php webm文件头,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,

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/8162656

http://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