Before the image data is ever loaded when a JPEG image is selected for viewing the markers must be read.  In a JPEG image, the very first marker is the SOI, or Start Of Image, marker.  This is the first "hey, I'm a JPEG" declaration by the file.  The JPEG standard, as written by the Joint Picture Expert's Group, specified the JPEG interchange format.  This format had several shortcomings for which the JFIF (JPEG File Interchange Format) was an attempted remedy.  The JFIF is the format used by almost all JPEG file readers/writers.  It tells the image readers, "Hey, I'm a JPEG that almost anyone can understand."

Most markers will have additional information following them.  When this is the case, the marker and its associated information is referred to as a "header."   In a header the marker is immediately followed by two bytes that indicate the length of the information, in bytes, that the header contains.  The two bytes that indicate the length are always included in that count.

A marker is prefixed by FF (hexadecimal).  The marker/header information that follows does not specify all known markers, just the essential ones for baseline JPEG.

A component is a specific color channel in an image.  For instance, an RGB image contains three components; Red, Green, and Blue.

1998 by James R. Weeks

Start of Image (SOI) marker -- two bytes (FFD8)

JFIF marker (FFE0)

Define Quantization table marker (FFDB)

Define Huffman table marker (FFC4)

Start of frame marker (FFC0)

The H and V sampling factors dictate the final size of the component they are associated with. For instance, the color space defaults to YCbCr and the H and V sampling factors for each component, Y, Cb, and Cr, default to 2, 1, and 1, respectively (2 for both H and V of the Y component, etc.) in the Jpeg-6a library by the Independent Jpeg Group. While this does mean that the Y component will be twice the size of the other two components--giving it a higher resolution, the lower resolution components are quartered in size during compression in order to achieve this difference. Thus, the Cb and Cr components must be quadrupled in size during decompression.

Start of Scan marker (FFDA)

Comment marker (FFFE)

End of Image (EOI) marker (FFD9)


JPEG is rather complex in this aspect, so we shall just give an overview of the basic principles (see the JPEG Book, chapter 7 for the full picture).
JPEG data is divided into segments, each of which starts with a 2-byte marker.
All markers are byte-aligned - they start on the byte boundaries of the transmission/storage medium. Any variable-length data which precedes a marker is padded with extra ones to achieve this.
The first byte of each marker is FFH FFH . The second byte defines the type of marker.
To allow for recovery in the presence of errors, it must be possible to detect markers without decoding all of the intervening data. Hence markers must be unique. To achieve this, if an FFH FFH byte occurs in the middle of a segment, an extra 00H 00H stuffed byte is inserted after it and 00H 00H is never used as the second byte of a marker.
Some important markers in the order they are often used are:
Name Code (hex) Purpose
SOI FFD8 Start of image.
COM FFFE Comment (segment ignored by decoder). Lseg Lseg , <Text comments>
DQT FFDB Define quantisation table(s). Lseg Lseg , < Qlum Qlum , Qchr Qchr . >
SOF0 SOF0 FFC0 Start of Baseline DCT frame. Lseg Lseg , <Frame size, no. of components (colours), sub-sampling factors, Q-table selectors>
DHT FFC4 Define Huffman table(s). Lseg Lseg , <DC Size and AC (Run,Size) tables for each component>
SOS FFDA Start of scan. Lseg Lseg , <Huffman table selectors for each component> <Entropy coded DCT blocks>
EOI FFD9 End of image.
In table 1 the data which follows each marker is shown between <> brackets. The first 2-byte word of most segments is the length (in bytes) of the segment, Lseg Lseg . The length of <Entropy coded DCT blocks>, which forms the main bulk of the compressed data, is not specified explicitly, since it may be determined by decoding the entropy codes. This also allows the data to be transmitted with minimal delay, since it is not necessary to determine the total length of the compressed data before any of the DCT block data can be sent.
Long blocks of entropy-coded data are rather prone to being corrupted by transmission errors. To mitigate the worst aspects of this, Restart Markers (FFD0 . FFD7) may be included at regular intervals (say at the start of each row of DCT blocks in the image) so that separate parts of the entropy coded stream may be decoded independently of errors in other parts. The restart interval, if required, is defined by a DRI (FFDD) marker segment. There are 8 restart markers, which are used in sequence, so that if one (or more) is corrupted by errors, its absence may be easily detected.
The use of multiple scans within each image frame and multiple frames within a given image allows many variations on the ordering and interleaving of the compressed data. For example: