Frame timing

apps/LoopThroughWithOpenGLCompositing/videoio/RenderOutputQueue.cpp

@@ -47,6 +47,18 @@ bool RenderOutputQueue::TryPop(RenderOutputFrame& frame)
 	return true;
 }
 
+bool RenderOutputQueue::DropOldestFrame()
+{
+	std::lock_guard<std::mutex> lock(mMutex);
+	if (mReadyFrames.empty())
+		return false;
+
+	ReleaseFrame(mReadyFrames.front());
+	mReadyFrames.pop_front();
+	++mDroppedCount;
+	return true;
+}
+
 void RenderOutputQueue::Clear()
 {
 	std::lock_guard<std::mutex> lock(mMutex);

apps/LoopThroughWithOpenGLCompositing/videoio/RenderOutputQueue.h

@@ -34,6 +34,7 @@ public:
 	void Configure(const VideoPlayoutPolicy& policy);
 	bool Push(RenderOutputFrame frame);
 	bool TryPop(RenderOutputFrame& frame);
+	bool DropOldestFrame();
 	void Clear();
 	RenderOutputQueueMetrics GetMetrics() const;
 

apps/LoopThroughWithOpenGLCompositing/videoio/VideoBackend.cpp

@@ -359,6 +359,12 @@ void VideoBackend::StartOutputProducerWorker()
 	if (mOutputProducerWorkerRunning)
 		return;
 
+	const double frameBudgetMilliseconds = State().frameBudgetMilliseconds;
+	const auto frameDuration = frameBudgetMilliseconds > 0.0
+		? std::chrono::duration_cast<RenderCadenceController::Duration>(
+			std::chrono::duration<double, std::milli>(frameBudgetMilliseconds))
+		: std::chrono::milliseconds(16);
+	mRenderCadenceController.Configure(frameDuration, std::chrono::steady_clock::now());
 	mLastOutputProductionCompletion = VideoIOCompletion();
 	mLastOutputProductionTime = std::chrono::steady_clock::time_point();
 	mOutputProducerWorkerStopping = false;
@@ -433,11 +439,16 @@ void VideoBackend::OutputProducerWorkerMain()
 
 		const RenderOutputQueueMetrics metrics = mReadyOutputQueue.GetMetrics();
 		RecordReadyQueueDepthSample(metrics);
-		const OutputProductionDecision decision = mOutputProductionController.Decide(BuildOutputProductionPressure(metrics));
+
-		if (decision.action != OutputProductionAction::Produce || decision.requestedFrames == 0)
+		const auto now = std::chrono::steady_clock::now();
+		RenderCadenceDecision cadenceDecision = mRenderCadenceController.Tick(now);
+		if (cadenceDecision.action == RenderCadenceAction::Wait)
 		{
+			const auto waitDuration = (std::min)(
+				std::chrono::duration_cast<std::chrono::milliseconds>(cadenceDecision.waitDuration),
+				OutputProducerWakeInterval());
 			std::unique_lock<std::mutex> lock(mOutputProducerMutex);
-			mOutputProducerCondition.wait_for(lock, OutputProducerWakeInterval());
+			mOutputProducerCondition.wait_for(lock, waitDuration);
 			if (mOutputProducerWorkerStopping)
 			{
 				mOutputProducerWorkerRunning = false;
@@ -454,16 +465,7 @@ void VideoBackend::OutputProducerWorkerMain()
 			completion = mLastOutputProductionCompletion;
 		}
 
-		const bool belowTargetDepth = metrics.depth < decision.targetReadyFrames;
+		const std::size_t producedFrames = ProduceReadyOutputFrames(completion, 1);
-		const auto now = std::chrono::steady_clock::now();
-		if (!belowTargetDepth &&
-			mLastOutputProductionTime != std::chrono::steady_clock::time_point() &&
-			now - mLastOutputProductionTime < OutputProducerWakeInterval())
-		{
-			continue;
-		}
-
-		const std::size_t producedFrames = ProduceReadyOutputFrames(completion, decision.requestedFrames);
 		if (producedFrames > 0)
 		{
 			mLastOutputProductionTime = std::chrono::steady_clock::now();
@@ -600,10 +602,6 @@ std::size_t VideoBackend::ProduceReadyOutputFrames(const VideoIOCompletion& comp
 	std::size_t producedFrames = 0;
 	while (producedFrames < maxFrames)
 	{
-		const OutputProductionDecision decision = mOutputProductionController.Decide(BuildOutputProductionPressure(metrics));
-		if (decision.action != OutputProductionAction::Produce)
-			break;
-
 		if (!RenderReadyOutputFrame(mVideoIODevice->State(), completion))
 			break;
 		++producedFrames;
@@ -634,7 +632,10 @@ bool VideoBackend::RenderReadyOutputFrame(const VideoIOState& state, const Video
 	VideoIOOutputFrame outputFrame;
 	const auto acquireStart = std::chrono::steady_clock::now();
 	if (!mSystemOutputFramePool.AcquireFreeSlot(outputSlot))
-		return false;
+	{
+		if (!mReadyOutputQueue.DropOldestFrame() || !mSystemOutputFramePool.AcquireFreeSlot(outputSlot))
+			return false;
+	}
 	outputFrame = outputSlot.frame;
 	const auto acquireEnd = std::chrono::steady_clock::now();
 

