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120 lines
3.8 KiB
Plaintext
120 lines
3.8 KiB
Plaintext
$Id$
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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 (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 for NTSC streams or 720x576 for PAL streams.
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Data packets start with a 4-byte header:
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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 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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* Subtitle image number (2 bytes).
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* Length of the rest of the packet (described in the next section).
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================
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SUBTITLE PACKETS
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================
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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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* Packet length (2 bytes)
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The assembled subtitle, with stripped sequence numbers
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* Option byte (1 byte)
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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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if cmd>>6 == 1 then a shift command and (cmd>>4)&3 is the
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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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the number 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. 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 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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