sync: masked waveform correlation, stretch sync, manual start time

- Waveform sync no longer treats uncached waveform regions as silence:
  the envelope extraction now reports a per-window validity mask and the
  correlation skips invalid windows on either side, improving accuracy
  for partially cached clips
- Add stretch/speed sync: AudioWaveformSync::EstimateStretchAndOffset
  searches a playback-rate range plus offset, and a new timeline
  context action 'Synchronize by Waveform (Adjust Speed)' applies the
  estimated rate as a clip speed change (with undo) when plain offset
  alignment is inconclusive
- Footage properties dialog gains a Source Start Time field so the
  value used by source-time sync can be viewed and edited manually
  instead of relying solely on auto-detected metadata; applied via an
  undo command, with Footage::ClearSourceStartTime() for removal
- Regression tests for masked correlation, stretch estimation, the
  envelope validity mask, and source-start-time set/clear

app/audio/audiowaveformsync.cpp

@@ -86,12 +86,26 @@ AudioWaveformSync::OffsetResult AudioWaveformSync::EstimateOffset(
 AudioWaveformSync::OffsetResult AudioWaveformSync::EstimateEnvelopeOffset(
 	const QVector<double> &reference, const QVector<double> &candidate,
 	size_t window_samples, int64_t max_offset_windows)
+{
+	return EstimateEnvelopeOffset(reference, candidate, QVector<bool>(),
+								  QVector<bool>(), window_samples,
+								  max_offset_windows);
+}
+
+AudioWaveformSync::OffsetResult AudioWaveformSync::EstimateEnvelopeOffset(
+	const QVector<double> &reference, const QVector<double> &candidate,
+	const QVector<bool> &reference_valid, const QVector<bool> &candidate_valid,
+	size_t window_samples, int64_t max_offset_windows)
 {
 	OffsetResult result;
 	if (reference.isEmpty() || candidate.isEmpty() || !window_samples) {
 		return result;
 	}
 
+	const auto is_valid = [](const QVector<bool> &mask, int size, int index) {
+		return mask.size() != size || mask.at(index);
+	};
+
 	double best_score = -2.0;
 	int64_t best_lag = 0;
 
@@ -106,23 +120,45 @@ AudioWaveformSync::OffsetResult AudioWaveformSync::EstimateEnvelopeOffset(
 			continue;
 		}
 
+		// Only windows marked valid on both sides participate in the score
 		double reference_mean = 0.0;
 		double candidate_mean = 0.0;
+		int valid_count = 0;
 		for (int i = 0; i < overlap; i++) {
-			reference_mean += reference.at(reference_start + i);
-			candidate_mean += candidate.at(candidate_start + i);
+			const int reference_index = reference_start + i;
+			const int candidate_index = candidate_start + i;
+			if (!is_valid(reference_valid, reference.size(),
+						  reference_index) ||
+				!is_valid(candidate_valid, candidate.size(), candidate_index)) {
+				continue;
+			}
+			reference_mean += reference.at(reference_index);
+			candidate_mean += candidate.at(candidate_index);
+			valid_count++;
 		}
-		reference_mean /= static_cast<double>(overlap);
-		candidate_mean /= static_cast<double>(overlap);
+
+		if (valid_count < 2) {
+			continue;
+		}
+
+		reference_mean /= static_cast<double>(valid_count);
+		candidate_mean /= static_cast<double>(valid_count);
 
 		double numerator = 0.0;
 		double reference_energy = 0.0;
 		double candidate_energy = 0.0;
 		for (int i = 0; i < overlap; i++) {
+			const int reference_index = reference_start + i;
+			const int candidate_index = candidate_start + i;
+			if (!is_valid(reference_valid, reference.size(),
+						  reference_index) ||
+				!is_valid(candidate_valid, candidate.size(), candidate_index)) {
+				continue;
+			}
 			const double reference_value =
-				reference.at(reference_start + i) - reference_mean;
+				reference.at(reference_index) - reference_mean;
 			const double candidate_value =
-				candidate.at(candidate_start + i) - candidate_mean;
+				candidate.at(candidate_index) - candidate_mean;
 			numerator += reference_value * candidate_value;
 			reference_energy += reference_value * reference_value;
 			candidate_energy += candidate_value * candidate_value;
@@ -149,4 +185,61 @@ AudioWaveformSync::OffsetResult AudioWaveformSync::EstimateEnvelopeOffset(
 	return result;
 }
 
