JPEG File Layout and Format


 

The File Layout


A JPEG file is partitioned by markers. Each marker is immediately preceded by an all 1 byte (0xff). Although there are more markers, We will discuss the following markers:

                                                                                                                                  

Marker Name

Marker Identifier

Description

SOI

0xd8

Start of Image

APP0

0xe0

JFIF application segment

APPn

0xe1 – 0xef

Other APP segments

DQT

0xdb

Quantization Table

SOF0

0xc0

Start of Frame

DHT

0xc4

Huffman Table

SOS

0xda

Start of Scan

EOI

0xd9

End of Image

     

If a 0xff byte occurs in the compressed image data either a zero byte (0x00) or a marker identifier follows it. Normally the only marker that should be found once the image data is started is an EOI. When a 0xff byte is found followed by a zero byte (0x00) the zero byte must be discarded.

 

A JPEG file consists of the eight following parts:

 

1.      A Start of Image SOI

2.      An APP0 Marker

1.      APP0 length

2.      Identifier

3.      Version

4.      Units for X & Y densities

5.      X density

6.      Y density

7.      Thumbnail horizontal pixels

8.      Thumbnail vertical pixels

9.      Thumbnail RGB bitmap

3.      APPn Markers where n can be form 1 to 15 (Optional)

1.      APPn length

2.      Application specific information

4.      One or more quantization tables DQT

1.      Quantization table length

2.      Quantization table number

3.      Quantization table

5.      A Start of Frame SOF0

1.      Start of Frame length

2.      Precision (Bits per pixel per color component)

3.      Image height

4.      Image width

5.      Number of color components

6.      For each component

1.      An ID

2.      A vertical sample factor

3.      A horizontal sample factor

4.      A quantization table#

6.      One or more huffman tables DHT

1.      Huffman table length

2.      Type, AC or DC

3.      Index

4.      A Bits table

5.      A Value table

7.      A Start of Scan SOS

1.      Start of Scan length

2.      Number of color components

3.      For each component

1.      An ID

2.      An AC table #

3.      An DC table #

4.      Compressed image data (Not included in Start of Scan length)

8.      An End of Image EOI

JPEG File Format

 

Header :

·        It occupies two bytes.

·        0xff, 0xd8 (SOI : Start Of Image ) (these two identify a JPEG/JFIF file)

 

  Segments or markers:

·        Following the SOI marker, there can be any number of “segments” or “markers” such as APP0,DQT, DHT, SOF, SOS and so on.

·        An APP0 segment is immediately follows the SOI marker.

 

Trailer:

·        It occupies two bytes.

·        0xff, 0xd9 (EOI: End of Image) (these two identify end of image).

 

Format of each segment:

 

       Header (4 bytes):

       0xff        1byte       identifies segment .

        n            1byte       type of segment.

       sh, sl       2bytes     size of the segment, including these two bytes, but not including the 0xff              and the type byte. Note, not intel order: high byte first, low byte last!

 

       Contents of the segment:  max. 65533 bytes.

 

Notes:

·         There are parameterless segments (denoted with a '*' below) that DON'T have a size     specification (and no contents), just 0xff and the type byte.

·         Any number of 0xff bytes between segments is legal and must be skipped.

Segment types:

 

SOI            0xd8   Start Of Image

 

APP0         0xe0   JFIF APP0 segment marker,

            APP15       0xef    ignore

 

SOF0        0xc0   Start Of Frame (baseline JPEG), for details see below

SOF1        0xc1   Start Of Frame (baseline JPEG), for details see below

SOF2        0xc2   usually unsupported

SOF3        0xc3   usually unsupported

SOF5        0xc5   usually unsupported

SOF6        0xc6   usually unsupported

SOF7        0xc7   usually unsupported

SOF9        0xc9   for arithmetic coding, usually unsupported

SOF10      0xca   usually unsupported

SOF11      0xcb   usually unsupported

 

SOF13      0xcd   usually unsupported

SOF14      0xce   usually unsupported

SOF15      0xcf   usually unsupported

 

DHT         0xc4   Define Huffman Table

DQT         0xdb   Define Quantization Table

SOS         0xda   Start Of Scan

 

JPG          0xc8   undefined/reserved (causes decoding error)

JPG0        0xf0   ignore (skip)

JPG13      0xfd   ignore (skip)

 

DAC        0xcc   Define Arithmetic Table, usually unsupported

 

DNL        0xdc   usually unsupported, ignore

DRI         0xdd   Define Restart Interval, for details see below

DHP        0xde   ignore (skip)

EXP        0xdf    ignore (skip)

 

*RST0    0xd0   RSTn are used for resync, may be ignored

*RST1    0xd1

*RST2    0xd2

*RST3    0xd3

*RST4    0xd4

*RST5    0xd5

*RST6    0xd6

*RST7    0xd7

*TEM     0x01  usually causes a decoding error, may be ignored

COM      0xfe    Comment, may be ignored  

 

EOI         0xd9   End Of Image

 

All other segment types are reserved and should be ignored (skipped).

 

SOF0 (Start Of Frame 0) marker:

 

            Field                                    Size                      Description

        

          Marker Identifier                 2 bytes    0xff, 0xc0 to identify SOF0 marker

 

          Length                                  2 bytes   This value equals to 8 + components*3 value

 

          Data precision                      1 byte     This is in bits/sample, usually 8

                                                                        (12 and 16 not supported by most software).

