JEITA CP-3451
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Figure 49 Structure of User Comment Chunk
As seen in Figure 49, the chunk data area storing the user comment starts with an 8-byte character code area,
where the character code is given. The rest of the chunk data area is the user comment area, used for writing
comments. The currently defined character codes are shown in Table 31. A registration system is used for
character codes to avoid duplication. When a character code is registered, a standard document is indicated in the
reference column to indicate the character format specification. If a character code is used for which there is no
clear specification like Shift-JIS in Japan, Undefined is designated.
The character code is written from the start of the character code area, and the unused portion is padded with
NULL ('00.H'). In order to match the format of the actual comments in the user comment part to that of the Exif
image file UserComment tag, NULL termination is not used even if the comment is written in ASCII text. In accord
with the chunk specification, however, a padding byte ('00.H') is used if the chunk data is not an even number of
bytes.
Table 31 Character Codes and Character Code Designation
Character Code
Code Designation (8 Bytes)
Reference
ASCII
41.H, 53.H, 43.H, 49.H, 49.H, 00.H, 00.H, 00.H
ITU-T T.50 IA5viii
JIS
4A.H, 49.H, 53.H, 00.H, 00.H, 00.H, 00.H, 00.H
JIS X0208-1990ix
Unicode
55.H, 4E.H, 49.H, 43.H, 4F.H, 44.H, 45.H, 00.H
Unicode Standard x
Undefined
00.H, 00.H, 00.H, 00.H, 00.H, 00.H, 00.H, 00.H
Undefined
[Notes and Additional Information]
The above chunks incorporated in exif list are used with a specific form type ('WAVE') and are therefore written in
all lower-case letters.
Only the ever chunk is mandatory; the others are optional. If an Exif image file related to an audio file exists, the
erel chunk is also mandatory.
When the exif list is read by a camera, information that cannot be recognized may be encountered, such as the
emnt chunk or an Undefined string in the eucm chunk. In that case the chunk is to be skipped.
An Exif reader and writer shall conform to the stipulations for an ordinary LIST chunk.
A general-purpose RIFF Form Reader will ignore the exif list. A dedicated Exif reader is required for reading the
exif list.
Figure 50 shows the exif list structure and a coding example. The ever chunk is mandatory and has a fixed length
(chunk data is 4 bytes). The erel chunk data is 13 bytes (odd), so a padding byte is added. The erel chunk is also
fixed length.