+AudioWaveformSync::StretchOffsetResult AudioWaveformSync::EstimateStretchAndOffset(
+	const QVector<double> &reference, const QVector<double> &candidate,
+	const QVector<bool> &reference_valid, const QVector<bool> &candidate_valid,
+	size_t window_samples, int64_t max_offset_windows, double min_rate,
+	double max_rate, double rate_step)
+{
+	StretchOffsetResult result;
+	if (reference.isEmpty() || candidate.isEmpty() || !window_samples ||
+		min_rate <= 0.0 || max_rate < min_rate || rate_step <= 0.0) {
+		return result;
+	}
+
+	double best_confidence = -2.0;
+
+	for (double rate = min_rate; rate <= max_rate + rate_step * 0.5;
+		 rate += rate_step) {
+		// Resample the candidate envelope so that window i of the resampled
+		// envelope corresponds to window i*rate of the original
+		const int resampled_size =
+			static_cast<int>(candidate.size() / rate);
+		if (resampled_size < 2) {
+			continue;
+		}
+
+		QVector<double> resampled(resampled_size);
+		QVector<bool> resampled_valid(resampled_size);
+		for (int i = 0; i < resampled_size; i++) {
+			const double position = i * rate;
+			const int lower = static_cast<int>(position);
+			const int upper =
+				std::min(lower + 1, static_cast<int>(candidate.size()) - 1);
+			const double fraction = position - lower;
+
+			resampled[i] = candidate.at(lower) * (1.0 - fraction) +
+						   candidate.at(upper) * fraction;
+
+			resampled_valid[i] =
+				(candidate_valid.size() != candidate.size() ||
+				 (candidate_valid.at(lower) && candidate_valid.at(upper)));
+		}
+
+		const OffsetResult offset = EstimateEnvelopeOffset(
+			reference, resampled, reference_valid, resampled_valid,
+			window_samples, max_offset_windows);
+
+		if (offset.valid && offset.confidence > best_confidence) {
+			best_confidence = offset.confidence;
+			result.valid = true;
+			result.rate = rate;
+			result.confidence = offset.confidence;
+			result.offset_samples = offset.offset_samples;
+		}
+	}
+
+	return result;
+}
+
 }

app/audio/audiowaveformsync.h

@@ -38,6 +38,16 @@ public:
 		bool valid = false;
 	};
 
+	struct StretchOffsetResult {
+		// Playback rate the candidate must be played at to align with the
+		// reference (e.g. 2.0 = candidate runs at half speed and needs to be
+		// sped up 2x)
+		double rate = 1.0;
+		int64_t offset_samples = 0;
+		double confidence = 0.0;
+		bool valid = false;
+	};
+
 	static QVector<double> ExtractRmsEnvelope(const core::SampleBuffer &samples,
 											  size_t window_samples);
 
@@ -50,6 +60,41 @@ public:
 											   const QVector<double> &candidate,
 											   size_t window_samples,
 											   int64_t max_offset_windows);
+
+	/**
+	 * @brief Offset estimation that ignores windows flagged as invalid
+	 *
+	 * @p reference_valid and @p candidate_valid mark which envelope windows
+	 * contain real data (e.g. actually cached waveform regions). Windows
+	 * flagged false on either side are excluded from the correlation instead
+	 * of being treated as silence, which improves accuracy when parts of the
+	 * waveform cache have not been generated yet. Empty masks are treated as
+	 * "all windows valid".
+	 */
+	static OffsetResult EstimateEnvelopeOffset(const QVector<double> &reference,
+											   const QVector<double> &candidate,
+											   const QVector<bool> &reference_valid,
+											   const QVector<bool> &candidate_valid,
+											   size_t window_samples,
+											   int64_t max_offset_windows);
+
+	/**
+	 * @brief Estimates a playback-rate change plus offset aligning the
+	 * candidate to the reference
+	 *
+	 * The candidate envelope is resampled at each candidate rate in
+	 * [min_rate, max_rate] (step rate_step) and correlated against the
+	 * reference. rate > 1 means the candidate runs slower than the reference
+	 * and must be sped up. The search is O(rates * lags * overlap), so
+	 * callers should bound max_offset_windows to a sensible range.
+	 */
+	static StretchOffsetResult
+	EstimateStretchAndOffset(const QVector<double> &reference,
+							 const QVector<double> &candidate,
+							 const QVector<bool> &reference_valid,
+							 const QVector<bool> &candidate_valid, size_t window_samples,
+							 int64_t max_offset_windows, double min_rate,
+							 double max_rate, double rate_step);
 };
 
