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JPEG File Interchange Format
Version 1.02

Why a File Interchange Format

JPEG File Interchange Format is a minimal file format which enables JPEG bitstreams to
be exchanged between a wide variety of platforms and applications.  This minimal format
does not include any of the advanced features found in the TIFF JPEG specification or any
application specific file format. Nor should it, for the only purpose of this simplified
format is to allow the exchange of JPEG compressed images.

JPEG File Interchange Format features

o      Uses JPEG compression
o      Uses JPEG interchange format compressed image representation
o      PC or Mac or Unix workstation compatible
o      Standard color space: one or three components. For three components, YCbCr
      (CCIR 601-256 levels)
o      APP0 marker used to specify Units, X pixel density, Y pixel density, thumbnail
o      APP0 marker also used to specify JFIF extensions
o      APP0 marker also used to specify application-specific information

JPEG Compression

Although any JPEG process is supported by the syntax of the JPEG File Interchange Format
(JFIF) it is strongly recommended that the JPEG baseline process be used for the purposes
of file interchange. This ensures maximum compatibility with all applications supporting
JPEG. JFIF conforms to the JPEG Draft International Standard (ISO DIS 10918-1).

The JPEG File Interchange Format is entirely compatible with the standard JPEG
interchange format;  the only additional requirement is the mandatory presence of the
APP0 marker right after the SOI marker.  Note that JPEG interchange format requires (as
does JFIF) that all table specifications used in the encoding process be coded in the
bitstream prior to their use.

Compatible across platforms

The JPEG File Interchange Format is compatible across platforms: for example, it does not
use any resource forks, supported by the Macintosh but not by PCs or workstations.


Standard color space

The color space to be used is YCbCr as defined by CCIR 601 (256 levels).  The RGB
components calculated by linear conversion from YCbCr shall not be gamma corrected
(gamma = 1.0).  If only one component is used, that component shall be Y.

APP0 marker used to identify JPEG FIF

The APP0 marker is used to identify a JPEG FIF file.  The JPEG FIF APP0 marker is
mandatory right after the SOI marker.

The JFIF APP0 marker is identified by a zero terminated string: "JFIF". The APP0 can be
used for any other purpose by the application provided it can be distinguished from the
JFIF APP0. 

The JFIF APP0 marker provides information which is missing from the JPEG stream:
version number, X and Y pixel density (dots per inch or dots per cm), pixel aspect ratio
(derived from X and Y pixel density), thumbnail.

APP0 marker used to specify JFIF extensions

Additional APP0 marker segment(s) can optionally be used to specify JFIF extensions.  If
used, these segment(s) must immediately follow the JFIF APP0 marker.  Decoders should
skip any unsupported JFIF extension segments and continue decoding.

The JFIF extension APP0 marker is identified by a zero terminated string:  "JFXX".  The
JFIF extension APP0 marker segment contains a 1-byte code which identifies the extension. 
This version, version 1.02, has only one extension defined:  an extension for defining
thumbnails stored in formats other than 24-bit RGB.

APP0 marker used for application-specific information

Additional APP0 marker segments can be used to hold application-specific information
which does not affect the decodability or displayability of the JFIF file.  Application-
specific APP0 marker segments must appear after the JFIF APP0 and any JFXX APP0
segments.  Decoders should skip any unrecognized application-specific APP0 segments.

Application-specific APP0 marker segments are identified by a zero terminated string
which identifies the application (not "JFIF" or "JFXX").  This string should be an
organization name or company trademark.  Generic strings such as dog, cat, tree, etc.
should not be used.



Conversion to and from RGB
 
Y, Cb, and Cr are converted from R, G, and B as defined in CCIR Recommendation 601
but are normalized so as to occupy the full 256 levels of a 8-bit binary encoding.  More
precisely:

Y   = 256 * E'y
Cb  = 256 * [ E'Cb ] + 128
Cr  = 256 * [ E'Cr ] + 128

where the E'y, E'Cb and E'Cb are defined as in CCIR 601.  Since values of E'y have a
range of 0 to 1.0 and those for  E'Cb and E'Cr have a range of -0.5 to +0.5,  Y, Cb, and Cr
must be clamped to 255 when they are maximum value.

