$Id$

The following information is culled from information from Julio
Sanchez Fernandez [1] by Rocky Bernstein.

SVCD subtitles (Philips Overlay Graphics Text or OGT) are transported
in an MPEG private stream with stream id 0x70. AC3 subtitle stream
numbers start at 0,x80 and go to 0,x87. (CVD subtitle stream numbers
start at stream ID 0 and go to 3.)

All numbers are in big-endian and the image is interlaced with a
resolution of 720x480 for NTSC streams or 720x576 for PAL streams.

Data packets start with a 4-byte header:

    * Stream number (1 byte). Only bits 0-3 are used as there can only
      be up to 8 subtitles.

    * Packet number(1 byte). The first packet starts at 0; the last
      packet of a subtitle has the 7th bit set. A subtitle can be
      broken up into several packets when it is multiplexed into the
      MPEG stream.

    * Subtitle image number (2 bytes).

    * Length of the rest of the packet (described in the next section). 

================
SUBTITLE PACKETS
================

The information below comes immediately after the packet header in the
same packet.

* Packet length (2 bytes) 

The assembled subtitle, with stripped sequence numbers

* Option byte (1 byte) 

This is normally set to 0x2e; bit 3 indicates that a field with the
display-time of the subtitle is present. What the other bits do is not
known.

* Time to display the subtitle (4 bytes) 

This in 1/90000'th of a second. This is only present if bit 3 above is
set. This value specifies how long the subtitle will be shown, not
when. When the subtitle is to be shown is specified in the PTS in the
PES header.

* X position (2 bytes) 

* Y position (2 bytes) 

* Width (2 bytes)

This value must be even.

* Height (2 bytes)

This value must be even.

* Unknown (1 byte) 

* 4 Color Palette Entries (16 bytes)

* A pallette entry has the following format:

    * Y (1 byte)
    * U (1 byte)
    * V (1 byte)
    * Transparency (1 byte). A 0 value means totally transparent and
      255 totally opaque.

* Command (1 byte) 

if cmd>>6 == 1 then a shift command and (cmd>>4)&3 is the
direction. 0=top, 1=left, 2 = right, 3 = bottom

* Odd-field data offset (2 bytes) 

Image data is stored interlaced with the even lines first. This offset
specifies where the first odd-line image data starts.

IMAGE DATA

Then subtitle image data follows the above control information. Image
lines are interlaced. That is line 0 is comes first, then line 2, then
line 4. After all of the even lines, comes line 1, then line 3,
etc.. The subtitle aspect ratio is not to be scaled. It is independent
of the aspect ratio of the underlying image which it is to be
overlayed on top of.

The image is encoded using two bits per pixel that select a palette
entry except that value 0 starts a limited run-length encoding for
color 0. When 0 is seen, the next two bits encode one less than
the number of pixels, so we can encode run lengths from 1 to 4. These
get filled with the color in palette entry 0.

The encoding of each line is padded to a whole number of bytes. The
first field is padded to an even byte length and the complete subtitle
is padded to a 4-byte multiple that always include one zero byte at
the end.

================
SUBTITLING TOOLS
================

Philips SVCD tools can create SVCD subtitles, but it only works under
NT4. It is not that easy to find anymore. (dvd.da.ru, doom9.net or
similar perhaps?). It can create good subtitled SVCDs. Subtitles are
created in a text editor. Another tool is called submux. Last version
I tried doesn't do renumber presentation time stamps properly, but
many media players (now) can tolerate this.

The subtitles you get from tools like "I-author" are not SVCD
subtitles, they are CVD subtitles. They look similar to DVD subtitles,
but the run-length encoding (RLE) and command format are different.