Commit add4e22d authored by Rocky Bernstein's avatar Rocky Bernstein

A start for what little public documentation there is for CVD and OGT

subtitles.
parent 5705ef28
$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.
$Id: svcd-ogt-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.
SVCD subtitles are transported in private streams with id 0x70 (AC3
has 0,x80+, 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)
Philips SVCD tools can create SVCD subtitles, but it only works under
NT4, (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.
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