JEITA CP-3451 - 68 - 4.8    Data Description 4.8.1   Stipulations on Compressed Image Size   The JPEG format is adopted for compressed images. Since JPEG compresses images in blocks of 8 x 8 pixels, preferably  the  pixel  number  set  as  the  width  and  height  of  the  image  should  be  exact  multiples  of  the  MCU (multiples of 16 for width, and multiples of 8 for height if Y:Cb:Cr = 4:2:2, or 16 if Y:Cb:Cr = 4:2:0). It is not required, however, that all image data be fit within this range.   If a restart marker is inserted every 4 MCU for the sake of transformation to Flashpix, the width of the image shall be an exact multiple of 4 MCU (64 pixels). There is no such restriction on height. Specifications on inserting data to adjust the recorded image size relative to the valid image size, or padding, are given below.   A.    Image size recording procedure in the Exif writer   An  Exif  writer  shall  record  a  compressed  image  with  the  image  width  information  and  height  information  set  in accord with the stipulations below, and padded as necessary.   When the image is padded in the direction of width, the padding data shall be appended at the right edge. Padding in the direction of height shall be appended at the bottom edge. The data used for padding is not specified. Stipulations on image width Image  width  information  consists  of  the  valid  image  width  tag  (PixelXDimension),  SOF  marker  information,  and JPEG compressed data. Of these three, the recording of a valid image width tag is mandatory. Image width is padded to the right in the following cases. The value used for padding is not specified. ·  When the image width is not an exact multiple of 16 and the JPEG block has unused capacity to be filled.   ·  When the image width is not a multiple of 64 and the insufficiency shall be made up in order to insert restart markers.