Block diagram
JPEG File Interchange Format, or JFIF, or, more colloquially known as just "JPEG" or "JPG", is a popular 2D bitmap image file format, offering lossy compression which works reasonably well with photographic images.
Format is organized as a container format, serving multiple "segments", each starting with a magic and a marker. JFIF standard dictates order and mandatory apperance of segments:
This page hosts a formal specification of JPEG (Joint Photographic Experts Group) File Interchange Format using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.
meta:
id: jpeg
title: JPEG (Joint Photographic Experts Group) File Interchange Format
file-extension:
- jpg
- jpeg
- jpe
- jif
- jfif
- jfi
xref:
ecma: TR/98
forensicswiki: jpeg
justsolve: JFIF
loc: fdd000017
mime: image/jpeg
pronom:
- fmt/42 # JFIF 1.00
- fmt/43 # JFIF 1.01
- fmt/44 # JFIF 1.02
wikidata: Q26329975
license: CC0-1.0
imports:
- exif
endian: be
doc: |
JPEG File Interchange Format, or JFIF, or, more colloquially known
as just "JPEG" or "JPG", is a popular 2D bitmap image file format,
offering lossy compression which works reasonably well with
photographic images.
Format is organized as a container format, serving multiple
"segments", each starting with a magic and a marker. JFIF standard
dictates order and mandatory apperance of segments:
* SOI
* APP0 (with JFIF magic)
* APP0 (with JFXX magic, optional)
* everything else
* SOS
* JPEG-compressed stream
* EOI
seq:
- id: segments
type: segment
repeat: eos
types:
segment:
seq:
- id: magic
contents: [0xff]
- id: marker
type: u1
enum: marker_enum
- id: length
type: u2
if: marker != marker_enum::soi and marker != marker_enum::eoi
- id: data
size: length - 2
if: marker != marker_enum::soi and marker != marker_enum::eoi
type:
switch-on: marker
cases:
'marker_enum::app0': segment_app0
'marker_enum::app1': segment_app1
'marker_enum::sof0': segment_sof0
'marker_enum::sos': segment_sos
- id: image_data
size-eos: true
if: marker == marker_enum::sos
enums:
marker_enum:
0x01: tem
0xc0: sof0 # start of frame 0
0xc1: sof1 # start of frame 1
0xc2: sof2 # start of frame 2
0xc3: sof3 # start of frame 3
0xc4: dht # define Huffman table
0xc5: sof5 # start of frame 5
0xc6: sof6 # start of frame 6
0xc7: sof7 # start of frame 7
0xd8: soi # start of image
0xd9: eoi # end of image
0xda: sos # start of scan
0xdb: dqt # define quantization table
0xdc: dnl # define number of lines
0xdd: dri # define restart interval
0xde: dhp # define hierarchical progression
0xe0: app0
0xe1: app1
0xe2: app2
0xe3: app3
0xe4: app4
0xe5: app5
0xe6: app6
0xe7: app7
0xe8: app8
0xe9: app9
0xea: app10
0xeb: app11
0xec: app12
0xed: app13
0xee: app14
0xef: app15
0xfe: com # comment
segment_app0:
seq:
- id: magic
type: str
encoding: ASCII
size: 5
- id: version_major
type: u1
- id: version_minor
type: u1
- id: density_units
type: u1
enum: density_unit
- id: density_x
type: u2
doc: Horizontal pixel density. Must not be zero.
- id: density_y
type: u2
doc: Vertical pixel density. Must not be zero.
- id: thumbnail_x
type: u1
doc: Horizontal pixel count of the following embedded RGB thumbnail. May be zero.
- id: thumbnail_y
type: u1
doc: Vertical pixel count of the following embedded RGB thumbnail. May be zero.
- id: thumbnail
size: thumbnail_x * thumbnail_y * 3
doc: Uncompressed 24 bit RGB (8 bits per color channel) raster thumbnail data in the order R0, G0, B0, ... Rn, Gn, Bn
enums:
density_unit:
0: no_units
1: pixels_per_inch
2: pixels_per_cm
segment_app1:
seq:
- id: magic
type: strz
encoding: ASCII
- id: body
type:
switch-on: magic
cases:
'"Exif"': exif_in_jpeg
segment_sof0:
seq:
- id: bits_per_sample
type: u1
- id: image_height
type: u2
- id: image_width
type: u2
- id: num_components
type: u1
- id: components
type: component
repeat: expr
repeat-expr: num_components
types:
component:
seq:
- id: id
type: u1
enum: component_id
doc: Component selector
- id: sampling_factors
type: u1
- id: quantization_table_id
type: u1
instances:
sampling_x:
value: (sampling_factors & 0xf0) >> 4
sampling_y:
value: sampling_factors & 0xf
segment_sos:
seq:
- id: num_components
type: u1
doc: Number of components in scan
- id: components
type: component
repeat: expr
repeat-expr: num_components
doc: Scan components specification
- id: start_spectral_selection
type: u1
doc: Start of spectral selection or predictor selection
- id: end_spectral
type: u1
doc: End of spectral selection
- id: appr_bit_pos
type: u1
doc: Successive approximation bit position high + Successive approximation bit position low or point transform
types:
component:
seq:
- id: id
type: u1
enum: component_id
doc: Scan component selector
- id: huffman_table
type: u1
# Extra wrapper for EXIF, as there is extra 0 byte that needs to be
# parsed. The actual EXIF specification is defined in external .ksy
# file, as it is shared with other formats (TIFF, PNG, XCF, etc).
exif_in_jpeg:
seq:
- id: extra_zero
contents: [0]
- id: data
size-eos: true
type: exif
enums:
component_id:
1: y
2: cb
3: cr
4: i
5: q