JPEG file Interchange Format

JPEG File Interchange FormatVersion 1.02September 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 FormatVersion 1.02Why a File Interchange FormatJPEG 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 featureso      Uses JPEG compressiono      Uses JPEG interchange format compressed image representationo      PC or Mac or Unix workstation compatibleo      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, thumbnailo      APP0 marker also used to specify JFIF extensionso      APP0 marker also used to specify application-specific informationJPEG CompressionAlthough 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 platformsThe 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 spaceThe 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 FIFThe 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 extensionsAdditional 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 informationAdditional 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'yCb  = 256 * [ E'Cb ] + 128Cr  = 256 * [ E'Cr ] + 128where 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  BCb  =   - 0.1687 R - 0.3313 G + 0.5    B + 128Cr  =     0.5    R - 0.4187 G - 0.0813 B + 128NOTE - 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 OrientationIn 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 encodingSpatial Relationship of ComponentsSpecification 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 beXoffseti[0,0] = ( Nsamplesref / Nsamplesi  ) / 2 - 0.5Yoffseti[0,0] = ( Nlinesref / Nlinesi  ) / 2 - 0.5whereNsamplesref 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 SpecificationThe 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', EOIJFIF Extension APP0 Marker SegmentImmediately 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 JPEGThis 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', EOIJFIF 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       (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 * YthumbnailJFIF 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:        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 * YthumbnailUseful tipso 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.