Sustainability of Digital Formats
Planning for Library of Congress Collections
Sustainability of Digital Formats
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| Introduction | Sustainability Factors | Content Categories | Format Descriptions | Contact | |
| Full name | JPEG file with Exif metadata (common name). Part of Exchangeable image file format for digital still cameras: Exif Version 2.2 |
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| Description |
Exif specifications define files mainly intended for digital photography and JPEG_EXIF is one of two file types defined in the specification. Employs baseline JPEG compression encoding (ISO/IEC 10918-1), which ensures that JPEG_EXIF files can be read by many image software applications. Exif's special contribution is metadata inserted as application marker segments (APP1 and APP2), representing a significant extension beyond JPEG's typical provision. Several Web sites, including http://sylvana.net/jpegcrop/exifpatch.html, report that the placement of application marker segments (APPn) relative to start of image (SOI) means that a valid JPEG_EXIF image cannot also be a valid JFIF image. The Exif specification also provides a method for recording thumbnails and allows for the inclusion of audio in the WAVE format, to support such things as a photographer's spoken annotations. |
| Production phase | Generally applied by cameras in initial-state picture creation; may also be used for middle- and final-state archiving or end-user delivery. |
| Relationship to other formats | |
| Contains | JPEG_DCT_BL, JPEG DCT Compression Encoding, Baseline |
| Affinity to | EXIF_2_2, Exif Exchangeable Image File Format, Version 2.2. Specification defines metadata elements for use in JPEG-EXIF. |
| Has subtype | JPEG_DCF, JPEG Encoded File, Design Rule for Camera File System |
| LC experience or existing holdings | Prints and Photographs Division holds a modest number of digital-camera generated photographs in JPEG_EXIF. |
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| LC preference | General preference for still images is for uncompressed; rich metadata always welcome. Thus, for digital-camera photographs, TIFF_UNC_EXIF would be preferred (TIFF_UNC acceptable); JPEG_EXIF is acceptable and preferred to JFIF (also acceptable). |
| Disclosure | Fully disclosed, non-proprietary standard. JPEG is an ISO/IEC standard; Exif is from JEITA (Japan Electronics and Information Technology Industries Association). |
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| Documentation | See JPEG_DCT and EXIF_2_2 |
| Adoption | Very widely adopted in digital cameras and supported by a number of image software applications. JPEG_EXIF may be more widely adopted than TIFF_UNC_EXIF. |
| Licensing and patents | See JPEG_DCT and EXIF_2_2 |
| Transparency | See JPEG_DCT_BL. |
| Self-documentation | Good support. The Exif IFD (Image File Directory) attribute set defined in the specification includes tags pertaining to the Exif version, color space, camera manufacturer, date and time, and an extensive set pertaining to "picture-taking conditions." (pp. 24-25) Also from the specification: "Related attribute information for . . . compressed . . . files is stored in the tag information format defined in TIFF Rev. 6.0. Information specific to the camera system and not defined in TIFF is stored in private tags registered for Exif. . . . The reason for using the TIFF Rev. 6.0 tag format in the compressed [JPEG] file APP1 segment is to facilitate exchange of attribute data between Exif compressed [JPEG] and uncompressed [TIFF] files." (p. 2) |
| External dependencies | None. |
| Technical protection considerations | None. |
| Still Image | |
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| Normal rendering | Good support. |
| Clarity (high image resolution) | Good. See JPEG_DCT_BL and see also Notes. |
| Color maintenance | Good. The Exif tag set permits the identification of files that use sRGB.1 There appears to be no tag for the ICC profile2 for a capture device. |
| Support for graphic effects and typography | No support for vector graphics. |
| Functionality beyond normal rendering | Supports thumbnails and audio content. |
| Sound | |
| Normal rendering | Good support. |
| Fidelity (high audio resolution) | Good, depending on encoding, sampling frequency, and word length. Exif supports the following encodings: LPCM and μ-Law PCM (conforming to ITU-T G.711) for uncompressed audio data; IMA ADPCM (Adaptive Differential PCM) for compressed audio data. Highest fidelity from LPCM is limited in this standard to CD quality (44.1 kHz sampling and 16 bit words). |
| Multiple channels | Stereo only. |
| Support for user-defined sounds, samples, and patches | None |
| Functionality beyond normal rendering | None. |
| Tag | Value | Note |
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| Filename extension | jpg |
Based on examination of files in Library of Congress collections |
| Internet Media Type | See note. | Although JPEG_EXIF files are not JFIF files, the same MIME types may be used, e.g., image/jpeg. Comments welcome. |
| Magic numbers | HEX: FF D8 FF E1 xx xx 45 78 69 66 00 ASCII: ÿØÿà..EXIF. |
Derived from the specification and other sources. |
| General | From the specification:"The combination of pixel composition and pixel sampling for image data shall be RGB 4:4:4 and either Y:Cb:Cr =4:2:2 or Y:Cb:Cr = 4:2:0. The pixel composition of image data shall also be 8 bits each. This specification is appliedsimilarly to thumbnails. As sampling points on the elements making up pixels, the Y and Cb,Cr sampling points may be either co-sited or centered. . . . In the case of Y:Cb:Cr = 4:2:2, spatial co-siting is recommended for the sake of improved image quality on TV systems. For Y:Cb:Cr = 4:2:0, spatial centering is recommended as per the TIFF default and is [sic; "as in the" may have been intended] most common format of the personal computer applications." (page 5) |
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| History | See EXIF_2_2. |
1The color space sRGB, standardized as IEC 61966-2-1, establishes an image viewing environment with a known color temperature (6500 degrees Kelvin) and gamma (2.2), thus increasing the user's ability to maintain color.
2The most effective color maintenance systems rely on the existence of an ICC (International Color Consortium) profile of the capture device, which can then be compared to profiles for output devices, permitting appropriate adjustments of image color.
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