This Is Not An Elaborately Large Quote I Am Just Writing Some TL;DR About Subtitle Formats To Explain Things As Requested By The Masses
While speaking to Eric over at Siren Visual and my bro Shadow Wolf at Supanova this weekend, the topic of various subtitle formats and how they impact visual typesetting and typography came up. Today, I’m going to be writing about the three main ways you could classify subtitle formats and how they work; namely, text-based subtitles, DVD IDX/SUB format, and the two (yes two!) BD subtitle formats, PGS/SUP and TTXML.
I’m super lazy and there is so much variation in test-based subtitling that I’ll sorta skim over this. Text-based subtitles are formats such as SRT or SSA. They are common in the ripping and fansubbing communities mostly for their ability to be turned on and off at will. Some are more complex than others. ASS (SSA V4+) for example is capable of rendering full text effects as well as vector graphics and with a container like Matroska, it can be packaged with fonts and used for full typography and visual effects as well as the subtitles themselves. SRT on the other hand is a much more basic format, it just stores lines with their times. There are many formats like this and they are used in many places and so I leave further research to the reader as this post is mostly aimed at DVD and Blu-Ray.
DVD uses a format that can be referred to as either IDX, Sub, or VobSub after the horrible renderer for Windows it used to have. DVD SUB uses 4 colours in a raw bitmap storage format. That said, the way your DVD player displays each of those colours is up to the manufacturer as the palette has 16 colours, and there are a further 16 contrast values, also with only 4 that can be used. The four colours are for the background, foreground, outline, and shadow. The background colour is generally an alpha field and so your sub-picture will overlay onto your actual video with transparency, and not covering it. Common ‘fill’ colours are white, yellow, and pale blue, while black is the most common outline. Most players have a transparent shadow, although black is fairly common too.
Blu-Ray on the other hand is a bit better. It uses 24-bit colours in its sub-picture format. This allows for a rather unique ability on a Blu-Ray, if anyone was to take advantage of it (Siren Visual I am looking at you.) Given a BD disk has so much space on it, yet most times that space isn’t utilised even CLOSE to fully, one could take advantage of this 24-bit colour (+alpha) to render full ‘soft’ typesetting onto the video. A studio could open up their compositing application of choice, do their thing, and then output a PNG sequence. Convert that to SUP, mux, and you now have full soft-sub typesetting on a BD release. I have yet to see ANYONE in the industry typeset at all, regardless of method, so this would be a real bonus on release quality.
The second format Blu-Ray has, and it seems a lot of people don’t know about this, is TTXML. TTXML shouldn’t be confused with the MP4 format’s Timed Text, which is usually referred to as TTXT. TTXML is a format mostly defined by Adobe, although barely supported by any software I have seen including Adobe’s own Flash player. It is a text-based format similar to Ogg Kate or SSA, only using XML. It is rather basic and from what I can tell (limited spec) it has no vectoring capability, but I assume SVG incorporation isn’t too difficult. It is capable of the general font selection, bolding, styling like outline and shadow, stretching, and basic text animation effects like karaoke by it’s time function, quite similar to the ASS \t flag if more basic. I have no idea if many hardware Blu-Ray players support this format, but I’m just putting it out there that it exists.
This concludes me writing walls of text about subtitles, it’s 11:48PM and my fingers are freezing. We’ll see if anyone takes interest in the Australian Blu-Ray Industry.
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That wasn’t as long as I was expecting. :P
About the typesetting: does SUP allow mid-alpha values, or was it just entirely on or off? Because that could mildly affect the ease of typesetting depending on the scene…
Honestly, I have no idea. I’ve never actually tried it myself. Guess I should do so. You could always render onto your vid itself if need be, or do some fancy masking.