JEITA CP-3451 - 64 - 4.7.2   Interoperability Structure of APP1 in Compressed Data APP1 consists of an APP1 marker indicating that it is an  application area, a  length code indicating its size, and primary image attribute information patterned after the TIFF structure. The APP1 segment cannot record more than 64 Kbytes, a limitation that shall be kept in mind when recording thumbnail images.   APP1 also records attribute information for JPEG compressed images. The features of this marker segment are described below.   A.    Order of APP1 recording APP1 is recorded immediately after the SOI marker indicating the start of the file (see Figure 6). B.    APP1 Interoperability structure APP1 consists of an APP1 marker, Exif ID code and the attribute information itself (see Figure 25). Address Offset Code Meaning (Hex) (Hex) +00 FF Marker Prefix +01 E1 APP1 +02 Length of field +04 45 'E' +05 78 'x' +06 69 'i' +07 66 'f' +08 00 NULL +09 00 Padding +0A Attribute information   Figure 25 Basic Structure of APP1 Marker Segment C.    Exif ID code The  Exif  ID  code  indicates  that  the  APP1  segment  Interoperability  is  Exif  format.  After  a  4-byte  code,  00.H  is recorded in 2 bytes. The reason for recording this code is to avoid duplication with other applications making use of JPEG application marker segments (APPn). D.    Attribute information   Attribute information is recorded in 2 IFDs (0th IFD, 1st IFD) following the TIFF structure, including the File Header. The  0th  IFD  records  compressed  image  attributes  (the  image  itself).  The  1st  IFD  may  be  used  for  thumbnail images. E.    TIFF header The TIFF header in APP1 constitutes 8 bytes of data, including the byte order and the Offset of IFD indicating the first IFD. The Offset is referenced to the address of the first byte of the Byte Order. For example, the Offset of IFD address in the TIFF Header is 4 (see Table 1). F.    Relation to JPEG marker segments Attribute information is recorded in essentially the same format as that used to record uncompressed data attribute information. In order to avoid duplication, however, attribute information recorded in a JPEG marker segment is not recorded in APP1.