: round/mirror/scaled) — no
+   divisions, no `Math.Round`; ±1 of the float reference (tests allow ±2).
+3. **Pump scratch pool** — `AcquireScratch`/`ReleaseScratch` (max 4, length-keyed, owned-by-reference so
+   bake-cache/social-bar/static-art arrays can never be captured). Release strictly AFTER
+   `SubmitFrameAsync` returns (stdin write copies). `FakeEncoder` now snapshots frames like the real
+   encoder (holds would race legitimate recycling).
+4. **Dead code kill:** the per-tick `fromScene` render in the transition branch fed NOTHING
+   (`BlendFrame` uses `TransitionService.FromFrame` captured at `Start`) — removed with the
+   `fromSceneProvider` seam + `MainViewModel.cs` call site. Transition cost halves as a side effect.

-1. **Take 4 (user records ~30s, record-only)** → ffprobe the file (expect frames ≈ 60×seconds, duration
-   ≈ wall time) + read `startup.log` `FramePump stats` (expect `n/300`, `avg render` ≤ ~16ms). If n/300
-   climbs only to ~45-55: next slice is the 8.3MB/frame LOH allocation → pool the master buffer.
-2. **Then code, one integration test per change** (queue from before, unchanged): radio pills /
-   record-OR-stream enforcement (TASK 18 ruling; `BeginGoLive(alsoRecord)` dies) → top bar Option A
-   **pending user approval** → empty-state ruling pending (disabled / rehearsal / default-record) →
-   SYNC slider placement ruling pending.
-3. **Un-asked questions** (don't nag; answers ready): the recorder said the desktop capture "didn't last
-   long enough to tell quality" — take 4 fixes that; "other issues" from takes 1/2 never listed.
+Bugs caught by the pixel probes before shipping (see MyMistakes take-4 follow-ups): first `Bilinear`
+double-shifted (both stages scaled → solid-255 sampled to ~1 → general path drew nothing); sentinel
+0xAB collided with a legitimate `x+y` value; pacing-fake synchronous completion hangs vstest (known,
+re-trod).

-## State of the app
+**Verification:** clean build 0 warnings (both projects); per-class vstest 59/59 (FramePump 11 incl. the
+new pooling test, SceneCompositor incl. NEW `Composite_OpaqueFullCover...`, SceneGraph, SocialBar, StretchMath,
+Camera/ScreenCapture/MediaVideoSource producers, WebcamOutputKey, SessionTeardown) + SourceNaming
+RealApp boot-smoke. Scope-check passed.

-Boots clean. Recording pipeline: render starvation fixed pending take-4 confirmation; webcam + social bar
-confirmed IN the output. ffmpeg cached at `%APPDATA%\ytLlive\tools\` (month-end pin). Test suite per-class
-green; do NOT run full-suite vstest (WASAPI teardown hang, pre-existing).
+## OPEN — next, in order
+
+1. **Take 5 (user, ~30s record-only):** read `%APPDATA%\ytLlive\startup.log` — expect
+   `FramePump stats: ≈300/300 frames per 5s, avg render ≤ ~10ms, avg submit ~1ms` + ffprobe frames ≈
+   60×duration. If render is under budget but `n/300` still short → suspect capture-side (the 1440p
+   `DownscaleBgra` per-frame alloc loop on the capture thread) or residual GC — next slice.
+   Also for the user to eyeball: webcam now at true 60fps, audio still in sync.
+2. **Unit B — the top bar + session logic (user spec 2026-09-04, decisions settled):**
+   - Two-line top bar. Line 1: center = REC + **LIVE** pills (text renamed from ON-AIR; pills become
+     mutually-exclusive RADIOS — record-OR-stream ruling), right = avatar + **Login/Logout** button
+     (no account status light). Line 2: centered primary **Start** (grayed while NO pill armed —
+     INVERTS the 2026-09-01 "unarmed Start records" rule; fix the map when landing) that becomes the
+     Stop/End button while active.
+   - Avatar right-click → **Change Account** (creator: "standard google thing ... on a portrait
+     right-click"). Login = `SignInCommand` direct (context menu on Start dies; "Choose Record Folder"
+     lives in gear → App Settings only).
+   - LIVE pill stays login-gated (`CanToggleOnAir` exists ✓ 3a.viii).
+   - REC+Start → `Microsoft.Win32.SaveFileDialog` (InitialDirectory = settings folder, default name
+     `ty-…-0000.mp4`, NATIVE overwrite prompt covers exists/validate, Enter confirms). Cancel →
+     abort + disarm pill (lit pill with no session is a lie). Up-front naming RETIRES the stop-time
+     rename modal (assumption stated; user's dialog answer was about Go-Live confirmation).
+   - LIVE+Start → **Go-Live dialog stays** as preflight: prefilled from Text-drawer `Broadcast.*`,
+     unfilled fields visibly prompted, explicit confirm → `PrepareAndStartLiveAsync` (user: "going
+     live is scary — confirmation allows back-out + testing up to go-live").
+   - Bottom-bar metrics init/maintain: `ResetHealth` + `HealthUpdated` exist — verify on take 5/6.
+   - F6 "start/end" hotkey routes through `HandleHotkey` — check it honors the new grayed-Start gate.
+   - ONE integration test (hermetic): pills↔button state machine + record-path seam (an
+     `internal static Func<SaveFileDialog-ish prompt>` override seam mirroring `RegistrarOverride`
+     — never pop real dialogs in tests).
+3. **Follow-ups recorded (don't fix opportunistically):** vertical-tier `BilinearScale` per-frame
+   alloc; cosmetic `FramePump: encoder stop failed: No process is associated` double-stop race;
+   1440p capture downscale alloc.

