JPEG File Interchange Format
| JPEG File Interchange Format | |
|---|---|
|
|
| File extension : |
.jpg, .jpeg, .jpe, .jfif
|
| MIME type : | image / jpeg |
| Magic number : | FF D8 |
| Developed by: | Joint Photographic Experts Group |
| Type: | Lossy graphic format |
| Standard (s) : | ISO / IEC 10918, ITU-T T.81, ITU-T T.83, ITU-T T.84, ITU-T T.86 |
| Website : | www.jpeg.org |
The JPEG File Interchange Format (JFIF) is a graphic format developed by Eric Hamilton in 1991 for storing images that have been compressed according to the JPEG standard. The filename extension usually used is jpg, less often jpeg, jpe or jfif . JFIF is the most widely used graphics format for photos on the World Wide Web .
The JPEG standard only defines the compression process, but not how the resulting image data must be stored so that they can be exchanged between different computer programs . For example, JPEG does not specify which color space to use. JFIF is a method of storing JPEG data; other options are the Still Picture Interchange File Format (SPIFF) published by the Joint Photographic Experts Group and JPEG Network Graphics . More commonly, a “JPEG file” means a JFIF file.
JFIF only makes use of a small part of the possibilities offered by JPEG: for example, only YCbCr is permitted as a color model and no longer RGB ; only Huffman coding is allowed for entropy coding . There are also some mechanisms for synchronization and resynchronization in the event of transmission errors.
File format
JFIF limits the size of JPEG images to 65,535 × 65,535 pixels.
JFIF image files are preceded by an SOI marker FF D8. These markers JFIF tag follows: FF E0 00 10 4A 46 49 46 00. The sequence 4A 46 49 46is the ASCII representation of "JFIF". Common image programs can also process files without a JFIF tag.
The JFIF files are divided into segments ( called tags in TIFF ). The segments are generally structured in the following form: A introduces the day. FF is a hexadecimal value and corresponds to decimal 255. The xx determines the type of tag. 256 * s1 + s2 specifies the length of the segment. Further images can be hidden in the segments, often a small preview image for a quick preview. Individual segments can appear multiple times.
FF xx s1 s2
FF xx
|
symbol | designation |
|---|---|---|
FF D8 |
SOI | Start of image |
FF E0 |
APP0 | JFIF day |
FF Cn |
SOF n | Start of Frame Marker, defines the type of compression: |
FF C0 |
SOF 0 | Baseline DCT |
FF C1 |
SOF 1 | Extended sequential DCT |
FF C2 |
SOF 2 | Progressive DCT |
FF C3 |
SOF 3 | Lossless (sequential) |
FF C4 |
DHT | Definition of the Huffman tables |
FF C5 |
SOF 5 | Differential sequential DCT |
FF C6 |
SOF 6 | Differential progressive DCT |
FF C7 |
SOF 7 | Differential lossless (sequential) |
FF C8 |
JPG | reserved for JPEG extensions |
FF C9 |
SOF 9 | Extended sequential DCT |
FF CA |
SOF 10 | Progressive DCT |
FF CB |
SOF 11 | Lossless (sequential) |
FF CD |
SOF 13 | Differential sequential DCT |
FF CE |
SOF 14 | Differential progressive DCT |
FF CF |
SOF 15 | Differential lossless (sequential) |
FF CC |
DAC | Definition of the arithmetic coding |
FF DB |
DQT | Definition of the quantization tables |
FF DD |
DRI | Define restart interval |
FF E1 |
APP1 | Exif data |
FF EE |
APP14 | Often for copyright entries |
FF En |
APP n | n = 2..F general pointer |
FF FE |
COM | Comments |
FF DA |
SOS | Start of Scan |
FF D9 |
EOI | End of image |
An example of a copyright tag:
FF EE 00 26 'File written by ...'
An example of a comment tag:
FF FE s1 s2 'This photo shot is dated ...'
Structure of a JFIF file:
| SOI segment |
FF D8
|
| APP0 segment |
FF E0 s1 s2 4A 46 49 46 00 v1 v2 ...
|
| ... further segments ... | |
| SOS segment |
FF DA
|
| Data | Compressed data |
| EOI segment |
FF D9
|
The length specifications of the segments also contain the space for the length specifications themselves: An empty segment therefore has the length 2, since this corresponds to the length of the components s1 and s2 themselves.
The start-of-scan (SOS) segment (marker FF DA) is immediately followed by the compressed data until the start of the next segment, which is indicated by the next marker. If a FFoccurs within the data , this is 00marked with a following (zero). Other values indicate the appearance of a new segment or marker. Exception: If FFone of the restart markers ( D0 - D7) follows this, the data continues directly behind it:FF DA ... daten ... FF D0 ... daten ...
Since a decoder is only interested in data between a start mark and an end mark, it is also possible to accommodate Exif , IPTC-IIM standard information and even entire XML or other files in the JFIF file using the data container method. However, the extensions are not JFIF-compliant. It is recommended to remove such files before publishing an image if they contain private data.
Exif metadata
Metadata in Exif format can also be saved in image files in JPEG format ; Many digital cameras store information about the manufacturer and software version here. License data can also be saved. The habit of some image recording (e.g. software in digital cameras) and image editing programs to save preview images in the Exif field, which are not necessarily adjusted when the image file is changed, attracted attention ; this means that parts of the image file that are not intended for publication can still be made public in low resolution.
literature
- Thomas W. Lipp: Graphic formats. Microsoft Press, Unterschleißheim 1997, ISBN 3-86063-391-0 .
- John Miano: Compressed Image File Formats. Addison-Wesley, Reading 2000, ISBN 0-201-60443-4 .
Web links
- JFIF specification, version 1.02 (PDF; 23 kB)