$Id: cvd-subtitles.txt,v 1.1 2004/01/04 16:25:00 rocky Exp $
The following information is culled from information from 
Julio Sanchez Fernandez (http://subhandler.sourceforge.net)
by Rocky Bernstein. 

We do not have information on the subtitle format used on CVD's except
the submux sample code and a couple of samples of dubious
origin. Thus, the information below is result of reading some code
whose correctness is not known and some experimentation.

CVD subtitles are different in several ways from SVCD OGT subtitles
(see see corresponding info on that.)

Image comes first and meta data is at the end.  So that the metadata
can be found easily, the subtitle packet starts with two bytes
(everything is big-endian again) that gives the total size of the
subtitle data and the offset to the metadata - i.e. size of the image
data plus the four bytes at the beginning.

Data for single screen subtitle may come in several non-contiguous
packets of a stream. From the scant data on the format, there is only
only way known to detect the first packet in a subtitle.  The first
packet seems to have a valid PTS while later packets for the same
image don't.

Image data comes interlaced and is run-length encoded (RLE). Each
field is a four-bit nibbles that is further subdivided in a two-bit
repeat count and a two-bit color number - up to three pixels can be
described in four bits.  What a 0 repeat count means is unknown.  It
might be used for RLE extension.  There is a special case of a 0
repeat count though.  When the full nibble is zero, the rest of the
line is filled with the color value in the next nibble.  It is unknown
what happens if the color value is greater than three.  The rest seems
to use a 4-entries palette.  It is not impossible that the fill-line
complete case above is not as described and the zero repeat count
means fill line.  The sample code never produces this, so it may be
untested.

Here is information given at the start of a subtitle:

SPU size        2 bytes
metadata offset 2 bytes

Although metadata information does not have to come in a fixed field
order, every metadata field consists of a tag byte followed by
parameters. In all cases known, the size including the tag byte is
exactly four bytes.

code    Meaning 
----    -------
0x0c 	Unknown

0x04 	24-bit subtitle duration in 1/90000ths of a second

0x17	upper left x, y position, each a 10-bit value, encoded:
           x = ((p[1]&0x0f)<<6) + (p[2]>>2)
           y = ((p[2]&0x03)<<8) + p[3];

0x1f	lower right x, y postion, each a 10-bit (0-1023) value,
        encoded as above 

0x24    3 bytes primary palette 0 - 1 byte for each of y, u, and v 
0x25    3 bytes primary palette 1 - 1 byte for each of y, u  and v
0x26    3 bytes primary palette 2 - 1 byte for each of y, u  and v
0x27    3 bytes primary palette 3 - 1 byte for each of y, u  and v

0x2c    3 bytes highlight palette 0 - 1 byte for each of y, u, and v 
0x2d    3 bytes highlight palette 1 - 1 byte for each of y, u  and v
0x2e    3 bytes highlight palette 2 - 1 byte for each of y, u  and v
0x2f    3 bytes highlight palette 3 - 1 byte for each of y, u  and v

0x37    3 bytes transparancy for primary palette - 1 byte for each 
	of y, u  and v

0x3f    3 bytes transparancy for highlight palette - 1 byte for each 
	of y, u  and v

0x47    Offset to start of even rows of interlaced image.
0x4f    Offset to start of odd rows of interlaced image.