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Revise. perhaps a little more detail too.
This commit is contained in:
Rocky Bernstein
parent
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commit
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@ -1,68 +1,118 @@
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$Id$
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The following information is culled from information from
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Julio Sanchez Fernandez (http://subhandler.sourceforge.net)
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by Rocky Bernstein.
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If you have any corrections or additions to make, please send them
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to rocky@panix.com.
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The following information is culled from information from Julio
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Sanchez Fernandez [1] by Rocky Bernstein.
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SVCD subtitles are transported in private streams with id 0x70 (AC3
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has 0,x80+, CVD subs 0+)
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SVCD subtitles (Philips Overlay Graphics Text or OGT) are transported
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in an MPEG private stream with stream id 0x70. AC3 subtitle stream
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numbers start at 0,x80 and go to 0,x87. (CVD subtitle stream numbers
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start at stream ID 0 and go to 3.)
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All numbers are in big-endian and the image is interlaced with a
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resolution of 720x480 (ntsc) or 720x576 (pal)
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resolution of 720x480 for NTSC streams or 720x576 for PAL streams.
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Data packets start with a 4 byte sequence number (Stream number(1),
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Packet number(1*), Subtitle number (2)) (*) this starts at 0, the last
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packet of a subtitle has the 7th bit set (the subtitle can be broken
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up into several packets when it is multiplexed into the mpeg stream)
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Data packets start with a 4-byte header:
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Subtitle Header (size in bytes):
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* Stream number (1 byte). Only bits 0-3 are used as there can only
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be up to 8 subtitles.
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Packet length (2) (the assembled subtitle, with stripped sequence
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numbers) Option byte (1) normally set to 0x2e, bit 3 indicates that a
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field with the display-time of the subtitle is present. what the
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other bits does is unknown.
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* Packet number(1 byte). The first packet starts at 0; the last
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packet of a subtitle has the 7th bit set. A subtitle can be
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broken up into several packets when it is multiplexed into the
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MPEG stream.
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Time to display the sub (4) in 1/90000'th of a second (only present if
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bit 3 above is set)
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Xpos (2)
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Ypos (2)
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Width (2*)
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Height (2*)
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* should be even
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* Subtitle image number (2 bytes).
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This only implies HOW LONG the subtitle will be shown, not when. That
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is given by the PTS in the PES header.
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* Length of the rest of the packet (described in the next section).
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Uknown Byte(1)
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================
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SUBTITLE PACKETS
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================
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4 Palette Entries formated like:
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Y (1)
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U (1)
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V (1)
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Transparency(1) (0 = transparent)
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The information below comes immediately after the packet header in the
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same packet.
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Command (1)
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>>6: 1 shift command, (cmd>>4)&3 is the direction (0=top, 1=left, 2 = right, 3
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>>= bottom)
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* Packet length (2 bytes)
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Odd-field data offset (2)
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The assembled subtitle, with stripped sequence numbers
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Then image data follows, and is formatted like this:
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2 bit codes are used so 01 means 1 pixel of color 1,
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10 means color 2 etc.
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* Option byte (1 byte)
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Lines are padded to complete bytes, field 0 is padded to an even byte
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length and finally subtitles are padded to a multiple of 4 bytes
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(always atleast 1 zero byte at the end)
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This is normally set to 0x2e; bit 3 indicates that a field with the
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display-time of the subtitle is present. What the other bits do is not
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known.
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* Time to display the subtitle (4 bytes)
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This in 1/90000'th of a second. This is only present if bit 3 above is
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set. This value specifies how long the subtitle will be shown, not
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when. When the subtitle is to be shown is specified in the PTS in the
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PES header.
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* X position (2 bytes)
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* Y position (2 bytes)
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* Width (2 bytes)
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This value must be even.
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* Height (2 bytes)
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This value must be even.
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* Unknown (1 byte)
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* 4 Color Palette Entries (16 bytes)
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* A pallette entry has the following format:
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* Y (1 byte)
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* U (1 byte)
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* V (1 byte)
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* Transparency (1 byte). A 0 value means totally transparent and
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255 totally opaque.
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* Command (1 byte)
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>>6: 1 shift command, (cmd>>4)&3 is the direction (0=top, 1=left, 2 = right, 3 >>= bottom)
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* Odd-field data offset (2 bytes)
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Image data is stored interlaced with the even lines first. This offset
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specifies where the first odd-line image data starts.
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IMAGE DATA
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Then subtitle image data follows the above control information. Image
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lines are interlaced. That is line 0 is comes first, then line 2, then
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line 4. After all of the even lines, comes line 1, then line 3,
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etc.. The subtitle aspect ratio is not to be scaled. It is independent
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of the aspect ratio of the underlying image which it is to be
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overlayed on top of.
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The image is encoded using two bits per pixel that select a palette
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entry except that value 0 starts a limited run-length encoding for
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color 0. When 0 is seen, the next two bits encode one less than
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thenumber of pixels, so we can encode run lengths from 1 to 4. These
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get filled with the color in palette entry 0.
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The encoding of each line is padded to a whole number of bytes. The
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first field is padded to an even byte length and the complete subtitle
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is padded to a 4-byte multiple that always include one zero byte at
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the end.
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================
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SUBTITLING TOOLS
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================
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Philips SVCD tools can create SVCD subtitles, but it only works under
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NT4, (and not that easy to find anymore.. dvd.da.ru, doom9.net or
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similar perhaps?), it can create good subtitled svcds and just open
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them in your favorite hex editor ^_^
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NT4. It is not that easy to find anymore. (dvd.da.ru, doom9.net or
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similar perhaps?). It can create good subtitled SVCDs. Subtitles are
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created in a text editor. Another tool is called submux. Last version
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I tried doesn't do renumber presentation time stamps properly, but
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many media players (now) can tolerate this.
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The subs you get from tools like "I-author" are not real SVCD
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subtitles, they are CVD sub and I don't know anything else about them
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except for that they look similar to dvd subs but the RLE encoding and
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command format is different.
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The subtitles you get from tools like "I-author" are not SVCD
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subtitles, they are CVD subtitles. They look similar to DVD subtitles,
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but the run-length encoding (RLE) and command format are different.
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