JPEG File Interchange Format
Version 1.02

September 1, 1992

Eric Hamilton
C-Cube Microsystems
1778 McCarthy Blvd.
Milpitas, CA 95035+1 408 944-6300
Fax: +1 408 944-6314
E-mail: eric@c3.pla.ca.us

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 tobe 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 anyapplication 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•••••••

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

JPEG Compression
Although any JPEG process is supported by the syntax of the JPEG File InterchangeFormat (JFIF) it is strongly recommended that the JPEG baseline process be used for thepurposes of file interchange. This ensures maximum compatibility with all applications
supporting JPEG. JFIF conforms to the JPEG Draft International Standard (ISO DIS10918-1).
The JPEG File Interchange Format is entirely compatible with the standard
JPEG interchange format; the only additional requirement is the mandatory presenceof 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 notuse any resource forks, supported by the Macintosh but not by PCs or workstations.

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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 ismandatory right after the SOI marker.
The JFIF APP0 marker is identified by a zero terminated string: "JFIF". The APP0 can beused for any other purpose by the application provided it can be distinguished from theJFIF 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. Ifused, these segment(s) must immediately follow the JFIF APP0 marker. Decoders shouldskip 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 theextension. 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. Applicationspecific APP0 marker segments must appear after the JFIF APP0 and any JFXX APP0segments. 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 beused.
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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. Moreprecisely:Y = 256 * E'yCb = 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 arange of 0 to 1.0 and those for E 'Cb and E 'Cr have a range of -0.5 to +0.5, Y, Cb, andCr 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 convertingan 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 imagesamples encoded in a JFIF file are located in the upper left hand corner of the image andencoding proceeds from left to right and top to bottom. Top-down orientation is used forboth 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.

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Spatial Relationship of Components
Specification of the spatial positioning of pixel samples within components relative to thesamples 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 aredefined with respect to the highest resolution component. Since components must besampled orthogonally (along rows and columns), the spatial position of the samples in agiven subsampled component may be determined by specifying the horizontal and verticaloffsets of the first sample, i.e. the sample in the upper left corner, with respect to thehighest 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 thespectral 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).
As an example, consider a 3 component image which is comprised of components having
the following dimensions:Component 1:
Component 2:
Component 3:

256 samples, 288 lines
128 samples, 144 lines
64 samples, 96 lines

In a JFIF file, centers of the samples are positioned as illustrated below:

Component 1
Component 2
Component 3

NOTE - This definition is compatible with industry standards such as PostcriptLevel 2 and QuickTime. This defintition is not compatible with the conventions
used by CCIR Recommendation 601-1 and other digital video formats. For theseformats, pre-processing of the chrominance components is necessary prior tocompression in order to ensure accurate reconstruction of the compressed image.
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JPEG File Interchange Format Specification
The syntax of a JFIF file conforms to the syntax for interchange format defined in Annex Bof ISO DIS 10918-1. In addition, a JFIF file uses APP0 marker segments and constrainscertain parameters in the frame header as defined below.
X’FF’, SOI
X’FF’, APP0, length, identifier, version, units, Xdensity, Ydensity, Xthumbnail,Ythumbnail, (RGB)nlength(2 bytes) Total APP0 field byte count, including the byte
count value (2 bytes), but excluding the APP0marker 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 shallhave zero parity (bit 7=0).
version(2 bytes) = X'0102'
The most significant byte is used for majorrevisions, 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 countYthumbnail (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 ]•••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•••
X’FF’, EOI
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JFIF Extension APP0 Marker Segment
Immediately following the JFIF APP0 marker segment may be a JFIF extension APP0marker. 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 APP0marker 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 shallhave zero parity (bit 7=0).
extension_code (1 byte) = Code which identifies the extension. In thisversion, 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 theextension. See below for a specification ofextension_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 lengthof 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 ofextension_data constrains parameters in the frame header as defined below:
X’FF’, SOI•••
X’FF’, SOFn, length, frame parameters
Number of components
Nf
1st component
C1
2nd component
C2
3rd component
C3•••
X’FF’, EOI
6= 1 or 3= 1 = Y component= 2 = Cb component= 3 = Cr component

JPEG File Interchange Format, Version 1.02

JFIF Extension: Thumbnail stored using one byte per pixelThis 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
Ythumbnailpalette(pixel)n(1 byte)
Thumbnail horizontal pixel count(1 byte)Thumbnail vertical pixel count(768 bytes) 24-bit RGB pixel values for the color palette.
The RGB values define the colors represented byeach value of an 8-bit binary encoding (0 - 255).
(n bytes)
8-bit values for the thumbnail pixels
n = Xthumbnail * Ythumbnail

JFIF Extension: Thumbnail stored using three bytes per pixelThis extension supports thumbnails stored using three bytes per pixel in the extension_data
field. The syntax of extension_data is:
XthumbnailYthumbnail(RGB)n(1 byte)
Thumbnail horizontal pixel count(1 byte)Thumbnail vertical pixel count(3n bytes) Packed (24-bit) RGB values for the thumbnail
pixels, n = Xthumbnail * Ythumbnail

Useful tips
• 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'.
• if you use APP0 elsewhere, be sure not to have the strings "JFIF" or "JFXX" right after
the APP0 marker.
• if you do not want to include a thumbnail, just program Xthumbnail = Ythumbnail = 0.
• 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 bedecodable.
• 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.

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