 }

app/dialog/footageproperties/footageproperties.cpp

@@ -22,6 +22,7 @@
 #include "footageproperties.h"
 
 #include <QGridLayout>
+#include <QHBoxLayout>
 #include <QLabel>
 #include <QComboBox>
 #include <QLineEdit>
@@ -58,6 +59,48 @@ FootagePropertiesDialog::FootagePropertiesDialog(QWidget *parent,
 	layout->addWidget(footage_name_field_, row, 1);
 	row++;
 
+	// Manual source start time: audio/timecode sync relies on this value,
+	// which is otherwise only auto-detected from file metadata
+	layout->addWidget(new QLabel(tr("Source Start Time:")), row, 0);
+
+	{
+		QHBoxLayout *start_time_layout = new QHBoxLayout();
+
+		source_start_time_enable_ = new QCheckBox(tr("Set"));
+		source_start_time_enable_->setChecked(footage_->HasSourceStartTime());
+		start_time_layout->addWidget(source_start_time_enable_);
+
+		source_start_time_spin_ = new QDoubleSpinBox();
+		source_start_time_spin_->setRange(-86400.0, 86400.0);
+		source_start_time_spin_->setDecimals(3);
+		source_start_time_spin_->setSuffix(QStringLiteral(" s"));
+		source_start_time_spin_->setValue(
+			footage_->HasSourceStartTime() ?
+				footage_->source_start_time().toDouble() :
+				0.0);
+		source_start_time_spin_->setEnabled(
+			source_start_time_enable_->isChecked());
+		start_time_layout->addWidget(source_start_time_spin_, 1);
+
+		QString detection_note;
+		if (footage_->HasSourceStartTime()) {
+			const QString &source = footage_->source_start_time_source();
+			detection_note =
+				(source == QStringLiteral("manual")) ?
+					tr("(set manually)") :
+					tr("(auto-detected: %1)").arg(source);
+		} else {
+			detection_note = tr("(not detected)");
+		}
+		start_time_layout->addWidget(new QLabel(detection_note));
+
+		connect(source_start_time_enable_, &QCheckBox::toggled,
+				source_start_time_spin_, &QDoubleSpinBox::setEnabled);
+
+		layout->addLayout(start_time_layout, row, 1);
+	}
+	row++;
+
 	layout->addWidget(new QLabel(tr("Tracks:")), row, 0, 1, 2);
 	row++;
 
@@ -165,6 +208,18 @@ void FootagePropertiesDialog::accept()
 		command->add_child(nrc);
 	}
 
+	// Apply source start time changes
+	{
+		const bool new_enabled = source_start_time_enable_->isChecked();
+		const rational new_time =
+			rational::fromDouble(source_start_time_spin_->value());
+		if (new_enabled != footage_->HasSourceStartTime() ||
+			(new_enabled && new_time != footage_->source_start_time())) {
+			command->add_child(new FootageSetSourceStartTimeCommand(
+				footage_, new_enabled, new_time, QStringLiteral("manual")));
+		}
+	}
+
 	for (int i = 0; i < footage_->GetTotalStreamCount(); i++) {
 		Track::Reference reference = footage_->GetReferenceFromRealIndex(i);
 		bool new_stream_enabled =
@@ -279,4 +334,44 @@ void FootagePropertiesDialog::StreamEnableChangeCommand::undo()
 	}
 }
 
+FootagePropertiesDialog::FootageSetSourceStartTimeCommand::
+	FootageSetSourceStartTimeCommand(Footage *footage, bool enabled,
+									 const rational &time,
+									 const QString &source)
+	: footage_(footage)
+	, new_enabled_(enabled)
+	, new_time_(time)
+	, new_source_(source)
+{
+}
+
+Project *
+FootagePropertiesDialog::FootageSetSourceStartTimeCommand::GetRelevantProject()
+	const
+{
+	return footage_->project();
+}
+
+void FootagePropertiesDialog::FootageSetSourceStartTimeCommand::redo()
+{
+	old_enabled_ = footage_->HasSourceStartTime();
+	old_time_ = footage_->source_start_time();
+	old_source_ = footage_->source_start_time_source();
+
+	if (new_enabled_) {
+		footage_->SetSourceStartTime(new_time_, new_source_);
+	} else {
+		footage_->ClearSourceStartTime();
+	}
+}
+
+void FootagePropertiesDialog::FootageSetSourceStartTimeCommand::undo()
+{
+	if (old_enabled_) {
+		footage_->SetSourceStartTime(old_time_, old_source_);
+	} else {
+		footage_->ClearSourceStartTime();
+	}
+}
+
 }

app/dialog/footageproperties/footageproperties.h

@@ -79,6 +79,30 @@ private:
 		bool new_enabled_;
 	};
 