 ## Landmines

 - testhost shares startup.log with the app — filter by time when triaging.
 - Stale testhost/exe locks the DLL (MSB3027): `taskkill /F /IM testhost.exe` / `ytLive.exe` first.
- Do NOT run full-suite vstest (hangs); do NOT claim suite totals — per-class only. verify.sh's step 2 IS
-  the full-suite hang — flow: clean build + per-class vstest + scope-check.
- Windows binaries (ffprobe/ffmpeg under `%APPDATA%\ytLlive\tools\`) need WINDOWS paths
-  (`C:\Users\...`), never `/mnt/c/...` — the mount path reads as "No such file or directory".
- Real-`MainWindow` tests MUST use `LayoutPathOverride` + temp DB; `VolumePushOverride` seam exists so
-  volume-slider tests never touch the machine's speakers.
- `AudioPipelineTests` was the "known failure" graveyard — a new failure there means a live-loop
-  regression; read the logged stack (throttled, with stack).
- Pacing-seam fakes must yield/await — synchronous completion runs the pump loop inline (see above).
+- Do NOT run full-suite vstest (WASAPI hang, pre-existing); flow = clean build + per-class + scope-check.
+- Pacing-seam fakes MUST await/yield (sync-completed task → pump runs inline on StartAsync → hang).
+- `FakeEncoder` snapshots submitted frames — keep any new fake encoder honest about buffer recycling.
+- Multi-stage fixed-point: shift only at the end (MyMistakes 2026-09-04).
+- Real-`MainWindow` tests: `LayoutPathOverride` + temp DB mandatory; `VolumePushOverride` for volume.
+- ffmpeg: month-end pinned build; `Startup.log` "Recording saved:" lines show the real final path.

 ## @ User note

-He does not want tangents acknowledged twice (rename question = noise; the ask was ALWAYS the recording).
-Keep responses SHORT; work the queue; one integration test per change; committing is expected, pushing is
-his word.
+Recording fix FIRST (his order), UX queue right behind — spec + settled decisions above, don't re-ask.
+Keep responses SHORT; one integration test per change; commit every unit; push on his word only.
@@ -75,6 +75,23 @@ Both halves were solved by OBS/libyuv long ago; do not re-derive:
   yield for exactly this reason). Assert the REQUESTED wait (< interval with a
   ≥cost-ms fake render) — never wall-clock rate, which flakes on loaded machines.

+   **Take-4 follow-ups (2026-09-04) — the symptom needed a second pass, so cite again:**
+   render was still 58.9ms after slice 1. Slice 2 (buffer pool + opaque-row memcpy +
+   integer bilinear) followed the same libyuv research
+   (https://chromium.googlesource.com/libyuv/libyuv/ — `row.cc`/`scale.cc` keep both
+   interpolation stages in ONE fixed-point scale; rounding constant only at the end).
+   My first `Bilinear` shifted stage 1 back to 8-bit AND shifted the final result >>16 —
+   double scaling turned solid-255 samples into ~1, i.e. the "fixed" general path drew
+   NOTHING (green webcam silently vanished from output; the pixel probes caught what
+   the eye in a 2x time-lapse would not). **Rule: multi-stage fixed point shifts only
+   at the end; verify against a uniform-255 sample before believing it.** Second trap:
+   a stale-byte sentinel test whose source pattern can generate the sentinel value
+   itself (0xAB was a legitimate `x+y` pixel) — pick the sentinel coprime/out-of-range
+   to every channel formula (0xFD: odd, not ×4, above the R max). Third: a fake encoder
+   that HOLDS submitted frames now must snapshot them (`Clone`) once the producer
+   legitimately recycles buffers — mirror the real consumer's copy semantics in the fake.
+
+
 ---

 ## Splitting a large file into partials — NEVER `awk … > SRC` while awking SRC
@@ -29,7 +29,8 @@ public sealed class SceneCompositor
        VideoFrame? socialBarFrame = null,
        int socialBarTop = 0,
        VideoFrame? staticBase = null,
-        int split = -1)
+        int split = -1,
+        byte[]? scratch = null)
    {
        if (scene == null) throw new ArgumentNullException(nameof(scene));
        if (frameFor == null) throw new ArgumentNullException(nameof(frameFor));
@@ -41,7 +42,9 @@ public sealed class SceneCompositor

        var cropW = options.SourceRectWidth;
        var cropH = options.SourceRectHeight;
-        var buffer = new byte[cropW * cropH * 4];
+        var buffer = scratch != null && scratch.Length == cropW * cropH * 4
+            ? scratch
+            : new byte[cropW * cropH * 4];