RGB to YCbCr Conversion
 
YCbCr (256 levels) can be computed directly from 8-bit RGB as follows:

Y   =     0.299  R + 0.587  G + 0.114  B
Cb  =   - 0.1687 R - 0.3313 G + 0.5    B + 128
Cr  =     0.5    R - 0.4187 G - 0.0813 B + 128

NOTE - Not all image file formats store image samples in the order R0, G0,
B0, ... Rn, Gn, Bn.  Be sure to verify the sample order before converting an
RGB file to JFIF.

 
YCbCr to RGB Conversion
 
RGB can be computed directly from YCbCr (256 levels) as follows:

R = Y                    + 1.402   (Cr-128)
G = Y - 0.34414 (Cb-128) - 0.71414 (Cr-128)
B = Y + 1.772   (Cb-128)


Image Orientation

In JFIF files, the image orientation is always top-down.  This means that the first image
samples encoded in a JFIF file are located in the upper left hand corner of the image and
encoding proceeds from left to right and top to bottom.  Top-down orientation is used for
both the full resolution image and the thumbnail image.

The process of converting an image file having bottom-up orientation to JFIF must include
inverting the order of all image lines before JPEG encoding


Spatial Relationship of Components

Specification of the spatial positioning of pixel samples within components relative to the
samples of other components is necessary for proper image post processing and accurate
image presentation.  In JFIF files, the position of the pixels in subsampled components are
defined with respect to the highest resolution component.  Since components must be
sampled orthogonally (along rows and columns), the spatial position of the samples in a
given subsampled component may be determined by specifying the horizontal and vertical
offsets of the first sample, i.e. the sample in the upper left corner, with respect to the
highest resolution component.

The horizontal and vertical offsets of the first sample in a subsampled component,
Xoffseti[0,0] and Yoffseti[0,0], is defined to be

Xoffseti[0,0] = ( Nsamplesref / Nsamplesi  ) / 2 - 0.5
Yoffseti[0,0] = ( Nlinesref / Nlinesi  ) / 2 - 0.5

where
Nsamplesref is the number of samples per line in the largest component,
Nsamplesi is the number of samples per line in the ith component,
Nlinesref is the number of lines in the largest component,
Nlinesi is the number of lines in the ith component.
 
Proper subsampling of components incorporates an anti-aliasing filter which reduces the
spectral bandwidth of the full resolution components.   Subsampling can easily be
accomplished using a symmetrical digital filter with an even number of taps (coefficients). 
A commonly used filter for 2:1 subsampling utilizes two taps (1/2,1/2).

NOTE - This definition is compatible with industry standards such as Postcript
Level 2 and QuickTime. This defintition is not compatible with the conventions
used by CCIR Recommendation 601-1 and other digital video formats.  For these
formats, pre-processing of the chrominance components is necessary prior to
compression in order to ensure accurate reconstruction of the compressed image.


JPEG File Interchange Format Specification

The syntax of a JFIF file conforms to the syntax for interchange format defined in Annex B
of ISO DIS 10918-1.  In addition, a JFIF file uses APP0 marker segments and constrains
certain parameters in the frame header as defined below.

  X'FF', SOI
        X'FF', APP0, length, identifier, version, units, Xdensity, Ydensity, Xthumbnail,
         Ythumbnail, (RGB)n
             length     (2 bytes)  Total APP0 field byte count, including the byte
                                   count value (2 bytes), but excluding the APP0
                                   marker itself
             identifier (5 bytes)  = X'4A', X'46', X'49', X'46', X'00'
                                   This zero terminated string ("JFIF") uniquely
                                   identifies this APP0 marker.  This string shall
                                   have zero parity (bit 7=0).
             version    (2 bytes)  = X'0102'
                                   The most significant byte is used for major
                                   revisions, the least significant byte for minor
                                   revisions. Version 1.02 is the current released
                                   revision.
             units      (1 byte)   Units for the X and Y densities.
                                   units = 0:  no units, X and Y specify the pixel
                                        aspect ratio
                                   units = 1:  X and Y are dots per inch
                                   units = 2:  X and Y are dots per cm
             Xdensity   (2 bytes)  Horizontal pixel density
             Ydensity   (2 bytes)  Vertical pixel density
             Xthumbnail (1 byte)   Thumbnail horizontal pixel count
             Ythumbnail (1 byte)   Thumbnail vertical pixel count
             (RGB)n     (3n bytes) Packed (24-bit) RGB values for the thumbnail
                                   pixels, n = Xthumbnail * Ythumbnail
       [ Optional JFIF extension APP0 marker segment(s) - see below ]
               o
               o
               o
       X'FF', SOFn, length, frame parameters
          Number of components Nf  = 1 or 3
          1st component    C1    = 1 = Y component
          2nd component    C2    = 2 = Cb component
          3rd component    C3    = 3 = Cr component
               o
               o
               o
  X'FF', EOI