        

          Image height                        2 bytes    This must be > 0

 

          Image Width                        2 bytes    This must be > 0

 

         Number of components        1 byte      Usually 1 = grey scaled, 3 = color YcbCr or YIQ

                                                                          4 = color CMYK

         Each component                   3 bytes     Read each component data of 3 bytes. It contains,

                                                             (component Id(1byte)(1 = Y, 2 = Cb, 3 = Cr, 4 = I, 5 = Q),   

                                                              sampling factors (1byte) (bit 0-3 vertical., 4-7 horizontal.),

                                                             quantization table number (1 byte)).

 

Remarks:     JFIF uses either 1 component (Y, greyscaled) or 3 components (YCbCr, sometimes called YUV, colour).

 

APP0 (JFIF segment marker) marker:

 

             Field                           Size                      Description

       

     Marker Identifier              2 bytes      0xff, 0xe0 to identify APP0 marker

 

     Length                              2 bytes      It must be >= 16

 

     File Identifier Mark          5 bytes     This identifies JFIF.

                                                    'JFIF'#0 (0x4a, 0x46, 0x49, 0x46, 0x00)

 

     Major revision number     1 byte      Should be 1, otherwise error

 

     Minor revision number     1 byte      Should be 0..2, otherwise try to decode anyway           

             

     Units for x/y densities      1 byte      0 = no units, x/y-density specify the aspect ratio instead

                                                               1 = x/y-density are dots/inch

                                                               2 = x/y-density are dots/cm

 

     X-density                          2 bytes    It should be <> 0

 

     Y-density                          2 bytes    It should be <> 0

 

     Thumbnail width              1 byte      -------

 

     Thumbnail height             1 byte      -------

 

     Bytes to be read              n bytes For thumbnail (RGB 24 bit), n = width*height*3 bytes should be read immediately followed by thumbnail height

 

 Remarks:

·         If there's no 'JFIF'#0, or the length is < 16, then it is probably not a JFIF segment and should be ignored.

·         Normally units=0, x-dens=1, y-dens=1, meaning that the aspect ratio is 1:1 (evenly scaled).

·         JFIF files including thumbnails are very rare, the thumbnail can usually be ignored.  If there's no thumbnail, then width=0 and height=0.If the length doesn't match the thumbnail size, a warning may be printed, then continue decoding.

DHT( Define Huffman Table) marker:

 

           Field                           Size                              Description

 

      Marker Identifier             2 bytes       0xff, 0xc4 to identify DHT marker

 

      Length                             2 bytes       This specify length of Huffman table

 

      HT information                 1 byte         bit 0..3 : number of HT (0..3, otherwise error)

                                                                            bit 4     : type of HT, 0 = DC table, 1 = AC table

                                                                bit 5..7 : not used, must be 0

 

      Number of Symbols        16 bytes       Number of symbols with codes of length 1..16,

                                                                 the sum(n) of these bytes is the total number of codes,

                                                                 which must be <= 256

      Symbols                           n bytes      Table containing the symbols in order of increasing

                                                                 code length ( n = total number of codes ).    

 

Remarks:    A single DHT segment may contain multiple HTs, each with its own information byte.

 

DRI (Define Restart Interval) marker:

 

          Field                         Size                   Description

 

   Marker Identifier            2 bytes      0xff, 0xdd identifies DRI marker

 

   Length                            2 bytes       It must be 4

 

   Restart interval               2 bytes       This is in units of MCU blocks, means that every n MCU

                                                             blocks a RSTn marker can be found. The first marker will

                                                             be RST0, then RST1 etc, after RST7 repeating from RST0.       

DQT (Define Quantization Table) marker:

 

          Field                        Size                   Description

 

   Marker Identifier          2 bytes        0xff, 0xdb identifies DQT

 

   Length                          2 bytes        This gives the length of QT.

 

   QT information            1 byte          bit 0..3: number of QT (0..3, otherwise error)

                                                            bit 4..7: precision of QT, 0 = 8 bit, otherwise 16 bit

 

    Bytes                           n bytes         This gives QT values, n = 64*(precision+1)

 

Remarks:

·         A single DQT segment may contain multiple QTs, each with its own information byte.

·         For precision=1 (16 bit), the order is high-low for each of the 64 words.

DAC (Define Arithmetic Table) marker:

·         Current software does not support arithmetic coding .

·         JPEG files using arithmetic coding can not be processed.

 

SOS (Start Of Scan) marker:

 

          Field                      Size               Description

 

    Marker Identifier       2 bytes      0xff, 0xda identify SOS marker

 

    Length                       2 bytes      This must be equal to 6+2*(number of components in scan).

 

    Number of

    Components in scan  1 byte        This must be >= 1 and <=4 (otherwise error), usually 1 or 3

 

    Each component        2 bytes      For each component, read 2 bytes. It contains,

                                                         1 byte   Component Id (1=Y, 2=Cb, 3=Cr, 4=I, 5=Q),

                                                         1 byte   Huffman table to use :

                                                   bit 0..3 : AC table (0..3)

                                                               bit 4..7 : DC table (0..3)

 

    Ignorable Bytes          3 bytes      We have to skip 3 bytes.

 

Remarks:    The image data (scans) is immediately following the SOS segment. 


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Last Revised: 5th July 2002 8:05:20