Commit 5d9dc8f5 authored by Rocky Bernstein's avatar Rocky Bernstein

Revise. perhaps a little more detail too.

parent da74604a
$Id$
The following information is culled from information from
Julio Sanchez Fernandez (http://subhandler.sourceforge.net)
by Rocky Bernstein.
If you have any corrections or additions to make, please send them
to rocky@panix.com.
The following information is culled from information from Julio
Sanchez Fernandez [1] by Rocky Bernstein.
SVCD subtitles are transported in private streams with id 0x70 (AC3
has 0,x80+, CVD subs 0+)
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 (ntsc) or 720x576 (pal)
resolution of 720x480 for NTSC streams or 720x576 for PAL streams.
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)
Data packets start with a 4-byte header:
Subtitle Header (size in bytes):
* Stream number (1 byte). Only bits 0-3 are used as there can only
be up to 8 subtitles.
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.
* 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.
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
* Subtitle image number (2 bytes).
This only implies HOW LONG the subtitle will be shown, not when. That
is given by the PTS in the PES header.
* Length of the rest of the packet (described in the next section).
Uknown Byte(1)
================
SUBTITLE PACKETS
================
4 Palette Entries formated like:
Y (1)
U (1)
V (1)
Transparency(1) (0 = transparent)
The information below comes immediately after the packet header in the
same packet.
Command (1)
>>6: 1 shift command, (cmd>>4)&3 is the direction (0=top, 1=left, 2 = right, 3
>>= bottom)
* Packet length (2 bytes)
Odd-field data offset (2)
The assembled subtitle, with stripped sequence numbers
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.
* Option byte (1 byte)
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)
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)
>>6: 1 shift command, (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
thenumber 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, (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.
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.
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