MKV的EBML格式

Just like XML, the specific "tags" (IDs in EBML parlance) used in an EBML implementation are arbitrary. However, the semantic of EBML outlines general data types and ID's.

The known basic types are:

• Signed Integer - Big-endian, any size from 1 to 8 octets
• Unsigned Integer - Big-endian, any size from 1 to 8 octets
• Float - Big-endian, defined for 4 and 8 octets (32, 64 bits)
• String - Printable ASCII (0x20 to 0x7E), zero-padded when needed
• UTF-8 - Unicode string, zero padded when needed (RFC 2279)
• Date - signed 8 octets integer in nanoseconds with 0 indicating the precise beginning of the millennium (at 2001-01-01T00:00:00,000000000 UTC)
• master-element - contains other EBML sub-elements of the next lower level
• Binary - not interpreted by the parser

As well as defining standard data types, EBML uses a system of Elements to make up an EBML "document." Elements incorporate an Element ID, a descriptor for the size of the element, and the binary data itself. Futher, Elements can be nested, or contain, Elements of a lower "level."

Element IDs are outlined as follows, beginning with the ID itself, followed by the Data Size, and then the non-interpreted Binary itself:

• Element ID coded with an UTF-8 like system : bits, big-endian
  

  1xxx xxxx	Class A IDs (2^7 -1 possible values) (base 0x8X)
  01xx xxxx xxxx xxxx	Class B IDs (2^14-1 possible values) (base 0x4X 0xXX)
  001x xxxx xxxx xxxx xxxx xxxx	Class C IDs (2^21-1 possible values) (base 0x2X 0xXX 0xXX)
  0001 xxxx xxxx xxxx xxxx xxxx xxxx xxxx	Class D IDs (2^28-1 possible values) (base 0x1X 0xXX 0xXX 0xXX)

  
  Some Notes:
  • The leading bits of the Class IDs are used to identify the length of the ID. The number of leading 0's + 1 is the length of the ID in octets. We will refer to the leading bits as the Length Descriptor.
  • Any ID where all x's are composed entirely of 1's is a Reserved ID, thus the -1 in the definitions above.
  • The Reserved IDs (all x set to 1) are the only IDs that may change the Length Descriptor.
• Data size, in octets, is also coded with an UTF-8 like system :
• bits, big-endian

  1xxx xxxx	- value 0 to 2^7-2
  01xx xxxx xxxx xxxx	- value 0 to 2^14-2
  001x xxxx xxxx xxxx xxxx xxxx	- value 0 to 2^21-2
  0001 xxxx xxxx xxxx xxxx xxxx xxxx xxxx	- value 0 to 2^28-2
  0000 1xxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx	- value 0 to 2^35-2
  0000 01xx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx	- value 0 to 2^42-2
  0000 001x xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx	- value 0 to 2^49-2
  0000 0001 xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx	- value 0 to 2^56-2

  Since modern computers do not easily deal with data coded in sizes greater than 64 bits, any larger Element Sizes are left undefined at the moment. Currently, the Element Size coding allows for an Element to grow to 72000 To, i.e. 7x10^16 octets or 72000 terabytes, which will be sufficient for the time being.

  There is only one reserved word for Element Size encoding, which is an Element Size encoded to all 1's. Such a coding indicates that the size of the Element is unknown, which is a special case that we believe will be useful for live streaming purposes. However, avoid using this reserved word unnecessarily, because it makes parsing slower and more difficult to implement.

• Data
  • Integers are stored in their standard big-endian form (no UTF-like encoding), only the size may differ from their usual form (24 or 40 bits for example).
  • The Signed Integer is just the big-endian representation trimmed from some 0x00 and 0xFF where they are not meaningful (sign). For example -2 can be coded as 0xFFFFFFFFFFFFFE or 0xFFFE or 0xFE and 5 can be coded 0x000000000005 or 0x0005 or 0x05.