JPEG/JFIF file format:
~~~~~~~~~~~~~~~~~~~~~~
- header (2 bytes): $ff, $d8 (SOI) (these two identify a JPEG/JFIF file)
- for JFIF files, an APP0 segment is immediately following the SOI marker, see below
- any number of "segments" (similar to IFF chunks), see below
- trailer (2 bytes): $ff, $d9 (EOI)
Segment format:
~~~~~~~~~~~~~~~
- header (4 bytes):
$ff identifies segment
n type of segment (one byte)
sh, sl size of the segment, including these two bytes, but not
including the $ff 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 $ff and the type byte.
- Any number of $ff bytes between segments is legal and must be skipped.
Segment types:
~~~~~~~~~~~~~~
*TEM = $01 usually causes a decoding error, may be ignored
SOF0 = $c0 Start Of Frame (baseline JPEG), for details see below
SOF1 = $c1 dito
SOF2 = $c2 usually unsupported
SOF3 = $c3 usually unsupported
SOF5 = $c5 usually unsupported
SOF6 = $c6 usually unsupported
SOF7 = $c7 usually unsupported
SOF9 = $c9 for arithmetic coding, usually unsupported
SOF10 = $ca usually unsupported
SOF11 = $cb usually unsupported
SOF13 = $cd usually unsupported
SOF14 = $ce usually unsupported
SOF14 = $ce usually unsupported
SOF15 = $cf usually unsupported
DHT = $c4 Define Huffman Table, for details see below
JPG = $c8 undefined/reserved (causes decoding error)
DAC = $
cc Define Arithmetic Table, usually unsupported
*RST0 = $d0 RSTn are used for resync, may be ignored
*RST1 = $d1
*RST2 = $d2
*RST3 = $d3
*RST4 = $d4
*RST5 = $d5
*RST6 = $d6
*RST7 = $d7
SOI = $d8 Start Of Image
EOI = $d9 End Of Image
SOS = $da Start Of Scan, for details see below
DQT = $
db Define Quantization Table, for details see below
DNL = $
dc usually unsupported, ignore
SOI = $d8 Start Of Image
EOI = $d9 End Of Image
SOS = $da Start Of Scan, for details see below
DQT = $
db Define Quantization Table, for details see below
DNL = $
dc usually unsupported, ignore
DRI = $dd Define Restart Interval, for details see below
DHP = $de ignore (skip)
EXP = $
df ignore (skip)
APP0 = $e0 JFIF APP0 segment marker, for details see below
APP15 = $ef ignore
JPG0 = $f0 ignore (skip)
JPG13 = $fd ignore (skip)
COM = $fe Comment, for details see below
All other segment types are reserved and should be ignored (skipped).
SOF0: Start Of Frame 0:
~~~~~~~~~~~~~~~~~~~~~~~
- $ff, $c0 (SOF0)
- length (high byte, low byte), 8+components*3
- data precision (1 byte) in bits/sample, usually 8 (12 and 16 not
supported by most software)
- image height (2 bytes, Hi-Lo), must be >0 if DNL not supported
- image width (2 bytes, Hi-Lo), must be >0 if DNL not supported
- number of components (1 byte), usually 1 = grey scaled, 3 = color YCbCr
or YIQ, 4 = color CMYK)
- for each component: 3 bytes
- component id (1 = Y, 2 = Cb, 3 = Cr, 4 = I, 5 = Q)
- sampling factors (bit 0-3 vert., 4-7 hor.)
- quantization table number