+	class FootageSetSourceStartTimeCommand : public UndoCommand {
+	public:
+		FootageSetSourceStartTimeCommand(Footage *footage, bool enabled,
+										 const rational &time,
+										 const QString &source);
+
+		virtual Project *GetRelevantProject() const override;
+
+	protected:
+		virtual void redo() override;
+		virtual void undo() override;
+
+	private:
+		Footage *footage_;
+
+		bool new_enabled_;
+		rational new_time_;
+		QString new_source_;
+
+		bool old_enabled_;
+		rational old_time_;
+		QString old_source_;
+	};
+
 	/**
    * @brief Stack of widgets that changes based on whether the stream is a video or audio stream
    */
@@ -89,6 +113,16 @@ private:
    */
 	QLineEdit *footage_name_field_;
 
+	/**
+   * @brief Whether a manual source start time should be used
+   */
+	QCheckBox *source_start_time_enable_;
+
+	/**
+   * @brief Source start time in seconds
+   */
+	QDoubleSpinBox *source_start_time_spin_;
+
 	/**
    * @brief Internal pointer to Media object (set in constructor)
    */

app/node/project/footage/footage.cpp

@@ -231,6 +231,13 @@ void Footage::SetSourceStartTime(const rational &time, const QString &source)
 	has_source_start_time_ = true;
 }
 
+void Footage::ClearSourceStartTime()
+{
+	source_start_time_ = rational();
+	source_start_time_source_.clear();
+	has_source_start_time_ = false;
+}
+
 void Footage::set_proxy_enabled(bool enabled)
 {
 	if (proxy_enabled_ != enabled) {

app/node/project/footage/footage.h

@@ -173,6 +173,11 @@ public:
 
 	void SetSourceStartTime(const rational &time, const QString &source);
 
+	/**
+	 * @brief Removes any source start time (auto-detected or manual)
+	 */
+	void ClearSourceStartTime();
+
 	bool proxy_enabled() const
 	{
 		return proxy_enabled_;

app/widget/timelinewidget/timelinewidget.cpp

@@ -1023,6 +1023,17 @@ void TimelineWidget::SynchronizeSelectedClipsBySourceTime()
 }
 
 void TimelineWidget::SynchronizeSelectedClipsByWaveform()
+{
+	SynchronizeSelectedClipsByWaveformInternal(false);
+}
+
+void TimelineWidget::SynchronizeSelectedClipsByWaveformWithSpeed()
+{
+	SynchronizeSelectedClipsByWaveformInternal(true);
+}
+
+void TimelineWidget::SynchronizeSelectedClipsByWaveformInternal(
+	bool allow_speed)
 {
 	if (!GetConnectedNode()) {
 		return;
@@ -1052,8 +1063,10 @@ void TimelineWidget::SynchronizeSelectedClipsByWaveform()
 		static_cast<int64_t>(sample_rate) * 10 * 60;
 	const int64_t max_offset_windows =
 		max_offset_samples / static_cast<int64_t>(window_samples);
-	const QVector<double> reference_envelope =
-		ExtractWaveformCacheEnvelope(reference, sample_rate, window_samples);
+
+	QVector<bool> reference_valid;
+	const QVector<double> reference_envelope = ExtractWaveformCacheEnvelope(
+		reference, sample_rate, window_samples, &reference_valid);
 
 	qDebug() << "TimelineWidget::SynchronizeSelectedClipsByWaveform: sample_rate="
 			 << sample_rate << "window_samples=" << window_samples
@@ -1063,27 +1076,62 @@ void TimelineWidget::SynchronizeSelectedClipsByWaveform()
 	struct SyncPlacement {
 		ClipBlock *clip = nullptr;
 		rational timeline_in;
+		double speed = 1.0;
 	};
 
 	QVector<SyncPlacement> placements;
-	placements.append({ reference.clip, reference.clip->in() });
+	placements.append({ reference.clip, reference.clip->in(), 1.0 });
 	for (const WaveformSyncClip &sync_clip : sync_clips) {
 		if (sync_clip.clip == reference.clip) {
 			continue;
 		}
 