        // staticBase != null: the caller pre-baked all static layers below `split`.
        // We copy it (never mutate the cached base) and composite elements[split..] on top.
@@ -200,13 +203,16 @@ public sealed class SceneCompositor
        CompositorOptions options,
        VideoFrame? socialBarFrame = null,
        int socialBarTop = 0,
-        VideoFrame? flashFrame = null)
+        VideoFrame? flashFrame = null,
+        byte[]? scratch = null)
    {
        var cropW = options.SourceRectWidth;
        var cropH = options.SourceRectHeight;
-        // We must not mutate the cached base: copy it into a fresh buffer.
-        var buffer = new byte[baseFrame.BgraPixels.Length];
-        Buffer.BlockCopy(baseFrame.BgraPixels, 0, buffer, 0, buffer.Length);
+        // We must not mutate the cached base: copy it into a fresh (or pooled) buffer.
+        var buffer = scratch != null && scratch.Length == baseFrame.BgraPixels.Length
+            ? scratch
+            : new byte[baseFrame.BgraPixels.Length];
+        Buffer.BlockCopy(baseFrame.BgraPixels, 0, buffer, 0, baseFrame.BgraPixels.Length);

        for (var i = split; i < scene.Elements.Count; i++)
        {
@@ -317,13 +323,34 @@ public sealed class SceneCompositor
        var y1 = Math.Min(dstH - 1, (int)Math.Ceiling(ey + eh));
        if (x0 > x1 || y0 > y1) return;

+        // Integer fixed-point sampling + blend (libyuv's shape): the float
+        // SampleBgra + BlendPixel pair cost ~8 Math.Round + ~8 divisions per
+        // pixel and dominated this path in takes 3–4. Row invariants hoisted,
+        // the y-range check moved out of the x loop, and the dst alpha is the
+        // same opaque-master invariant as the row helpers (every write = 255).
+        var w = src.Width;
+        var hgt = src.Height;
+        var sxMax = w - 1;
+        var syMax = hgt - 1;
+        var invScale = 1f / scale;
+        var op = (int)(Math.Clamp(opacity, 0f, 1f) * 256);
+        if (op <= 0) return;
+        var s = src.BgraPixels;
+
        for (var y = y0; y <= y1; y++)
        {
-            for (var x = x0; x <= x1; x++)
+            var py = y - ey;
+            if (py < oy || py > oy + drawnH) continue;
+            var syRaw = Math.Clamp((py - oy) * invScale, 0f, syMax);
+            var syi = (int)syRaw;
+            var sy1i = Math.Min(syi + 1, syMax);
+            var fy8 = (int)((syRaw - syi) * 256);
+            var fyInv = 256 - fy8;
+            var di = (y * dstW + x0) * 4;
+            for (var x = x0; x <= x1; x++, di += 4)
            {
                var px = x - ex; // element space
-                var py = y - ey;
-                if (px < ox || px > ox + drawnW || py < oy || py > oy + drawnH) continue;
+                if (px < ox || px > ox + drawnW) continue;
                if (isRound)
                {
                    var dx = px - cx;
@@ -331,15 +358,56 @@ public sealed class SceneCompositor
                    if (dx * dx + dy * dy > radius * radius) continue;
                }

-                var sx = (px - ox) / scale;
-                var sy = (py - oy) / scale;
-                if (isMirror) sx = src.Width - 1 - sx;
-                var sample = StretchMath.SampleBgra(src.BgraPixels, src.Width, src.Height, sx, sy);
-                BlendPixel(dst, (y * dstW + x) * 4, sample, opacity);
+                var sxRaw = (px - ox) * invScale;
+                if (isMirror) sxRaw = sxMax - sxRaw;
+                sxRaw = Math.Clamp(sxRaw, 0f, sxMax);
+                var sxi = (int)sxRaw;
+                var sx1i = Math.Min(sxi + 1, sxMax);
+                var fx8 = (int)((sxRaw - sxi) * 256);
+                var fxInv = 256 - fx8;
+
+                var p00 = (syi * w + sxi) * 4;
+                var p10 = (syi * w + sx1i) * 4;
+                var p01 = (sy1i * w + sxi) * 4;
+                var p11 = (sy1i * w + sx1i) * 4;
+                var sb = Bilinear(s[p00], s[p10], s[p01], s[p11], fx8, fxInv, fy8, fyInv);
+                var sg = Bilinear(s[p00 + 1], s[p10 + 1], s[p01 + 1], s[p11 + 1], fx8, fxInv, fy8, fyInv);
+                var sr = Bilinear(s[p00 + 2], s[p10 + 2], s[p01 + 2], s[p11 + 2], fx8, fxInv, fy8, fyInv);
+                var saFull = Bilinear(s[p00 + 3], s[p10 + 3], s[p01 + 3], s[p11 + 3], fx8, fxInv, fy8, fyInv);
+
+                var sa = saFull * op >> 8;
+                if (sa >= 255)
+                {
+                    dst[di] = (byte)sb;
+                    dst[di + 1] = (byte)sg;
+                    dst[di + 2] = (byte)sr;
+                    dst[di + 3] = 255;
+                }
+                else if (sa > 0)
+                {
+                    var inv = 255 - sa;
+                    dst[di] = (byte)((sb * sa + dst[di] * inv + 127) / 255);
+                    dst[di + 1] = (byte)((sg * sa + dst[di + 1] * inv + 127) / 255);
+                    dst[di + 2] = (byte)((sr * sa + dst[di + 2] * inv + 127) / 255);
+                    dst[di + 3] = 255;
+                }
            }
        }
    }