apps/LoopThroughWithOpenGLCompositing/videoio/VideoBackend.h

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "OutputProductionController.h"
+#include "RenderCadenceController.h"
 #include "RenderOutputQueue.h"
 #include "SystemOutputFramePool.h"
 #include "VideoBackendLifecycle.h"
@@ -105,6 +106,7 @@ private:
 	VideoBackendLifecycle mLifecycle;
 	VideoPlayoutPolicy mPlayoutPolicy;
 	OutputProductionController mOutputProductionController;
+	RenderCadenceController mRenderCadenceController;
 	RenderOutputQueue mReadyOutputQueue;
 	SystemOutputFramePool mSystemOutputFramePool;
 	std::unique_ptr<VideoIODevice> mVideoIODevice;

docs/PHASE_7_7_RENDER_CADENCE_PLAYOUT_DESIGN.md

@@ -2,7 +2,15 @@
 
 ## Status
 
-Proposed.
+In progress.
+
+Implemented so far:
+
+- real DeckLink buffered-frame telemetry is exposed separately from synthetic scheduler lead
+- pure `RenderCadenceController` exists with non-GL tests
+- `SystemOutputFramePool` now exposes the Phase 7.7 state vocabulary: `Free`, `Rendering`, `Completed`, `Scheduled`
+- the output producer now uses `RenderCadenceController` to render one output frame per cadence tick
+- DeckLink scheduling remains a separate top-up pass capped by the configured preroll target
 
 Phase 7.5 and 7.6 proved useful pieces individually:
 
@@ -38,6 +46,16 @@ DeckLink playout scheduler
 
 The system-memory frame buffer becomes the contract between render timing and device timing.
 
+Core principle:
+
+- The render cadence should be stable and boring.
+- If the selected output mode is 59.94 fps, the render producer should attempt to render at 59.94 fps.
+- It should not speed up just because the DeckLink buffer is empty.
+- It should not slow down because DeckLink is full or because completed frames have not drained.
+- Completed-but-unscheduled frames are a latest-N cache. Old completed frames may be dropped/recycled to keep rendering at cadence.
+- Scheduled frames are protected until DeckLink completes them.
+- The only normal reason for the render cadence to deviate is that rendering/GPU work itself overruns the frame budget.
+
 ## Non-Goals
 
