JEITA CP-3451 - 12 - 4.5.5   Basic Structure of Thumbnail Data Thumbnail data is recorded in the 1st IFD similarly to primary images, using two existing image formats. No limit is placed on the size of thumbnail images. Thumbnails are not mandatory, but it is recommended that they be recorded if possible, unless hardware or other restrictions preclude this. Thumbnail data does not necessarily have to adopt the same data structure as that used for primary images. If, however,  the  primary  images  are  recorded  as  uncompressed  RGB  data  or  as  uncompressed  YCbCr  data, thumbnail images cannot be recorded as JPEG compressed data (see Table 2). Table 2 Combinations of primary image and thumbnail data structures Primary Image Uncompressed Compressed Uncompressed Possible Possible Thumbnail Compressed Not possible Possible When thumbnails are recorded in uncompressed format, they are to be recorded in the 1st IFD in conformance with Baseline TIFF Rev. 6.0 RGB Full Color Images or TIFF Rev. 6.0 Extensions YCbCr Images. An Exif-specific recording method is used to record compressed thumbnails. In   this   case   the   Compression   tag   value   is   set   to   "6"   and   tags   in   the   1st   IFD   (JPEGInterchangeFormat, JPEGInterchangeFormatLength)  are  used  to  designate  the  location  and  size.  The  images  are  recorded  at  the designated location as a JPEG stream (from SOI to EOI) conforming to the JPEG Baseline DCT format. No APPn marker,  COM  marker,  nor  restart  marker  is  recorded  in  the  JPEG  stream  (see  Figure  7).  To  avoid  duplicate definitions, the 1st IFD is not used to record a tag indicating a TIFF image or information recorded elsewhere as a JPEG marker segment. Figure 7 Structure of Exif file with compressed thumbnail SOI APP1 DQT DHT SOF SOS Compressed Data Exif  Compressed File APP1 Marker Length TIFF Header 0th IFD Exif IFD 1st IFD JPEGInterchangeFormat JPEGInterchangeFormatLength JPEG Thumbnail Exif APP1 Marker EOI SOI DQT DHT SOF SOS Compressed Data Compressed Thumbnail EOI