+    /// <summary>Two-stage bilinear in 8.8 fixed point; result within ±1 of the
+    /// float StretchMath.SampleBgra (well inside every pixel test's ±2 tolerance).
+    /// The first stage must NOT shift back to 8 bits — shifting twice collapsed
+    /// every solid-255 sample to 1 (green overwrites silently degraded to no-ops;
+    /// caught by the SceneCompositor/SceneGraph pixel probes 2026-09-04).</summary>
+    private static int Bilinear(int tl, int tr, int bl, int br, int fx8, int fxInv, int fy8, int fyInv)
+    {
+        // Both stages in one 16.8 scale: top/bot ≤ 65280, ×256 + round ≤ 16.8M — int-safe.
+        var top = tl * fxInv + tr * fx8;
+        var bot = bl * fxInv + br * fx8;
+        return (top * fyInv + bot * fy8 + 32768) >> 16;
+    }
+
    /// <summary>Row walk for a 1:1 aligned blit at integer offset (ex, ey): per-pixel
    /// branch on source alpha — opaque rows copy straight, transparent pixels are
    /// skipped, partial alpha blends in integer fixed-point (libyuv's pattern). The
@@ -357,17 +425,38 @@ public sealed class SceneCompositor
        var y1 = Math.Min(dstH, ey + src.Height);
        if (x0 >= x1 || y0 >= y1) return;

-        var rowLen = x1 - x0;
-        for (var y = y0; y < y1; y++)
+        // The take-4 fix: a full-canvas opaque capture (the Live backdrop) is ONE
+        // memcpy, not 2.07M managed loop iterations — the row walk below still cost
+        // ~58ms/frame dominated by bounds-check arithmetic and GC of the per-pixel
+        // loop's slowness, i.e. the same starvation shape as take 3, one floor lower.
+        if (op == 256 && src.IsOpaque && x0 == 0 && y0 == 0 && x1 == dstW && y1 == dstH)
        {
-            var si = ((y - ey) * src.Width + (x0 - ex)) * 4;
-            var di = (y * dstW + x0) * 4;
-            if (op == 256) BlendRowOpaque(dst, di, src.BgraPixels, si, rowLen);
-            else BlendRowWeighted(dst, di, src.BgraPixels, si, rowLen, op);
+            Buffer.BlockCopy(src.BgraPixels, 0, dst, 0, dstW * dstH * 4);
+            return;
+        }
+
+        var d = dst.AsSpan();
+        var s = src.BgraPixels.AsSpan();
+        var rowLen = x1 - x0;
+        if (op == 256)
+        {
+            for (var y = y0; y < y1; y++)
+            {
+                var si = ((y - ey) * src.Width + (x0 - ex)) * 4;
+                BlendRowOpaque(d, (y * dstW + x0) * 4, s, si, rowLen);
+            }
+        }
+        else
+        {
+            for (var y = y0; y < y1; y++)
+            {
+                var si = ((y - ey) * src.Width + (x0 - ex)) * 4;
+                BlendRowWeighted(d, (y * dstW + x0) * 4, s, si, rowLen, op);
+            }
        }
    }

-    private static void BlendRowOpaque(byte[] dst, int di, byte[] src, int si, int count)
+    private static void BlendRowOpaque(Span<byte> dst, int di, ReadOnlySpan<byte> src, int si, int count)
    {
        for (var n = 0; n < count; n++)
        {
@@ -392,7 +481,7 @@ public sealed class SceneCompositor
        }
    }

-    private static void BlendRowWeighted(byte[] dst, int di, byte[] src, int si, int count, int op)
+    private static void BlendRowWeighted(Span<byte> dst, int di, ReadOnlySpan<byte> src, int si, int count, int op)
    {
        for (var n = 0; n < count; n++)
        {
@@ -30,10 +30,40 @@ public sealed class FramePump : IDisposable
    private readonly Func<TimeSpan, CancellationToken, Task> _pacingDelay;
    private readonly Func<(VideoFrame? Frame, SocialBarPosition Position)>? _socialBar;
    private readonly TransitionService? _transition;
-    private readonly Func<Scene?>? _fromSceneProvider;
    private readonly SceneGraph? _sceneGraph;
    private readonly SceneCompositor _compositor = new();

+    // Master-buffer scratch pool (take-4 starvation fix, slice 2 of 2): a fresh
+    // 8.3MB byte[] every tick is ~500MB/s of LOH churn — GC stalls masquerading
+    // as render cost. The pump pools ONLY buffers it handed out (reference-equality
+    // set), so bake-cache / social-bar / static-cache frames are never touched;
+    // SubmitFrameAsync copies the bytes to the encoder's stdin before returning,
+    // so recycling after submit is safe (MyMistakes recipe).
+    private readonly HashSet<byte[]> _ownedScratch = new(ReferenceEqualityComparer.Instance);
+    private readonly List<byte[]> _freeScratch = new();
+    private const int MaxScratchPooled = 4;
+
+    private byte[] AcquireScratch(int size)
+    {
+        for (var i = _freeScratch.Count - 1; i >= 0; i--)
+        {
+            if (_freeScratch[i].Length != size) continue;
+            var buffer = _freeScratch[i];
+            _freeScratch.RemoveAt(i);
+            return buffer;
+        }
+        var fresh = new byte[size];
+        _ownedScratch.Add(fresh);
+        return fresh;
+    }
+
+    private void ReleaseScratch(byte[]? buffer)
+    {
+        if (buffer == null || !_ownedScratch.Contains(buffer)) return;
+        if (_freeScratch.Count >= MaxScratchPooled || _freeScratch.Contains(buffer)) return;
+        _freeScratch.Add(buffer);
+    }
+
    private readonly object _gate = new();
    private IFfmpegEncoder? _encoder;
    private CancellationTokenSource? _cts;
@@ -56,7 +86,6 @@ public sealed class FramePump : IDisposable
        Func<TimeSpan, CancellationToken, Task>? pacingDelay = null,
        Func<(VideoFrame? Frame, SocialBarPosition Position)>? socialBar = null,
        TransitionService? transition = null,
-        Func<Scene?>? fromSceneProvider = null,
        SceneGraph? sceneGraph = null)
    {
        _sceneProvider = sceneProvider ?? throw new ArgumentNullException(nameof(sceneProvider));
@@ -68,7 +97,6 @@ public sealed class FramePump : IDisposable
        _pacingDelay = pacingDelay ?? ((delay, ct) => Task.Delay(delay, ct));
        _socialBar = socialBar;
        _transition = transition;
-        _fromSceneProvider = fromSceneProvider;
        _sceneGraph = sceneGraph;
    }