+		QVector<bool> candidate_valid;
 		const QVector<double> candidate_envelope = ExtractWaveformCacheEnvelope(
-			sync_clip, sample_rate, window_samples);
-		const AudioWaveformSync::OffsetResult offset =
-			AudioWaveformSync::EstimateEnvelopeOffset(reference_envelope,
-																  candidate_envelope,
-																  window_samples,
-																  max_offset_windows);
+			sync_clip, sample_rate, window_samples, &candidate_valid);
+
+		// Skip uncached (zero-filled) windows on both sides so partially
+		// cached waveforms don't drag the correlation down
+		AudioWaveformSync::OffsetResult offset =
+			AudioWaveformSync::EstimateEnvelopeOffset(
+				reference_envelope, candidate_envelope, reference_valid,
+				candidate_valid, window_samples, max_offset_windows);
+
+		double speed = 1.0;
+
+		if (allow_speed && (!offset.valid || offset.confidence < 0.6)) {
+			// Plain offset alignment is inconclusive; the clips may run at
+			// different speeds (e.g. 24fps vs 25fps pull-down). Search a
+			// rate range with a tighter offset radius to keep the search
+			// interactive.
+			const int64_t stretch_radius_windows = std::min<int64_t>(
+				max_offset_windows,
+				(static_cast<int64_t>(sample_rate) * 30) /
+					static_cast<int64_t>(window_samples));
+			const AudioWaveformSync::StretchOffsetResult stretch =
+				AudioWaveformSync::EstimateStretchAndOffset(
+					reference_envelope, candidate_envelope, reference_valid,
+					candidate_valid, window_samples, stretch_radius_windows,
+					0.75, 1.34, 0.005);
+			qDebug() << "TimelineWidget::SynchronizeSelectedClipsByWaveform: "
+						"stretch estimate valid="
+					 << stretch.valid << "rate=" << stretch.rate
+					 << "confidence=" << stretch.confidence;
+
+			if (stretch.valid &&
+				stretch.confidence > (offset.valid ? offset.confidence : 0.0)) {
+				speed = stretch.rate;
+				offset.valid = true;
+				offset.confidence = stretch.confidence;
+				offset.offset_samples = stretch.offset_samples;
+			}
+		}
+
 		qDebug() << "TimelineWidget::SynchronizeSelectedClipsByWaveform: candidate"
 				 << sync_clip.clip << "envelope_size="
-				 << candidate_envelope.size() << "offset_valid="
-				 << offset.valid << "offset_samples=" << offset.offset_samples
-				 << "confidence=" << offset.confidence;
+				 << candidate_envelope.size() << "offset_valid=" << offset.valid
+				 << "offset_samples=" << offset.offset_samples
+				 << "confidence=" << offset.confidence << "speed=" << speed;
 		if (!offset.valid) {
 			continue;
 		}
@@ -1095,7 +1143,7 @@ void TimelineWidget::SynchronizeSelectedClipsByWaveform()
 				 << "valid=" << placement.valid << "timeline_in="
 				 << placement.timeline_in.toDouble();
 		if (placement.valid && placement.timeline_in >= 0) {
-			placements.append({ sync_clip.clip, placement.timeline_in });
+			placements.append({ sync_clip.clip, placement.timeline_in, speed });
 		}
 	}
 
@@ -1111,6 +1159,13 @@ void TimelineWidget::SynchronizeSelectedClipsByWaveform()
 	for (const SyncPlacement &placement : placements) {
 		command->add_child(new TrackReplaceBlockWithGapCommand(
 			placement.clip->track(), placement.clip, false));
+
+		if (placement.speed != 1.0) {
+			command->add_child(new NodeParamSetStandardValueCommand(
+				NodeKeyframeTrackReference(
+					NodeInput(placement.clip, ClipBlock::kSpeedInput)),
+				placement.clip->speed() * placement.speed));
+		}
 	}
 
 	TimelineWidgetSelections new_selections;
@@ -1120,9 +1175,15 @@ void TimelineWidget::SynchronizeSelectedClipsByWaveform()
 			placement.clip->track()->Index(), placement.clip,
 			placement.timeline_in));
 
+		// A speed change scales the clip's timeline length accordingly
+		const rational placed_length =
+			placement.speed == 1.0 ?
+				placement.clip->length() :
+				rational::fromDouble(placement.clip->length().toDouble() /
+									 placement.speed);
 		new_selections[placement.clip->track()->ToReference()].insert(
 			TimeRange(placement.timeline_in,
-					  placement.timeline_in + placement.clip->length()));
+					  placement.timeline_in + placed_length));
 	}
 
 	command->add_child(
@@ -1719,6 +1780,13 @@ void TimelineWidget::ShowContextMenu()
 		connect(sync_by_waveform, &QAction::triggered, this,
 				&TimelineWidget::SynchronizeSelectedClipsByWaveform);
 
