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324 lines
9.6 KiB
Text
324 lines
9.6 KiB
Text
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DVD subtitles
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---------------
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0. Introduction
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1. Basics
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2. The data structure
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3. Reading the control header
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4. Decoding the graphics
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5. What I do not know yet / What I need
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6. Thanks
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7. Changes
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The latest version of this document can be found here:
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http://www.via.ecp.fr/~sam/doc/dvd/
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0. Introduction
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One of the last things we missed in DVD decoding under my system was the
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decoding of subtitles. I found no information on the web or Usenet about them,
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apart from a few words on them being run-length encoded in the DVD FAQ.
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So we decided to reverse-engineer their format (it's completely legal in
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France, since we did it on interoperability purposes), and managed to get
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almost all of it.
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1. Basics
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DVD subtitles are hidden in private PS packets (0x000001ba), just like AC3
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streams are.
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Within the PS packet, there are PES packets, and like AC3, the header for the
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ones containing subtitles have a 0x000001bd header.
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As for AC3, where there's an ID like (0x80 + x), there's a subtitle ID equal
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to (0x20 + x), where x is the subtitle ID. Thus there seems to be only
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16 possible different subtitles on a DVD (my Taxi Driver copy has 16).
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I'll suppose you know how to extract AC3 from a DVD, and jump to the
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interesting part of this documentation. Anyway you're unlikely to have
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understood what I said without already being familiar with MPEG2.
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2. The data structure
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A subtitle packet, after its parts have been collected and appended, looks
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like this :
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+----------------------------------------------------------+
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| 0 2 size |
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| +----+------------------------+-----------------+ |
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| |size| data packet | control | |
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| +----+------------------------+-----------------+ |
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| a subtitle packet |
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+----------------------------------------------------------+
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size is a 2 bytes word, and data packet and control may have any size.
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Here is the structure of the data packet :
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+----------------------------------------------------------+
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| 2 4 S0+2 |
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| +----+------------------------------------------+ |
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| | S0 | data | |
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| +----+------------------------------------------+ |
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| the data packet |
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+----------------------------------------------------------+
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S0, the data packet size, is a 2 bytes word.
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Finally, here's the structure of the control packet :
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+----------------------------------------------------------+
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| S0+2 S0+4 S1 size |
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| +----+---------+---------+--+---------+--+---------+ |
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| | S1 |ctrl seq |ctrl seq |..|ctrl seq |ff| end seq | |
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| +----+---------+---------+--+---------+--+---------+ |
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| the control packet |
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+----------------------------------------------------------+
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To summarize :
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- S1, at offset S0+2, the position of the end sequence
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- several control sequences
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- the 'ff' byte
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- the end sequence
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3. Reading the control header
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The first thing to read is the control sequences. There are several
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types of them, and each type is determined by its first byte. As far
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as I know, each type has a fixed length.
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* type 0x01 : '01' - 1 byte
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it seems to be an empty control sequence.
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* type 0x03 : '03wxyz' - 3 bytes
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this one has the palette information ; it basically says 'encoded color 0
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is the wth color of the palette, encoded color 1 is the xth color, aso.
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* type 0x04 : '04wxyz' - 3 bytes
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I *think* this is the alpha channel information ; I only saw values of 0 or f
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for those nibbles, so I can't really be sure, but it seems plausable.
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* type 0x05 : '05xxxXXXyyyYYY' - 7 bytes
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the coordinates of the subtitle on the screen :
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xxx is the first column of the subtitle
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XXX is the last column of the subtitle
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yyy is the first line of the subtitle
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YYY is the last line of the subtitle
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thus the subtitle's size is (XXX-xxx+1) x (YYY-yyy+1)
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* type 0x06 : '06xxxxyyyy' - 5 bytes
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xxxx is the position of the first graphic line, and yyyy is the position of
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the second one (the graphics are interlaced, so it helps a lot :p)
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The end sequence has this structure:
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xxxx yyyy 02 ff (ff)
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it ends with 'ff' or 'ffff', to make the whole packet have an even length.
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FIXME: I absolutely don't know what xxxx is. I suppose it may be some date
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information since I found it nowhere else, but I can't be sure.
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yyyy is equal to S1 (see picture).
