FLAC Metadata

The introduction of FLAC is here present on Wikipedia.

Structure

Here is the structure of FLAC's metadata, the original format structure of FLAC is here present on SourceForge.net (You may need a proxy if you are in China mainland). :

Size	Name				Description
32		File Header 		Four fixed bytes: 0x66 0x4C 0x61 0x43 (fLaC)
		Metadata Block
			Header
1				Last flag	1: Last Block, 0: Not last block.
7				Block type	0: STREAMINFO
							1: PADDING
							2: APPLICATION
							3: SEEKTABLE
							4: VORBIS_COMMENT
							5: CUESHEET
							6: PICTURE
							7-126 : Reserved
							127: Invalid
24			Length			The length of metadata, exclude the header.
??			Data			The data of metadata.

The type 4(Vorbis Comment), 5(Cue Sheet), and 6(Picture) is important for us to get the tag information of the FLAC media.

Vorbis Commit

Here is a sample of Vorbis Commit metadata block's data:

Offset      0  1  2  3  4  5  6  7   8  9  A  B  C  D  E  F
00000020                                              0B 00                   
00000030   00 00 4C 61 76 66 35 33  2E 32 34 2E 30 07 00 00     Lavf53.24.0   
00000040   00 73 00 00 00 41 52 54  49 53 54 3D 42 69 42 69    s   ARTIST=BiBi
00000050   EF BD 9E E7 B5 A2 E7 80  AC E7 B5 B5 E9 87 8C 28   锝炵耽鐎档閲?
00000060   43 56 2E E5 8D 97 E6 A2  9D E6 84 9B E4 B9 83 29   CV.?楁?鎰涗箖)
00000070   E3 80 81 E8 A5 BF E6 9C  A8 E9 87 8E E7 9C 9F E5   銆 瑗挎湪閲庣湡?
00000080   A7 AB 28 43 56 2E 50 69  6C 65 29 E3 80 81 E7 9F   Й(CV.Pile)銆 鐭
00000090   A2 E6 BE A4 E3 81 AB E3  81 93 28 43 56 2E E5 BE   ㈡兢? ?CV.寰
000000A0   B3 E4 BA 95 E9 9D 92 E7  A9 BA 29 20 66 72 6F 6D   充簳?掔┖) from
000000B0   20 CE BC 27 73 EF BD 9E  13 00 00 00 54 49 54 4C    渭's锝?   TITL
000000C0   45 3D 43 75 74 69 65 20  50 61 6E 74 68 65 72 0D   E=Cutie Panther 
000000D0   00 00 00 54 4F 54 41 4C  54 52 41 43 4B 53 3D 35      TOTALTRACKS=5
000000E0   60 00 00 00 41 6C 62 75  6D 3D 54 56 E3 82 A2 E3   `   Album=TV銈
000000F0   83 8B E3 83 A1 E3 80 8C  E3 83 A9 E3 83 96 E3 83   儖銉°€屻儵銉栥?
00000100   A9 E3 82 A4 E3 83 96 EF  BC 81 E3 80 8D E3 83 A6   ┿偆銉栵?銆 銉?
00000110   E3 83 8B E3 83 83 E3 83  88 E3 82 B7 E3 83 B3 E3   銉嬨儍銉堛偡銉炽
00000120   82 B0 E3 83 AB 20 32 6E  64 20 73 65 73 73 69 6F   偘銉?2nd sessio
00000130   6E E3 80 8C 43 75 74 69  65 20 50 61 6E 74 68 65   n銆孋utie Panthe
00000140   72 E3 80 8D 0B 00 00 00  47 65 6E 72 65 3D 41 6E   r銆     Genre=An
00000150   69 6D 65 09 00 00 00 44  41 54 45 3D 32 30 31 33   ime    DATE=2013
00000160   0D 00 00 00 54 52 41 43  4B 4E 55 4D 42 45 52 3D       TRACKNUMBER=
00000170   31                                                 1	

There's a string like 'Lavf53.24.0' at the begin, should jump over. It's a 4-byte length+string data+4-byte unknown data.

Then all the data is in the following structure, it's a Pascal-style string:

4-bytes length + text data in UTF-8

Picture

The structure of picture frame is simple:

Offset      0  1  2  3  4  5  6  7   8  9  A  B  C  D  E  F
00000170      06 02 D9 25 00 00 00  03 00 00 00 0A 69 6D 61      ?        ima
00000180   67 65 2F 6A 70 65 67 00  00 00 00 00 00 01 F4 00   ge/jpeg       ?
00000190   00 01 F4 00 00 00 18 00  00 00 00 00 02 D8 FB FF     ?         佧
000001A0   D8 FF E0 00 10 4A 46 49  46 (Binary Data)          ?? JFIF

Picture metadata block start with 4-bytes type. In the sample is 00 00 00 03(0x00000175~0x00000178).

Here is a list of the image type index, it's the same as ID3v2:

0 - Other
1 - 32x32 pixels 'file icon' (PNG only)
2 - Other file icon
3 - Cover (front)
4 - Cover (back)
5 - Leaflet page
6 - Media (e.g. label side of CD)
7 - Lead artist/lead performer/soloist
8 - Artist/performer
9 - Conductor
10 - Band/Orchestra
11 - Composer
12 - Lyricist/text writer
13 - Recording Location
14 - During recording
15 - During performance
16 - Movie/video screen capture
17 - A bright coloured fish
18 - Illustration
19 - Band/artist logotype
20 - Publisher/Studio logotype

Following a 4-bytes length mime type discription string size. In the example, the MIME-Type is 'image/jpeg'.

Then is a 4-bytes length description size. In the sample it's empty, so it's 0.

Then follows these 4-bytes data: width, height, depth, index color num.

After that, it's 4-bytes image data size and image binary data.