@@ -211,6 +239,8 @@ public sealed class FramePump : IDisposable
        {
            _log?.Invoke($"FramePump: dispose failed: {ex.Message}");
        }
+        _freeScratch.Clear();
+        _ownedScratch.Clear();
    }

    private async Task PumpAsync(EncoderOptions options, CancellationToken ct)
@@ -273,23 +303,25 @@ public sealed class FramePump : IDisposable
                    }

                    VideoFrame frame;
+                    var scratchSize = compositorOptions.SourceRectWidth
+                        * compositorOptions.SourceRectHeight * 4;
+                    // The transition "from" frame lives in TransitionService.FromFrame
+                    // (captured at Start by the VM). The old per-tick fromScene render
+                    // here was dead weight — a full extra scene composite every
+                    // transition tick that BlendFrame never read; removed with the
+                    // pooling change because its buffer's only consumer was its own release.
                    renderSw.Restart();
-                    if (_transition is { Active: true } transition && _fromSceneProvider != null)
+                    var scratch = AcquireScratch(scratchSize);
+                    frame = RenderScene(scene, compositorOptions, socialBarFrame, socialBarTop, scratch);
+                    if (_transition is { Active: true } transition)
                    {
-                        var fromScene = _fromSceneProvider();
-                        var fromFrame = fromScene != null
-                            ? RenderScene(fromScene, compositorOptions, socialBarFrame, socialBarTop)
-                            : RenderScene(scene, compositorOptions, socialBarFrame, socialBarTop);
-                        var toFrame = RenderScene(scene, compositorOptions, socialBarFrame, socialBarTop);
-                        frame = transition.BlendFrame(toFrame);
+                        frame = transition.BlendFrame(frame);
                        transition.Tick(lastTick.Elapsed.TotalMilliseconds);
-                        lastTick.Restart();
-                    }
-                    else
-                    {
-                        frame = RenderScene(scene, compositorOptions, socialBarFrame, socialBarTop);
-                        lastTick.Restart();
                    }
+                    // Restarted EVERY frame (transition or not) — the old per-frame reset
+                    // is what stops a transition that begins after idle from inheriting
+                    // a giant ElapsedMs and completing instantly on its first tick.
+                    lastTick.Restart();
                    renderSw.Stop();
                    renderTicks += renderSw.ElapsedTicks;

@@ -300,6 +332,11 @@ public sealed class FramePump : IDisposable
                    await encoder.SubmitFrameAsync(frame, ct);
                    submitSw.Stop();
                    submitTicks += submitSw.ElapsedTicks;
+                    // Submit copied the bytes — everything from this tick is recyclable.
+                    // Release AFTER submit, and the free-list Contains guard makes the
+                    // Cut path (BlendFrame returns toFrame itself, aliasing scratch) safe.
+                    ReleaseScratch(frame.BgraPixels);
+                    ReleaseScratch(scratch);
                    statFrames++;
                    ReportStats();

@@ -349,15 +386,18 @@ public sealed class FramePump : IDisposable
    /// <summary>Render a scene, using the baked-crust optimization when a <see cref="SceneGraph"/>
    /// is wired in: bake/cache the static layers below the split point, then composite the
    /// dynamic/above-split layers per frame. Without a SceneGraph, falls back to a full render
-    /// (identical output — see SceneCompositorTests).</summary>
+    /// (identical output — see SceneCompositorTests). <paramref name="scratch"/> is a pooled
+    /// master buffer when supplied; the fully-static path returns the bake-cache frame itself
+    /// (never pooled — the release side checks owned-by-reference).</summary>
    private VideoFrame RenderScene(
        Scene scene,
        CompositorOptions options,
        VideoFrame? socialBarFrame,
-        int socialBarTop)
+        int socialBarTop,
+        byte[]? scratch = null)
    {
        if (_sceneGraph == null)
-            return _compositor.Render(scene, _frameResolver, null, options, socialBarFrame, socialBarTop);
+            return _compositor.Render(scene, _frameResolver, null, options, socialBarFrame, socialBarTop, scratch: scratch);

        var split = _sceneGraph.GetSplitPoint(scene);
        if (split == scene.Elements.Count)
@@ -372,11 +412,11 @@ public sealed class FramePump : IDisposable
        if (baseFrame != null)
        {
            return SceneCompositor.CompositeLayers(
-                baseFrame, scene, split, _frameResolver, options, socialBarFrame, socialBarTop);
+                baseFrame, scene, split, _frameResolver, options, socialBarFrame, socialBarTop, scratch: scratch);
        }

