How Do Blu-Ray Subtitles Work?

by Justin Sevakis,

John asked:

What are the technical differences between creating subtitles for DVDs and Blu-ry Discs? And were HD-DVDs any better or worse?

Subtitles are a pain in the ass. No other part of making a DVD or Blu-ray is as complicated, error-prone, or as likely to be called out on mistakes by fans. They're also one of the hardest parts of making a DVD or a Blu-ray, due mostly to how complicated they are.

What all three of those formats had in common was that they get rendered as bitmap graphics, which then get layered on top of the video. DVD is so old that only four colors are used, and one of those colors has to be "clear" so that you can see the video from under it. One color needs to be black, for the outline of the text. Some discs use both colors for subtitles (yellow and green, for example), while others use one for anti-aliasing, to make the text look less jaggy. Subtitles are fed into the authoring program as a pile of either BMP or TIFF files, along with a timecoded script to note each image's start and end timecodes. Most TV anime runs about 400 subtitle graphics per episode.

Blu-ray and HD-DVD subtitles work pretty much the same way, except they can now utilize a 256 color palette, and are fed into authoring as transparent PNG files. Obviously, those formats also allow for a mix of resolutions and frame rates, so the subtitles have to be rendered in a way that matches the video.

One major issue is that only one of these subtitle graphics can be displayed at a time, so if there are any overlapping subtitles (say, a character holds up a sign while talking), they have to be chopped up into different graphics (i.e. line_0001.png has only the first subtitle, line_0002.png has both, and line_0003.png has only the second subtitle). There are myriad software packages out there that let you make subtitles and render them as graphics, and most are pretty terrible.

When a player reads a video track, the video, audio and subtitles are all "muxed" together into a Program Stream. The subtitles get temporarily loaded ("buffered") into RAM after they're loaded off the disc, to get ready to show on screen at the right time. Unfortunately, Blu-ray players were designed not to have enough RAM to accomplish this well, so if you have too many subtitles displaying too quickly, the whole thing goes to hell: the subtitle renderer on the player crashes, and starts displaying gibberish, or turns off entirely. (Some DVD players have a similar issue, as Right Stuf discovered when they made their epic His and Her Circumstances DVDs.) There's also a related issue with subtitles blinking every time they refresh, which can be worked around, with great difficulty.

Blu-rays were originally supposed to fix all of this by allowing text script subtitles instead of graphics based ones, but that feature ended up not working at all, and no disc that I know of uses it. Did I mention that Blu-ray is an ungodly mess of a format? For what it's worth, I have no idea if HD-DVD had these issues -- that format didn't last long enough for anybody to start pushing the envelope.

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    Justin Sevakis has worked in the anime business for over 20 years. He's the original founder of Anime News Network, and owner of the video production company MediaOCD. You can follow him on Twitter at @worldofcrap.

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