JFIF Extension APP0 Marker Segment

Immediately following the JFIF APP0 marker segment may be a JFIF extension APP0
marker.  This JFIF extension APP0 marker segment may only be present for JFIF versions
1.02 and above.  The syntax of the JFIF extension APP0 marker segment is:

        X'FF', APP0, length, identifier, extension_code, extension_data
           length   (2 bytes)    Total APP0 field byte count, including the byte
                               count value (2 bytes), but excluding the APP0
                               marker itself
           identifier  (5 bytes)    = X'4A', X'46', X'58', X'58', X'00'
                               This zero terminated string ("JFXX") uniquely
                               identifies this APP0 marker.  This string shall
                               have zero parity (bit 7=0).
           extension_code (1 byte)    = Code which identifies the extension.  In this
                               version, the following extensions are defined:
                                  = X'10'   Thumbnail coded using JPEG
                                  = X'11'   Thumbnail stored using 1 byte/pixel
                                  = X'13'   Thumbnail stored using 3 bytes/pixel
           extension_data (variable)    = The specification of the remainder of the JFIF
                               extension APP0 marker segment varies with the
                               extension. See below for a specification of
                               extension_data for each extension.

JFIF Extension:  Thumbnail coded using JPEG

This extension supports thumbnails compressed using JPEG.  The compressed thumbnail 
immediately follows the extension_code (X'10') in the extension_data field and the length
of the compressed data must be included in the JFIF extension APP0 marker length field.

The syntax of the extension_data field conforms to the syntax for interchange format defined
in Annex B of ISO DIS 10918-1.  However, no "JFIF" or "JFXX" marker segments shall
be present.  As in the full resolution image of the JFIF file, the syntax of extension_data
constrains parameters in the frame header as defined below:

  X'FF', SOI
               o
               o
               o
       X'FF', SOFn, length, frame parameters
            Number of components    Nf    = 1 or 3
            1st component    C1    = 1 = Y component
            2nd component    C2    = 2 = Cb component
            3rd component    C3    = 3 = Cr component
               o
               o
               o
  X'FF', EOI



JFIF Extension:  Thumbnail stored using one byte per pixel

This extension supports thumbnails stored using one byte per pixel and a color palette in
the extension_data field.  The syntax of extension_data is:

       Xthumbnail       (1 byte)    Thumbnail horizontal pixel count
       Ythumbnail       (1 byte)    Thumbnail vertical pixel count
       palette          (768 bytes)  24-bit RGB pixel values for the color palette.
                                     The RGB values define the colors represented by
                                     each value of an 8-bit binary encoding (0 - 255).
       (pixel)n        (n bytes)     8-bit values for the thumbnail pixels
                                     n = Xthumbnail * Ythumbnail

JFIF Extension:  Thumbnail stored using three bytes per pixel

This extension supports thumbnails stored using three bytes per pixel in the extension_data
field.  The syntax of extension_data is:

       Xthumbnail       (1 byte)    Thumbnail horizontal pixel count
       Ythumbnail       (1 byte)    Thumbnail vertical pixel count
       (RGB)n           (3n bytes)  Packed (24-bit) RGB values for the thumbnail
                                    pixels, n = Xthumbnail * Ythumbnail

Useful tips

o you can identify a JFIF file by looking for the following sequence: X'FF', SOI, X'FF',
APP0, <2 bytes to be skipped>, "JFIF", X'00'.

o if you use APP0 elsewhere, be sure not to have the strings "JFIF" or "JFXX" right after
the APP0 marker.

o if you do not want to include a thumbnail, just program Xthumbnail = Ythumbnail = 0.

o be sure to check the version number in the special APP0 field.  In general, if the major
version number of the JFIF file matches that supported by the decoder, the file will be
decodable.

o if you only want to specify a pixel aspect ratio, put 0 for the units field in the special
APP0 field. Xdensity and Ydensity can then be programmed for the desired aspect ratio.
Xdensity = 1, Ydensity = 1 will program a 1:1 aspect ratio.  Xdensity and Ydensity should
always be non-zero.