=head1 CVD (Chaoji VCD) Subtitle Information

The following information is culled from information from Julio
Sánchez Fernández (L<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.

Please, if you have any corrections or additions to make, send
them to rocky@panix.com.

CVD subtitles are different in several ways from SVCD OGT subtitles.
See
L<http://www.vcdimager.org/pub/vcdimager/manuals/0.7/svcd-ogt-subtitles.html>
for information on SVCD subtitles.

The image data 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.

As with OGT subtitles, Data for single screen subtitle may come in
several non-contiguous packets of a stream.  From the scant data on
the format, the 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.

The 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 not known.  It
might be used for RLE extension.  There is a special case of a 0
repeat count that has been inferred: 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 possible that the
fill-line general case above is not as described and the zero repeat
count means fill line.  But since this hasn't been ever seen, the
preceding is only conjecture.

=head2 Format of a Subtitle Header

Here is information given at the start of a subtitle:

  SPU size        2 bytes
  metadata offset 2 bytes

Although metadata information does come in a fixed field order, every
metadata field consists of a tag byte followed by parameters. In all
cases seen, 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 position, 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 transparency for primary palette - 1 byte for each 
  	  of y, u  and v

  0x3f    3 bytes transparency 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.

I<$Id: cvd-subtitles.pod,v 1.3 2006/02/07 02:57:35 rocky Exp $>
