webm EBML文件头分析
EBML element
分析webm格式，主要是对ebml element的理解，EBML是类似于XML那样的层次化结构，每一个元素都有它的ID、值，在二进制存储中每个元素的排列是
ID，长度，值
Element IDs (also called EBML 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)
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
去掉001这样的前缀，xxx就是实际需要的element id, element data size的值。
Data
element id, element data size 都是以001这样开头，element data直接跟在element data size，没有了前面的001这样的前缀。
android 中的 EBML header的ID( external/libvpx/libmkv/EbmlIDs.h)：
EBML = 0x1A45DFA3,
EBMLVersion = 0x4286,
EBMLReadVersion = 0x42F7,
EBMLMaxIDLength = 0x42F2,
EBMLMaxSizeLength = 0x42F3,
DocType = 0x4282,
DocTypeVersion = 0x4287,
DocTypeReadVersion = 0x4285,
//segment
Segment = 0x18538067,
Segment = 0x18538067,
所以判断一个文件是否webm文件需要满足两个主要条件：
- 是否有EBML文件头0x1A45DFA3
- doctype是不是webm
分析webm文件头
这是通过ghex程序拷贝的一个文件的ebml文件头信息（ghex打开文件后，可以通过save as菜单把hex保存为html）：
1a 45 df a3 01 00 00 00 00 00 00 1f 42 86 81 01 42 f7 |
81 01 42 f2 81 04 42 f3 81 08 42 82 84 77 65 62 6d 42 |
87 81 02 42 85 81 02 18 53 80 67 01 00 00 00 00 18 ab |
这个文件的EBML header可以这样理解：
Element ID：1a 45 df a3
Element data size : 01 [0000 0001, 8个字节]
Element data: 00 00 00 00 00 00 1f [十进制是31，表示了后面所有Element总长度（字节）,所以对于EBML header 的level 0，data的内容就是header中sub element的总字节数]
以42 82为例分析doctype：
Element ID：42 82 Element data size : 84 [84二进制就是1000 0100，去掉1，后面就是000 0100，十进制是4，表示后面的数据占四个字节]Element data: 77 65 62 6d [对应的ascii字符就是w e b m]
gstreamer中gsttypefindfunctions.c 中 EBML 文件头解析的部分代码如下：
/* EBML typefind helper */
static gboolean
ebml_check_header (GstTypeFind * tf, const gchar * doctype, int doctype_len)
{
/* 4 bytes for EBML ID, 1 byte for header length identifier */
guint8 *data = gst_type_find_peek (tf, 0, 4 + 1);
gint len_mask = 0x80, size = 1, n = 1, total;
if (!data)
return FALSE;
/* ebml header? */
if (data[0] != 0x1A || data[1] != 0x45 || data[2] != 0xDF || data[3] != 0xA3)
return FALSE;
/* length of header */
total = data[4];
/*
* len_mask binary: 1000 0000, while循环 total & len_mask 就可计算出前面0的个数,
* 碰到1结束循环，size的值刚好就是ebml head element的字节数。
*/
while (size <= 8 && !(total & len_mask)) {
size++;
len_mask >>= 1;
}
if (size > 8) /* 得出ebml header(level 0) data 的字节数 */
return FALSE;
total &= (len_mask - 1);
while (n < size)
total = (total << 8) | data[4 + n++];
/* get new data for full header, 4 bytes for EBML ID,
* EBML length tag and the actual header */
data = gst_type_find_peek (tf, 0, 4 + size + total);
if (!data)
return FALSE;
/* only check doctype if asked to do so */
if (doctype == NULL || doctype_len == 0)
return TRUE;
/* the header must contain the doctype. For now, we don't parse the
* whole header but simply check for the availability of that array
* of characters inside the header. Not fully fool-proof, but good
* enough. */
for (n = 4 + size; n <= 4 + size + total - doctype_len; n++)
if (!memcmp (&data[n], doctype, doctype_len))
return TRUE;
return FALSE;
}
调用ebml_check_header的时候指定参数doctype为"matroska", "webm"即可。
static void
matroska_type_find (GstTypeFind * tf, gpointer ununsed)
{
if (ebml_check_header (tf, "matroska", 8))
gst_type_find_suggest (tf, GST_TYPE_FIND_MAXIMUM, MATROSKA_CAPS);
else if (ebml_check_header (tf, NULL, 0))
gst_type_find_suggest (tf, GST_TYPE_FIND_LIKELY, MATROSKA_CAPS);
}
参考:
多媒体封装格式详解---MKV【1】【2】【3】
http://blog.csdn.net/tx3344/article/details/8162656http://blog.csdn.net/tx3344/article/details/8176288
http://blog.csdn.net/tx3344/article/details/8203260
MKV的EBML格式
http://tigersoldier.is-programmer.com/2008/6/30/ebml-in-mkv.4052.html
MKV文件格式
http://blog.chinaunix.net/uid-12845622-id-311943.html
Matroska文件解析之SimpleBlock
http://www.cnblogs.com/tangdoudou/archive/2012/05/14/2499063.html
工具MKVtoolnix:
http://www.cinker.com/2009/01/13/mkv-movie-split-merge-mkvtoolnix/
mkv ebml官方文档：
http://www.matroska.org/technical/specs/index.html