JPEG File Interchange Format
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The JPEG File Interchange Format (JFIF) is an image file format standard. It is a format for exchanging JPEG encoded files compliant with the JPEG Interchange Format (JIF) standard. It solves some of JIF's limitations in regard to simple JPEG encoded file interchange. As with all JIF compliant files, image data in JFIF files is compressed using the techniques in the JPEG standard, hence JFIF is sometimes referred to as "JPEG/JFIF".
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[edit] Purpose
JFIF defines a number of details that are left unspecified by the JPEG standard (ISO/IEC IS 10918-1 | ITU-T Recommendation T.81]):
[edit] Component sample registration
JPEG allows multiple components (such as Y, Cb, and Cr) to have different resolutions, but it doesn't define how those differing sample arrays should be aligned. The JFIF standard requires samples to be sited "interstitially" — meaning the decoder can treat each component array as representing an array of equal-sized rectangular pixels sampled in their centers, with each array having the same exterior boundaries as the image. This is convenient for computer users, but is not the alignment used in MPEG-2 and most video applications.
[edit] Resolution and aspect ratio
The JPEG standard does not include any method of coding the resolution or aspect ratio of an image. JFIF provides resolution or aspect ratio information using an application segment extension to JPEG. It uses Application Segment #0, with a segment header of 'JFIF\x00', and specifies that this must be the first segment in the file, hence making it simple to recognise a JFIF file. Exif images recorded by digital cameras generally do not include this segment, but typically comply in all other respects with the JFIF standard.
[edit] Color space
JPEG does not define which color encoding is to be used for images. JFIF defines the color model to be used: either Y for greyscale, or YCbCr as defined by CCIR 601. Since this is not an absolute color space — unless an ICC profile, colorspace metadata, or an sRGB tag is provided and interpreted – a decoded JFIF image will be in a device-dependent RGB colorspace. Hence, JFIF does not by itself provide a mechanism for accurately transporting color-managed images across the Internet.
[edit] History
The standard was established on March 1, 1991 in a meeting at C-Cube Microsystems involving representitives of many companies, including C-Cube Microsystems, Radius, NeXT, Storm Tech, the PD JPEG group, Sun, and Handmade Software. The standard appears to have lost ownership, since C-Cube Microsystems are now defunct, and further development of the standard is dead. The latest version is v1.02, published September 1, 1992.
[edit] MIME Type
In 1996, RFC 2046 specified that the image format used for transmitting JPEG images across the internet should be JFIF. The MIME type of "image/jpeg" must be encoded as JFIF. In practice, however, virtually all Internet software can decode any baseline JIF image that uses Y or YCbCr components, whether it is JFIF compliant or not.
[edit] Exif/Photoshop compatibility
Formally, the Exif and JFIF standards are incompatible. This is because both specify that their particular application segment (APP0 for JFIF, APP1 for Exif) must be the first in the image file. In practice, many programs and digital cameras produce files with both application segments included. This will not affect the image decoding for most decoders, but poorly designed JFIF or Exif parsers may not recognise the file properly.
JFIF is compatible with Adobe Photoshop's JPEG "Information Resource Block" extensions, and IPTC Information Interchange Model metadata, since JFIF does not preclude other application segments, and the Photoshop extensions are not required to be the first in the file. However, Photoshop generally saves CMYK buffers as four-component "Adobe JPEGs" that are not conformant with JFIF. Since these files are not in a YCbCr color space, they are typically not decodable by Web browsers and other Internet software.
[edit] Exif comparison
The newer Exif standard provides almost all the features of the JFIF standard within its feature set. In particular, images can be tagged with an absolute color space, sRGB.
[edit] File format structure
The JFIF meta data resides in the JPEG Application Segment APP0, having the zero-terminated ASCII string "JFIF" as segment header.
[edit] JFIF segment format
| Field | Size (bytes) | Description |
|---|---|---|
| APP0 marker | 2 | Always equals 0xFFE0 |
| Length | 2 | Length of segment excluding APP0 marker |
| Identifier | 5 | Always equals "JFIF" (with zero following) (0x4A46494600) |
| Version | 2 | First byte is major version (currently 0x01), Second byte is minor version (currently 0x02) |
| Density units | 1 | Units for pixel density fields
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| X density | 2 | Integer horizontal pixel density |
| Y density | 2 | Integer vertical pixel density |
| Thumbnail width (tw) | 1 | Horizontal size of embedded JFIF thumbnail in pixels |
| Thumbnail height (th) | 1 | Vertical size of embedded JFIF thumbnail in pixels |
| Thumbnail data | 3 × tw × th | Uncompressed 24 bit RGB raster thumbnail |
[edit] JFIF extension (JFXX) segment format
An optional second application segment allows a thumbnail image to be embedded using several different image formats (to save space).
| Field | Size (bytes) | Description | |||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| APP0 marker | 2 | Always equals 0xFFE0 | |||||||||||||||||||||||||||
| Length | 2 | Length of segment excluding APP0 marker | |||||||||||||||||||||||||||
| Identifier | 5 | Always equals "JFXX" (with zero following) (0x4A46585800) | |||||||||||||||||||||||||||
| Thumbnail format | 1 | Specifies what data format is used for the thumbnail:
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| Thumbnail data | Variable |
JPEG
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[edit] External links
- JPEG File Interchange Format Specification v1.02, September 1, 1992.
