[MPlayer-dev-eng] SVCD subtitles ?

[Bjorn Stahl]

Well, here goes:

SVCD subtitles are transported in private streams with id 0x70 (AC3 has 0,x80+, DVD subs 0x20+ and CVD subs 0+)
All numbers are in big-endian and the image is interlaced with a resolution of 720x480 (ntsc) or 720x576 (pal)

Data packets start with a 4 byte sequence number (Stream number(1), Packet number(1*), Subtitle number (2))
(*) this starts at 0, the last packet of a subtitle has the 7th bit set (the subtitle can be broken up into several packets
when it is multiplexed into the mpeg stream)

Subtitle Header (size in bytes):
Packet length (2) (the assembled subtitle, with stripped sequence numbers)
Option byte (1) normally set to 0x2e, bit 3 indicates that a field with the display-time of the subtitle is present.
what the other bits does is unknown.

Time to display the sub (4) in 1/90000'th of a second (only present if bit 3 above is set)
Xpos (2)
Ypos (2)
Width (2*)
Height (2*)
* should be even

This only implies HOW LONG the subtitle will be shown, not when. That is given by the PTS in the PES header.

Uknown Byte(1) 

4 Palette Entries formated like:
Y (1)
U (1)
V (1)
Transparency(1)    (0 = transparent)

Command (1) 
>>6: 1 shift command, (cmd>>4)&3 is the direction (0=top, 1=left, 2 = right, 3 = bottom)

Odd-field data offset (2)

Then image data follows, and is formatted like this:
2 bit codes are used so 01 means 1 pixel of color 1,
10 means color 2 etc.

Lines are padded to complete bytes, field 0 is padded to an even byte length and finally subtitles are padded
to a multiple of 4 bytes (always atleast 1 zero byte at the end)

If anyone is interested in finding out more, take a look at phillips svcd tools (only works under NT4 afaik, (and not that easy to find anymore.. dvd.da.ru, doom9.net or similar perhaps?), it can create good subtitled svcds and just open them in your favorite hex editor ^_^

The subs you get from tools like "I-author" are not real SVCD subtitles, they are CVD sub and I don't know anything else about them except for that they look similar to dvd subs but the RLE encoding and command format is different.