These images are put over the top of the video frame when displayed. Much like the way characters on older text-based computer interfaces, they're actually just a collection of dots on a grid. They're actually encoded as raster graphics. As a matter of fact subtitles on DVDs aren't actually text. Although it's normal for all subtitles to start out this way, that doesn't mean that's how they're stored. Since text is also some of the easiest data to store and compress it makes sense to store subtitles as simple text files or a text stream within a video file. Images Most subtitles consist purely of text characters. ![]() Then we'll take a look at three specific subtitle formats VobSub, SubRip, and SubStation Alpha. First we'll be considering the difference between image and text based subtitles. But we'll still be considering two categories. For our purposes we'll be focusing on subtitles that exist as separate streams in a video container, or even separate files entirely. They can be hard coded or separate streams, depending on whether they're encoded as part of the video or simply muxed into the same container. ![]() They can be mandatory or selectable, depending on whether they can be turned off. Subtitle Categories There are many ways to categorize subtitles. With a little bit of reading it becomes easy to figure them out. Ironically, for most people subtitles are much easier to grasp than other multimedia elements. Even they're found on nearly every DVD, with the complexity of dealing with video, audio, and menus it's common for people to leave them for last, and often they get ignored completely. Subtitles may be the least understood aspect of digital video.
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