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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 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,
thumbnail, 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.
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