 - Do not hide failure by repeating frames as the primary strategy.
@@ -64,6 +82,14 @@ That means the system can be full and still look wrong, because "full" is not ti
 ### Target Shape
 
 ```text
+Startup / warmup
+  render cadence starts first
+  render thread produces warmup frames at the selected cadence
+  completed system-memory queue reaches warmup target
+  DeckLink preroll is scheduled from completed frames
+  DeckLink playback starts with a filled buffer
+
+Steady state
 RenderCadenceController
   owns output frame tick: frame 0, 1, 2...
   owns render target time
@@ -73,7 +99,8 @@ RenderCadenceController
 PlayoutFrameStore
   owns free / rendering / completed / scheduled slots
   tracks frame number, render time, completion time, and schedule state
-  exposes completed frames to DeckLink scheduler
+  exposes latest completed frames to DeckLink scheduler
+  may drop/recycle oldest unscheduled completed frames when render cadence needs space
 
 DeckLinkPlayoutScheduler
   owns DeckLink schedule time
@@ -111,14 +138,28 @@ Rules:
 
 - If the render thread is early, it waits/yields.
 - If it is slightly late, it renders the next frame immediately and records lateness.
-- If it is badly late, policy may skip render ticks before rendering the newest frame.
+- If it is badly late because render/GPU work overran the frame budget, policy may skip render ticks before rendering the newest frame.
-- Skipping render ticks is a render-cadence decision, not a DeckLink stream-time jump.
+- Skipping render ticks is an overrun policy, not a buffer-fill strategy.
 - DeckLink schedule time should remain continuous unless a deliberate device recovery policy says otherwise.
 
+Non-rule:
+
+- The render producer must not render faster than the selected cadence to refill DeckLink.
+- DeckLink should start only after warmup/preroll has filled enough completed frames.
+- If the DeckLink buffer drains in steady state, that is a real timing failure to measure, not a signal for the render thread to sprint.
+
 ## Buffer Model
 
 Use a fixed system-memory slot pool.
 
+The completed portion of the pool is not a strict consume-before-render queue. It is a latest-N rendered-frame cache:
+
+- render cadence writes one frame per selected output tick
+- if completed-but-unscheduled frames are full, the oldest completed frame is disposable
+- DeckLink scheduling consumes from the completed cache when it needs frames
+- frames already scheduled to DeckLink are never recycled until completion
+- if all slots are scheduled/in flight, cadence may miss because there is genuinely no safe system-memory target
+
 Suggested starting values:
 
 - completed-frame target: 2-4 frames
@@ -266,14 +307,14 @@ Before more scheduling changes, measure the real device buffer.
 
 Deliverables:
 
-- call DeckLink `GetBufferedVideoFrameCount()` after schedule/completion where available
+- [x] call DeckLink `GetBufferedVideoFrameCount()` after schedule/completion where available
-- expose `actualDeckLinkBufferedFrames`
+- [x] expose `actualDeckLinkBufferedFrames`
-- keep `scheduledLeadFrames` but label it synthetic/internal
+- [x] keep `scheduledLeadFrames` but label it synthetic/internal
-- record schedule-call duration and failures
+- [x] record schedule-call duration and failures
 
 Exit criteria:
 
-- runtime telemetry distinguishes app completed queue, system scheduled slots, synthetic lead, and actual DeckLink buffer depth
+- [x] runtime telemetry distinguishes app completed queue, system scheduled slots, synthetic lead, and actual DeckLink buffer depth
 
 ### Step 2: Rename Existing Queues To Match Their Roles
 
@@ -295,17 +336,17 @@ Add a pure timing helper first.
 
 Responsibilities:
 
-- compute next render tick
+- [x] compute next render tick
-- track frame duration
+- [x] track frame duration
-- report early/late/drift
+- [x] report early/late/drift
-- decide whether to render, wait, or skip render ticks
+- [x] decide whether to render, wait, or skip render ticks
 
 Tests:
 
-- exact cadence advances
+- [x] exact cadence advances
-- late ticks are measured
+- [x] late ticks are measured
-- large lateness can skip according to policy
+- [x] large lateness can skip according to policy
-- no dependency on GL or DeckLink
+- [x] no dependency on GL or DeckLink
 
 ### Step 4: Move Output Production To Cadence Ticks
 
@@ -313,15 +354,36 @@ Replace queue-pressure-only production with cadence-driven production.
 
 Initial behavior:
 
-- render at selected output cadence
+- [x] render at selected output cadence
-- produce into system-memory slots
+- [x] produce into system-memory slots
-- publish completed frames
+- [x] publish completed frames
-- pause when completed queue is at max depth
+- [x] recycle/drop oldest unscheduled completed frames when cadence needs a slot
+- [ ] only wait when every safe slot is scheduled/in flight
 
 Exit criteria:
 
 - output rendering continues without DeckLink completions
 - output rendering does not schedule DeckLink directly
+- completed-frame buffering behaves as latest-N, not consume-before-render
+
+### Step 4a: Add Warmup Before DeckLink Playback
+
+DeckLink output should not start consuming before the render cadence has prepared an initial cushion.
+
+Initial behavior:
+
+- configure DeckLink output without starting scheduled playback
+- start the render cadence producer
+- render warmup frames at the selected cadence, not faster
+- wait until completed-frame depth reaches `targetWarmupFrames`
+- schedule those completed frames as DeckLink preroll
+- call `StartScheduledPlayback()`
+
+Exit criteria:
+
+- startup does not require the render producer to catch up by rendering faster than cadence
+- DeckLink begins playback with a real completed-frame buffer
+- if warmup cannot fill within a bounded timeout, startup enters degraded state with telemetry
 
 ### Step 5: Make DeckLink Scheduler A Separate Top-Up Loop
 

tests/RenderOutputQueueTests.cpp

@@ -94,6 +94,29 @@ void TestOverflowReleasesDroppedFrame()
 	Expect(gReleasedFrames == 1, "pop transfers ownership without releasing");
 }
 
+void TestDropOldestFrameReleasesFrame()
+{
+	gReleasedFrames = 0;
+	VideoPlayoutPolicy policy;
+	policy.maxReadyFrames = 2;
+	RenderOutputQueue queue(policy);
+
+	queue.Push(MakeOwnedFrame(1));
+	queue.Push(MakeOwnedFrame(2));
+
+	Expect(queue.DropOldestFrame(), "oldest ready frame can be explicitly dropped");
+	Expect(gReleasedFrames == 1, "explicit drop releases oldest frame");
+
+	RenderOutputQueueMetrics metrics = queue.GetMetrics();
+	Expect(metrics.depth == 1, "explicit drop reduces queue depth");
+	Expect(metrics.droppedCount == 1, "explicit drop increments dropped count");
+
+	RenderOutputFrame frame;
+	Expect(queue.TryPop(frame), "newest frame remains after explicit drop");
+	Expect(frame.frameIndex == 2, "explicit drop keeps newest frame");
+	Expect(!queue.DropOldestFrame(), "empty queue cannot drop a frame");
+}
+
 void TestUnderrunIsCounted()
 {
 	RenderOutputQueue queue;
@@ -169,6 +192,7 @@ int main()
 	TestQueuePreservesOrdering();
 	TestBoundedQueueDropsOldestFrame();
 	TestOverflowReleasesDroppedFrame();
+	TestDropOldestFrameReleasesFrame();
 	TestUnderrunIsCounted();
 	TestConfigureShrinksDepthToNewCapacity();
 	TestConfigureReleasesTrimmedFrames();