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Example of a control header :
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----
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0A 0C 01 03 02 31 04 0F F0 05 00 02 CF 00 22 3E 06 00 06 04 E9 FF 00 93 0A 0C 02 FF
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----
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Let's decode it. First of all, S1 = 0x0a0c.
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The control sequences are :
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01
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Nothing to say about this one
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03 02 31
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Color 0 is 0, color 1 is 2, color 2 is 3, and color 3 is 1.
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04 0F F0
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Colors 0 and 3 are transparent, and colors 2 and 3 are opaque (not sure of this one)
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05 00 02 CF 00 22 3E
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The first column is 0x000, the last one is 0x2cf, the first line is 0x002, and
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the last line is 0x23e. Thus the subtitle's size is 0x2d0 x 0x23d.
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06 00 06 04 E9
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The first encoded image starts at offset 0x006, and the second one starts at 0x04e9.
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And the end sequence is :
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00 93 0A 0C 02 FF
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Which means... well, not many things now. We can at least verify that S1 (0x0a0c) is
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there.
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4. Decoding the graphics
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The graphics are rather easy to decode (at least, when you know how to do it - it
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took us one whole week to figure out what the encoding was :p).
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The picture is interlaced, for instance for a 40 lines picture :
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line 0 ---------------#----------
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line 2 ------#-------------------
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...
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line 38 ------------#-------------
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line 1 ------------------#-------
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line 3 --------#-----------------
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...
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line 39 -------------#------------
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When decoding you should get:
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line 0 ---------------#----------
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line 1 ------------------#-------
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line 2 ------#-------------------
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line 3 --------#-----------------
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...
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line 38 ------------#-------------
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line 39 -------------#------------
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Computers with weak processors could choose only to decode even lines
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in order to gain some time, for instance.
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The encoding is run-length encoded, with the following alphabet:
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0xf
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0xe
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0xd
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0xc
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0xb
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0xa
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0x9
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0x8
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0x7
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0x6
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0x5
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0x4
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0x3-
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0x2-
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0x1-
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0x0f-
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0x0e-
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0x0d-
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0x0c-
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0x0b-
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0x0a-
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0x09-
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0x08-
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0x07-
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0x06-
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0x05-
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0x04-
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0x03--
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0x02--
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0x01--
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0x0000
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'-' stands for any other nibble. Once a sequence X of this alphabet has
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been read, the pixels can be displayed : (X >> 2) is the number of pixels
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to display, and (X & 0x3) is the color of the pixel.
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For instance, 0x23 means "8 pixels of color 3".
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"0000" has a special meaning : it's a carriage return. The decoder should
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do a carriage return when reaching the end of the line, or when encountering
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this "0000" sequence. When doing a carriage return, the parser should be
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reset to the next even position (it cannot be nibble-aligned at the start
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of a line).
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After a carriage return, the parser should read a line on the other
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interlaced picture, and swap like this after each carriage return.
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Perhaps I don't explain this very well, so you'd better have a look at
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the enclosed source.
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5. What I do not know yet / What I need
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I don't know what's in the end sequence yet.
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Also, I don't know exactly when to display subtitles, and when to remove them.
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I don't know if there are other types of control sequences (in my programs I consider
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0xff as a control sequence type, as well as 0x02. I don't know if it's correct or not,
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so please comment on this).
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I don't know what the "official" color palette is.
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I don't know how to handle transparency information.
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I don't know if this document is generic enough.
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So what I need is you :
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- if you can, patch this document or my programs to fix strange behaviour with your subtitles.
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- send me your subtitles (there's a program to extract them enclosed) ; the first 10 KB
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of subtitles in a VOB should be enough, but it would be cool if you sent me one subtitle
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file per language.
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6. Thanks
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Thanks to Michel Lespinasse <walken@via.ecp.fr> for his great help on understanding
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the RLE stuff, and for all the ideas he had.
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Thanks to mass (David Waite) and taaz (David I. Lehn) from irc at
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openprojects.net for sending me their subtitles.
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7. Changes
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20000116: added the 'changes' section.
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20000116: added David Waite's and David I. Lehn's name.
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20000116: changed "x0" and "x1" to "S0" and "S1" to make it less confusing.
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--
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Paris, January 16th 2000
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Samuel Hocevar <sam@via.ecp.fr>
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