        // No static base (first layer is dynamic or empty scene) — full render.
-        return _compositor.Render(scene, _frameResolver, null, options, socialBarFrame, socialBarTop);
+        return _compositor.Render(scene, _frameResolver, null, options, socialBarFrame, socialBarTop, scratch: scratch);
    }

    private void OnProcessFailed(object? sender, string message)
@@ -227,7 +227,8 @@ public sealed class MediaCaptureFrameSource : ICameraFrameSource
            {
                var pixels = new byte[capacity];
                Marshal.Copy(pixelsPtr, pixels, 0, (int)capacity);
-                FrameAvailable?.Invoke(new VideoFrame(bitmap.PixelWidth, bitmap.PixelHeight, pixels));
+                // MediaCapture video carries no alpha plane — the OS fills 255.
+                FrameAvailable?.Invoke(new VideoFrame(bitmap.PixelWidth, bitmap.PixelHeight, pixels) { IsOpaque = true });
            }
        }
        catch (Exception ex)
@@ -177,12 +177,13 @@ public sealed class ScreenCaptureFrameSource : IScreenCaptureSource
            var scale = Math.Min(MaxBackgroundWidth / (double)sw, MaxBackgroundHeight / (double)sh);
            var dw = Math.Max(1, (int)(sw * scale));
            var dh = Math.Max(1, (int)(sh * scale));
-            return new VideoFrame(dw, dh, DownscaleBgra(data, sw, sh, srcStride, dw, dh));
+            // DWM delivers an opaque surface (alpha 255); bilinear keeps it 255.
+            return new VideoFrame(dw, dh, DownscaleBgra(data, sw, sh, srcStride, dw, dh)) { IsOpaque = true };
        }

        var pixels = new byte[count];
        Marshal.Copy(data, pixels, 0, pixels.Length);
-        return new VideoFrame(sw, sh, pixels);
+        return new VideoFrame(sw, sh, pixels) { IsOpaque = true };
    }

    // Bilinear downscale to the master frame. Reads each source row pair through
@@ -13,6 +13,13 @@ public sealed class VideoFrame
    public byte[] BgraPixels { get; }
    public int Stride => Width * 4;

+    /// <summary>Producer contract: every alpha byte in <see cref="BgraPixels"/> is 255
+    /// (DWM capture surfaces and MediaCapture video carry no alpha — the OS fills 255).
+    /// Lets the compositor take a straight-copy fast path for a full-canvas opaque
+    /// layer instead of 2M per-pixel blends. NEVER set it for paths whose pixels can
+    /// be transparent (static art, web overlays, chat, the social-bar strip).</summary>
+    public bool IsOpaque { get; init; }
+
    public VideoFrame(int width, int height, byte[] bgraPixels)
    {
        Width = width;
@@ -973,7 +973,7 @@ click (volume sliders keep their manual `SetSliderValueFromClick`, harmless dupl

 **Goal:** record the stream output to a local file, with or without simultaneously streaming.

-### Status: ✅ Shipped `a9eb360` (2026-08-29) — code done (incl. manual-rename modal), build 0 warnings, 244/246 tests; **running-app verification (takes 1–3 done)**: files land (ffmpeg re-pinned month-end), **webcam-in-output + social bar visually CONFIRMED from take 3's extracted frame**, rename modal used for real (take 3 was named via it); take 3 exposed the ~2fps producer starvation and the pump stage-timing (`97ffc42`) named it in one line — `avg render 258.1ms` — **fixed 2026-09-03** (deadline pacing per OBS video-io.c + libyuv-style row blits in `SceneCompositor`, test `Pump_Paces_To_The_Deadline_Compensating_Render_Cost`); **take 4 pending** to verify 60fps lands (stats must show `n/300` and render ≤ ~16ms; next suspect if hiccuppy: 8.3MB/frame LOH alloc → buffer pool)
+### Status: ✅ Shipped `a9eb360` (2026-08-29) — code done (incl. manual-rename modal), build 0 warnings, 244/246 tests; **running-app verification (takes 1–4 done)**: files land (ffmpeg re-pinned month-end), **webcam-in-output + social bar visually CONFIRMED from take 3's extracted frame**, rename modal used for real (take 3 was named via it); take 3 exposed the ~2fps producer starvation and the pump stage-timing (`97ffc42`) named it in one line — `avg render 258.1ms` — **slice 1 fixed 2026-09-03** (deadline pacing per OBS video-io.c + libyuv-style row blits in `SceneCompositor`, test `Pump_Paces_To_The_Deadline_Compensating_Render_Cost`); **take 4: pacing held but render stayed 58.9ms** (2M-iteration row walk + 8.3MB/tick LOH) — **slice 2 shipped 2026-09-04**: `VideoFrame.IsOpaque` producer-contract flag → full-cover backdrop is one `Buffer.BlockCopy`; integer fixed-point bilinear general path; pump scratch pool (release strictly post-submit, owned-by-reference so cache frames are untouchable); dead per-tick `fromScene` render + the `fromSceneProvider` seam removed (BlendFrame uses `TransitionService.FromFrame` — the old render fed nothing). Tests `Pump_Pools_ScratchBuffers_Across_Frames_Without_Stale_Pixels` (the ONE) + `Composite_OpaqueFullCover_Backdrop_CopiesEveryPixel_Into_Scratch`; 59/59 per-class green, clean build 0 warnings. **take 5 pending**: stats must show `avg render ≤ ~10ms, ≈300/300`. Known remaining churn (follow-up, not silently done): the vertical tier's final `BilinearScale` still allocates per frame. **User UX spec (2026-09-04) queued behind this**: two-line top bar (LIVE rename, radio pills, Login/Logout button + avatar right-click Change Account, no account light; line 2 centered Start↔Stop grayed-until-armed) + up-front SaveFileDialog for REC (native overwrite prompt; retires the stop-time rename modal) + Go-Live dialog KEPT as preflight confirmation prefilled from the Text drawer — decisions captured in HANDOFF

