[MPlayer-dev-eng] Closed captions

[Bryan Henderson]

On 6/1/08, Uoti Urpala <uoti.urpala at pp1.inet.fi> wrote:
> On Sat, 2008-05-31 at 03:17 +0000, Bryan Henderson wrote:
>> Another difference with the existing DVD CC function is that it
>> renders the captioning as subtitles.  There's a lot of difference
>> between CC and subtitles; you lose a lot in the translation.
>
> What kind of differences are you talking about?

Subtitles are little more than lines of text.  The input stream
describes the text that has to go on the screen, and when, and Mplayer
(with the user) decides how to put it there.

NTSC Closed Caption commands are low level display commands, like "put
the character 'I' at column 7, Row 2."  Or "erase the screen."  They
control attributes of each character including italic, blinking,
underscore, and foreground and background color.  There's a mode where
text scrolls up smoothly in a window.  Because the characters are
addressable, the font has to be a certain size and fixed-width.
There's a particular, somewhat unusual, character set and code.

NTSC Closed Captions and Teletext are alike in this way.

> And which subtitle format?
Is there something you couldn't translate to a subtitle format
> like SSA/ASS?

The DVD CC code doesn't render the closed caption information in a
subtitle file format; it feeds the information to the Mplayer
'subtitle' object and the subtitle OSD object takes it from there and
puts it on the screen.  In order to use that facility, it essentially
reads the characters being drawn on the screen and forms simple lines
of text out of them, discarding much of the information.

>Is there something you couldn't translate to a subtitle format
>like SSA/ASS?

I don't know anything about SSA/ASS, but assuming it can represent all
the information in a Closed Caption stream, how does that relate to
getting Mplayer to display CC information from the MPEG source?

BTW, I know little about subtitles.  I've played a little with SRT
because there's a program (ccextractor) that converts IVTV embedded
MPEG closed caption (the format I'm working with) to an SRT file --
with much loss, of course.