Dive media: Extract thumbnails from videos with ffmpeg

Extract thumbnails using ffmpeg.
Behavior is controlled by three new preferences fields:
 - extract_video_thumbnails (bool): if true, thumbnails are calculated.
 - extract_video_thumbnail_position (int 0..100): position in video
   where thumbnail is fetched.
 - ffmpeg_executable (string): path of ffmpeg executable.

If ffmpeg refuses to start, extract_video_thumbnails is set to false
to avoid unnecessary churn.

Video thumbnails are marked by an overlay.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/videoframeextractor.cpp

@@ -0,0 +1,120 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "videoframeextractor.h"
+#include "imagedownloader.h"
+#include "core/pref.h"
+#include "core/dive.h"		// for report_error()!
+
+#include <QtConcurrent>
+#include <QProcess>
+
+// Note: this is a global instead of a function-local variable on purpose.
+// We don't want this to be generated in a different thread context if
+// VideoFrameExtractor::instance() is called from a worker thread.
+static VideoFrameExtractor frameExtractor;
+VideoFrameExtractor *VideoFrameExtractor::instance()
+{
+	return &frameExtractor;
+}
+
+VideoFrameExtractor::VideoFrameExtractor()
+{
+	// Currently, we only process one video at a time.
+	// Eventually, we might want to increase this value.
+	pool.setMaxThreadCount(1);
+}
+
+void VideoFrameExtractor::extract(QString originalFilename, QString filename, duration_t duration)
+{
+	QMutexLocker l(&lock);
+	if (!workingOn.contains(originalFilename)) {
+		// We are not currently extracting this video - add it to the list.
+		workingOn.insert(originalFilename, QtConcurrent::run(&pool, [this, originalFilename, filename, duration]()
+				 { processItem(originalFilename, filename, duration); }));
+	}
+}
+
+void VideoFrameExtractor::fail(const QString &originalFilename, duration_t duration, bool isInvalid)
+{
+	if (isInvalid)
+		emit invalid(originalFilename, duration);
+	else
+		emit failed(originalFilename, duration);
+	QMutexLocker l(&lock);
+	workingOn.remove(originalFilename);
+}
+
+void VideoFrameExtractor::clearWorkQueue()
+{
+	QMutexLocker l(&lock);
+	for (auto it = workingOn.begin(); it != workingOn.end(); ++it)
+		it->cancel();
+	workingOn.clear();
+}
+
+// Trivial helper: bring value into given range
+template <typename T>
+T clamp(T v, T lo, T hi)
+{
+	return v < lo ? lo : v > hi ? hi : v;
+}
+
+void VideoFrameExtractor::processItem(QString originalFilename, QString filename, duration_t duration)
+{
+	// If video frame extraction is turned off (e.g. because we failed to start ffmpeg),
+	// abort immediately.
+	if (!prefs.extract_video_thumbnails) {
+		QMutexLocker l(&lock);
+		workingOn.remove(originalFilename);
+		return;
+	}
+
+	// Determine the time where we want to extract the image.
+	// If the duration is < 10 sec, just snap the first frame
+	duration_t position = { 0 };
+	if (duration.seconds > 10) {
+		// We round to second-precision. To be sure that we don't attempt reading past the
+		// video's end, round down by one second.
+		--duration.seconds;
+		position.seconds = clamp(duration.seconds * prefs.extract_video_thumbnails_position / 100,
+					 0, duration.seconds);
+	}
+	QString posString = QString("%1:%2:%3").arg(position.seconds / 3600, 2, 10, QChar('0'))
+					       .arg((position.seconds % 3600) / 60, 2, 10, QChar('0'))
+					       .arg(position.seconds % 60, 2, 10, QChar('0'));
+
+	QProcess ffmpeg;
+	ffmpeg.start(prefs.ffmpeg_executable, QStringList {
+		"-ss", posString, "-i", filename, "-vframes", "1", "-q:v", "2", "-f", "image2", "-"
+	});
+	if (!ffmpeg.waitForStarted()) {
+		// Since we couldn't sart ffmpeg, turn off thumbnailing
+		// TODO: call the proper preferences-functions
+		prefs.extract_video_thumbnails = false;
+		report_error(qPrintable(tr("ffmpeg failed to start - video thumbnail creation suspended")));
+		qDebug() << "Failed to start ffmpeg";
+		return fail(originalFilename, duration, false);
+	}
+	if (!ffmpeg.waitForFinished()) {
+		qDebug() << "Failed waiting for ffmpeg";
+		report_error(qPrintable(tr("failed waiting for ffmpeg - video thumbnail creation suspended")));
+		return fail(originalFilename, duration, false);
+	}
+
+	QByteArray data = ffmpeg.readAll();
+	QImage img;
+	img.loadFromData(data);
+	if (img.isNull()) {
+		qInfo() << "Failed reading ffmpeg output";
+		// For debugging:
+		//qInfo() << "stdout: " << QString::fromUtf8(data);
+		ffmpeg.setReadChannel(QProcess::StandardError);
+		// For debugging:
+		//QByteArray stderr_output = ffmpeg.readAll();
+		//qInfo() << "stderr: " << QString::fromUtf8(stderr_output);
+		return fail(originalFilename, duration, true);
+	}
+
+	emit extracted(originalFilename, img, duration, position);
+	QMutexLocker l(&lock);
+	workingOn.remove(originalFilename);
+}