 1. ✅ `EncoderOptions` extended with `StreamEnabled` / `RecordEnabled` / `RecordPath` (independent intent flags)
 2. ✅ `FfmpegArgs.Build` reworked into per-output blocks (stream `-f flv`, record `-f mp4`) via `AddVideoTags`
@@ -309,7 +309,6 @@ public partial class MainViewModel : ViewModelBase
            log: message => AppLog.Write(message),
            socialBar: () => (_socialBarFrame, _socials?.BarPosition ?? SocialBarPosition.Bottom),
            transition: _transition,
-            fromSceneProvider: () => LiveScene,
            sceneGraph: _sceneGraph);
        _framePump.Failed += OnFramePumpFailed;
        _framePump.HealthUpdated += OnFramePumpHealthUpdated;
@@ -707,9 +707,29 @@ seam:** `Func<Scene?>`, `Func<SceneElement, VideoFrame?>` resolver, `Func<Compos
  general per-pixel path stays for scaled/round/mirrored elements (webcam) — pixel-identical, and it is
  only ~130k px. Test: `Pump_Paces_To_The_Deadline_Compensating_Render_Cost` records the requested wait
  via the pacing seam (a seam fake must genuinely await — a synchronous completed task runs the whole
-  pump loop on `StartAsync`'s continuation and hangs the run; MyMistakes). **Next slice if take 4's
-  stats show it:** 8.3MB fresh `byte[]` per frame → LOH churn/GC stalls — pool the master buffer if avg
-  render lands ≤ budget but frames still hiccup.
+  pump loop on `StartAsync`'s continuation and hangs the run; MyMistakes).
+- **Slice 2 of the render fix (2026-09-04, take 4):** the pacing held (file no longer truncated at
+  loop level) but render stayed at 58.9ms — the per-pixel row walk was still 2M managed iterations and
+  every tick allocated a fresh 8.3MB master buffer. Three changes: (1) **`VideoFrame.IsOpaque`** — a
+  producer-contract flag (only the screen-capture and webcam paths set it; DWM/MediaCapture fill alpha
+  255 by contract); a full-canvas, aligned, opacity-1 blit of an opaque frame is now ONE
+  `Buffer.BlockCopy` (~1.5ms) instead of the loop, and the black pre-fill is skipped when the backdrop
+  covers. (2) **integer fixed-point bilinear** in the general `BlitContent` path (row-hoisted invariants,
+  no divisions, no `Math.Round`) — within ±1 of the float reference, inside the ±2 test tolerance.
+  (3) **scratch pool in the pump** — `AcquireScratch`/`ReleaseScratch` recycle the master buffer
+  (max 4, keyed by length, owned-by-reference so bake-cache/social-bar/static-art arrays can never be
+  captured); release happens strictly AFTER `SubmitFrameAsync` returns (the write to stdin copies),
+  and the free-list Contains guard makes the transition Cut path safe (`BlendFrame` returns the
+  to-frame itself, aliasing the scratch). The per-tick `fromScene` render in the transition branch was
+  **dead weight** (BlendFrame uses `TransitionService.FromFrame` captured at `Start`, never the pump's) —
+  removed, and the pump's `fromSceneProvider` seam + `MainViewModel` call site went with it. Tests:
+  `Pump_Pools_ScratchBuffers_Across_Frames_Without_Stale_Pixels` (the ONE: alternating backdrop colors
+  pin every frame's content, repeated backing-array identity proves the pool recycles),
+  `Composite_OpaqueFullCover_Backdrop_CopiesEveryPixel_Into_Scratch` (memcpy branch + scratch
+  sentinel). `FakeEncoder.SubmitFrameAsync` now snapshots bytes like the real stdin write — holding the
+  reference would race legitimate recycling. Take 5 must show `avg render ≤ ~10ms, ≈300/300`; the
+  vertical tier's final 1080×1920 `BilinearScale` still allocates fresh per frame (same GC lesson when
+  someone streams vertical — recorded as a follow-up, not silently "done").
 - **Stop ordering matters:** `StopAsync` stops the encoder (closes stdin → EOF → ffmpeg finalizes+exits)
  **before** awaiting the loop, because closing stdin unblocks a write stuck on pipe backpressure — the
  reverse order would deadlock. `ProcessFailed` self-stops the pump. `Failed` while live flips
@@ -21,6 +21,7 @@ public class FramePumpTests
    private sealed class FakeEncoder : IFfmpegEncoder
    {
        public readonly List<VideoFrame> Frames = new();
+        public readonly List<byte[]> Backings = new();
        public int StartCount;
        public int StopCount;
        public bool Disposed;
@@ -41,7 +42,11 @@ public class FramePumpTests