+		QAction *sync_by_waveform_speed =
+			menu.addAction(tr("Synchronize by Waveform (Adjust Speed)"));
+		sync_by_waveform_speed->setEnabled(
+			GetSelectedWaveformSyncClips(selected).size() >= 2);
+		connect(sync_by_waveform_speed, &QAction::triggered, this,
+				&TimelineWidget::SynchronizeSelectedClipsByWaveformWithSpeed);
+
 		menu.addSeparator();
 
 		if (ClipBlock *clip = dynamic_cast<ClipBlock *>(selected.first())) {

app/widget/timelinewidget/timelinewidget.h

@@ -109,6 +109,8 @@ public:
 
 	void SynchronizeSelectedClipsByWaveform();
 
+	void SynchronizeSelectedClipsByWaveformWithSpeed();
+
 	void GenerateProxiesForSelectedClips();
 
 	void SetSelectedClipsProxyEnabled(bool enabled);
@@ -345,6 +347,8 @@ private:
 
 	bool PasteInternal(bool insert);
 
+	void SynchronizeSelectedClipsByWaveformInternal(bool allow_speed);
+
 	TimelineAndTrackView *AddTimelineAndTrackView(Qt::Alignment alignment);
 
 	QHash<Node *, Node *>

app/widget/timelinewidget/timelinewidgetwaveformsync.cpp

@@ -81,20 +81,26 @@ GetSelectedWaveformSyncClips(const QVector<Block *> &blocks)
 
 QVector<double> ExtractWaveformCacheEnvelope(const WaveformSyncClip &clip,
 											 int sample_rate,
-											 size_t window_samples)
+											 size_t window_samples,
+											 QVector<bool> *valid_mask)
 {
 	QVector<double> envelope;
 	if (sample_rate <= 0 || !window_samples) {
 		return envelope;
 	}
 
+	if (valid_mask) {
+		valid_mask->clear();
+	}
+
 	const rational window_time(static_cast<int>(window_samples), sample_rate);
 
 	// Only trust regions that have actually been validated. Unvalidated cache
 	// returns zero samples, which both drags the correlation score down and
 	// can produce false peaks if one clip happens to have more cached data
-	// than another. Using zero placeholders keeps every envelope aligned to
-	// the same absolute timeline.
+	// than another. Zero placeholders keep every envelope aligned to the same
+	// absolute timeline, while the validity mask lets the correlation skip
+	// those placeholders entirely.
 	const TimeRangeList validated_ranges =
 		clip.waveform->GetValidatedRanges().Intersects(clip.media_range);
 
@@ -103,8 +109,10 @@ QVector<double> ExtractWaveformCacheEnvelope(const WaveformSyncClip &clip,
 		const rational length = qMin(window_time, clip.media_range.out() - t);
 		const TimeRange window(t, t + length);
 
+		const bool window_valid = validated_ranges.contains(window);
+
 		double peak = 0.0;
-		if (validated_ranges.contains(window)) {
+		if (window_valid) {
 			const AudioVisualWaveform::Sample summary =
 				clip.waveform->GetSummaryFromTime(t, length);
 
@@ -118,6 +126,9 @@ QVector<double> ExtractWaveformCacheEnvelope(const WaveformSyncClip &clip,
 		}
 
 		envelope.append(peak);
+		if (valid_mask) {
+			valid_mask->append(window_valid);
+		}
 	}
 	return envelope;
 }

app/widget/timelinewidget/timelinewidgetwaveformsync.h

@@ -71,11 +71,15 @@ GetSelectedWaveformSyncClips(const QVector<Block *> &blocks);
  * @brief Extract a peak envelope from the validated regions of a waveform cache.
  *
  * Windows that have not been cached yet are filled with zero so that every
- * envelope stays aligned to the same absolute timeline.
+ * envelope stays aligned to the same absolute timeline; when @p valid_mask is
+ * provided it receives one flag per window marking whether the window was
+ * actually cached, allowing the correlation to skip uncached regions instead
+ * of treating them as silence.
  */
 QVector<double> ExtractWaveformCacheEnvelope(const WaveformSyncClip &clip,
 											 int sample_rate,
-											 size_t window_samples);
+											 size_t window_samples,
+											 QVector<bool> *valid_mask = nullptr);
 
 } // namespace TimelineWaveformSync