What kind of subtitles to use?
What standard do you think we should use?
Since Lib-Ray will be an international standard, languages and localization are going to be a design challenge. The most obvious issue is that Lib-Ray releases will need to accommodate subtitle tracks in an adequate format.
There are multiple formats for subtitles, and this is something that is going to take some more thought before being completely settled. My first choice was to use the "Kate" standard (or "OggKate") which is a very capable format that supports text and vector graphics. By far the most widely-used, though, is "SubRip" or "SRT" format, which is a very basic (unicode) text format.
There is also a format called WebVTT which is associated with the web standards and HTML5 (and is very similar to SRT in syntax). And I have heard of other formats as well.
Of course, whatever standard is used, there is software to convert between different formats, so the main issue is what each format can represent. Some allow for font, position, and color changes (all supported by Kate), while SRT supports only plain text.
Another question is whether subtitles should be embedded in the media file itself, or provided in separate files. For Kate format, it probably makes sense to put them in the media file. Either is possible with SRT format, but separate files are probably more common. WebVTT is apparently expected to be in external files.
From the point of view of making the media file more transportable on its own, it's better to put the subtitles into it. On the other hand, this makes them harder to update, and with the decision to switch to read-write Flash media, the idea of making subtitles easier to patch is an attractive advantage to providing them in separate files.
It seems to me that this is too wide open to leave unspecified, but it may be necessary to include more than one option.
The following article shows how I dealt with subtitles in my 0.1 and 0.2 prototypes of "Sintel", where they were encoded as an OggKate stream for inclusion within the video player. These worked fine in VLC, which I was using for testing at the time.
Creating Subtitles from SRT Sources for an Ogg Video with kateenc
Originally published in Free Software Magazine:: http://fsmsh.com/3540
Sun, 2011-05-01 22:18 -- Terry Hancock
One of the more interesting aspects of Ogg Video is that it allows an essentially unlimited number of subtitle tracks to be included. This is especially useful for free-culture videos, since they are generally released globally, and there are often contributed subtitles. In fact, for "Sintel", I was able to find 44 subtitle files. I will be including them all as Ogg Kate streams in my prototype "Lib-Ray" version of "Sintel", and in this column I will demonstrate the use of several command line utilities useful for this, especially the kateenc tool for creating the streams.
The main source for the "Sintel" material is a download directory on the Xiph.org website. If you've been following this series, you'll recognize this as the same site I got the PNG frames and audio soundtracks from. This site also has the original .srt format subtitle files for the nine languages that are included on the DVD version of Sintel.
Since then, however, 36 additional .srt files have been provided by the community, for a total of 45 different subtitle tracks. These are collected at a different site. I'll be using all of these.
There are also Ogg streams to carry subtitles. The most popular, and the one I'm going to be using, is Kate. As with Theora and FLAC, the command line tool for manipulating this format (kateenc) is included in the Debian archive (as part of the libkate-tools package).
There are also Ogg streams to carry subtitles. The most popular, and the one I'm going to be using, is Kate
The first problem I encounter with the .srt files from Sintel is that they do not use consistent encodings (I didn't realize this until kateenc choked on some of the files -- it expects UTF-8 encoded files!). Using the file command, I can see this right away:
sintel_afr.srt: ISO-8859 text, with CRLF line terminators
sintel_ar.srt: ISO-8859 text, with CRLF line terminators
sintel_bg.srt: ISO-8859 text, with CRLF line terminators
sintel_bn.srt: UTF-8 Unicode text
sintel_chs.srt: ISO-8859 text, with CRLF line terminators
sintel_cn.srt: Little-endian UTF-16 Unicode text, with CRLF, CR line terminators
sintel_cz.srt: UTF-8 Unicode text
sintel_da.srt: ISO-8859 English text, with CRLF line terminators
sintel_eo.srt: UTF-8 Unicode (with BOM) text, with CRLF line terminators
I started by collecting these into directories for each of the major encodings, but unfortunately, this is a little hard to untangle. In particular, the ISO-8859 encoded files use different code pages according to language, so you have to recognize the language codes in the file names (or look them up in an ISO-639 language code table) and figure out the correct page. For example, ar is the code for Arabic, and this means that we should decode from ISO-8859-6. Or so I thought -- actually attempting this results in an error. Opening the file up in Iceweasel, I noticed some weird characters that didn't make sense. It provides some other options for encoding, and with "Windows-1256" it actually looked like Arabic. So, that's what I'll use (in fact, it turned out that the majority of these files that file identified as ISO-8859 were actually in one of various Windows encodings -- they were probably submitted in the default encoding of the user who contributed the translations).
This can be converted with the iconv command line tool:
$ iconv -f WINDOWS-1256 -t UTF-8 sintel_ar.srt -o ../UTF-8/sintel_ar.srt
I didn't figure out any way to automate this, so I just went through the files one-by-one to convert the encodings appropriately (fortunately, they were not all this hard). I'm not going to go through this in detail, but in the end, I had all 44 of my subtitle files in one directory with UTF-8 encodings.
At this point, a tcsh loop is handy for processing the files in bulk to get my Ogg Kate streams:
> foreach lang ( af ar bg bn cz da de en eo es fi_ep fi_FI fi_ps fr gl gr he hr hu id it jp ko ku la lv mk ml nb_NO nl pa pl pt ro ru sk sr th tr uk vi zh_CN zh zh_TW )
foreach? echo $lang
foreach? kateenc -t srt -c SUB -l $lang -o ../../OGG/sintel_$lang.ogg sintel_$lang.srt
Note that by providing this code to kateenc via the -l option, it will identify the subtitle track correctly by language. This will allow the player to identify the tracks correctly by language (I tested this in VLC, and it provides the full-name of each language in that language on the subtitles pull-down menu for the user to select from).
Fixing bugs in the SRT files
At this point I must be honest -- many of these .srt files had bugs. As a result, I got quite a number of syntax warnings from kateenc. I had to go back and fix a lot of these files in order to get them to work smoothly.
I had to go back and fix a lot of these files in order to get them to work smoothly
When everything is working smoothly, the code above will simply list the language/country code extensions. If something goes wrong, though, the echo line will tell which language files was being processed, so you can check it out.
Some of the errors I found on inspection:
- Two subtitle blocks run together without an intervening blank line (kateenc skips the second one and then complains about "non-consecutive ids"). Fix by adding the blank line.
- Extra blank lines inside a subtitle block, resulting in a syntax error.
- Incorrect time codes (typos?) resulting in nonsensical time intervals (reversed, or of zero-length)
- Non-standard annotations, such as author information. These can be converted into normal subtitles and placed at the end (they'll appear near the end titles), thus preserving the correct attribution.
- Unicode byte-order marks at the beginning seem to confuse kateenc in a couple of cases, so I just removed them.
At this point we have 44 Ogg files with Kate streams in them. Combined with the audio and video streams from before, we'll be ready to assemble them into a single multimedia file, which will be the goal of my next column.
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