Subtitler in Qt

A certain company needed a way to easily get text-based subtitle files in a batch fashion. The DVDs were available for ripping such subtitles, but the necessary OCR work proved too time-consuming and cumbersome, even with solutions like SubRip. So, I figured that 1000s of individuals on the Internet already gladly spend their days OCRing DVD subtitles into text formats, and sure enough databases like Subscene exist. So, I built a program to wrap Subscene’s web interface, and what better way to do this is there than QtWebKit?

I capture the downloads by using QtWebKit’s unsupportedContent signal and parse the downloads by using QNetworkAccessManager’s finished signal. I then run the zip archive through QtIOCompressor‘s code for unzipping zips, then extract and parse the subtitles.

A significant issue is that a lot of the subtitles on Subscene are from torrents that have videos which are cut off at the beginning. The solution is to shift each subtitle in the file by a constant time interval if neccessary, so I built an interface to easily see and shift timing:

As you can see, it’s possible to shift all the subtitles. Phonon made this page a breeze. One significant challenge was writing a search algorithm that would find the correct Subtitle based on a time falling in a certain range. It needed to be super speedy and designed for sequential searches. I came up with the following, which could use a bit of improvement:

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QString SubtitleParser::subtitleAtTime(quint64 time)
{
	QLinkedList<Subtitle>::const_iterator i;
	bool forward = true;
	for (i = m_searchPosition; i != m_subtitles.constEnd(); ) {
		if (i->isContainedIn(time)) {
			m_searchPosition = i;
			return i->text();
		}
		if (i->isAfter(time)) {
			if (i == m_subtitles.constBegin())
				break;
			--i;
			forward = false;
		} else {
			if (!forward)
				break;
			++i;
		}
	}
	return QString(' ');
}

Another challenge was dealing with QWizard and QWizardPage, particularly issues with fields and custom buttons. The field/property system is just too cumbersome to work with; there ought to be a better way of establishing centralized data with straight up pointers. The buttons on QWizard all want to be global, so adding a custom button to a single page proved a bit hackish. It’s a cool API, but just a little bit limiting. If anyone has some suggestions, let me know.

As always, the code is available on my gitweb, and I encourage you to try it out, read the code, and send me suggestions in the comments below.

Though this was designed to work on Linux, Qt, being delightfully cross-platform, also makes it possible to run the Subtitler on OSX. Check out the OSX screencast:

Direct YouTube Link