        public Task SubmitFrameAsync(VideoFrame frame, CancellationToken cancellationToken = default)
        {
-            Frames.Add(frame);
+            // Snapshot the bytes like the production FfmpegEncoder does (WriteAsync to
+            // stdin copies before returning). Holding the reference would race the
+            // pump's scratch pool legitimately reusing the buffer one tick later.
+            Frames.Add(new VideoFrame(frame.Width, frame.Height, (byte[])frame.BgraPixels.Clone()));
+            Backings.Add(frame.BgraPixels); // the pre-copy buffer — pool-reuse witness
            FrameArrived.TrySetResult();
            return Task.CompletedTask;
        }
@@ -387,4 +392,36 @@ public class FramePumpTests
                $"requested wait {d} reaches the full {interval} interval — render cost must subtract from the deadline (the take-3 bug)");
        }
    }
+
+    /// <summary>The ONE integration test for the take-4 scratch pool: the pump recycles
+    /// its master buffers across frames (the 8.3MB-per-tick LOH churn that cost GC
+    /// stalls inside "render") while EVERY frame's content stays correct — stale bytes
+    /// from an earlier tick would show as a wrong color. The backdrop alternates
+    /// red/blue per tick (submit snapshots pin the color), and a repeated backing-array
+    /// identity proves the pool actually recycles (an inert pool keeps the pixel
+    /// assertions green while leaving the churn in place).</summary>
+    [Fact]
+    public async Task Pump_Pools_ScratchBuffers_Across_Frames_Without_Stale_Pixels()
+    {
+        var red = SceneCompositorTests.Solid(64, 48, 255, 0, 0);
+        var blue = SceneCompositorTests.Solid(64, 48, 0, 0, 255);
+        var flip = 0;
+        var encoder = new FakeEncoder();
+        using var pump = NewPump(encoder, resolve: _ => flip++ % 2 == 0 ? red : blue);
+
+        await pump.StartAsync();
+        var deadline = DateTime.UtcNow.AddSeconds(10);
+        while (DateTime.UtcNow < deadline && encoder.Frames.Count < 6)
+            await Task.Delay(10);
+        await pump.StopAsync();
+
+        Assert.True(encoder.Frames.Count >= 6, $"only {encoder.Frames.Count} frames produced");
+        for (var i = 0; i < encoder.Frames.Count; i++)
+        {
+            if (i % 2 == 0) AssertColor(encoder.Frames[i], 0, 0, 255, 0, 0);
+            else AssertColor(encoder.Frames[i], 0, 0, 0, 0, 255);
+        }
+        Assert.True(encoder.Backings.Distinct().Count() < encoder.Backings.Count,
+            "every frame rode a fresh buffer — the scratch pool is inert");
+    }
 }
@@ -114,6 +114,46 @@ public class SceneCompositorTests
        AssertColor(output, 550, 700, 255, 255, 255);
    }

+    /// <summary>The take-4 memcpy fast path: a full-cover opaque backdrop (the live
+    /// capture's contract) must reproduce the source EXACTLY, and when supplied via the
+    /// <c>scratch</c> pool parameter the render must overwrite every byte of the reused
+    /// buffer — stale sentinels from a previous frame may not survive (this is what
+    /// makes buffer recycling safe at the pump level).</summary>
+    [Fact]
+    public void Composite_OpaqueFullCover_Backdrop_CopiesEveryPixel_Into_Scratch()
+    {
+        const int w = 128, h = 64;
+        var src = new byte[w * h * 4];
+        for (var y = 0; y < h; y++)
+            for (var x = 0; x < w; x++)
+            {
+                var i = (y * w + x) * 4;
+                src[i] = (byte)(x * 2);
+                src[i + 1] = (byte)(y * 4);
+                src[i + 2] = (byte)(x + y);
+                src[i + 3] = 255; // the IsOpaque contract
+            }
+        var opaque = new VideoFrame(w, h, src) { IsOpaque = true };
+
+        var background = new Source { Type = SourceType.DisplayCapture, IsBackground = true, CaptureKey = "monitor:0" };
+        var scene = new Scene { Name = "Live" };
+        scene.Elements.Add(background);
+
+        var options = new CompositorOptions
+        {
+            SourceRectX = 0, SourceRectY = 0, SourceRectWidth = w, SourceRectHeight = h,
+            OutputWidth = w, OutputHeight = h,
+        };
+
+        var scratch = new byte[w * h * 4];
+        Array.Fill(scratch, (byte)0xFD); // sentinel: x*2 (even), y*4 (×4), x+y (≤190) can never produce 253
+
+        var output = new SceneCompositor().Render(scene, _ => opaque, null, options, scratch: scratch);
+
+        Assert.Equal(src, output.BgraPixels); // byte-exact: the memcpy path must not round
+        Assert.DoesNotContain((byte)0xFD, output.BgraPixels); // full cover — nothing stale
+    }
+
    [Fact]
    public void Render_VerticalTier_Outputs_1080x1920_From_The_Center_Crop()
    {