Digital Negative ( DNG ) is an open raw image format owned by Adobe used for digital photography. It was launched on September 27 2004. The launch was accompanied by the first version of the DNG specification, plus various products including a free of charge DNG Converter utility. All Adobe photo manipulation software (such as Adobe Photoshop and Adobe Lightroom) released since the launch supports DNG.
DNG is based on the TIFF/EP standard format, and mandates significant use of metadata. Exploitation of the file format is royalty free; Adobe have published a license allowing anyone to exploit DNG, and have also stated that there are no known intellectual property encumbrances or license requirements for DNG. Adobe stated that if there was a consensus that DNG should be controlled by a standards body, they were open to the idea. Adobe have submitted DNG to ISO for incorporation into their revision of TIFF/EP.
Rationale for DNG
This section is not a rationale for raw image formats. It is a rationale for DNG, given that raw image formats exist. It describes the objectives for DNG, lists its characteristics that facilitate and enable these objectives, and provides assessments of whether these objectives are met.
The implication is that DNG is unusual and satisfies objectives that other raw image formats do not. This section cites verification that DNG has relevant characteristics, but it is hard to cite verification that other formats don't have them. Therefore, anyone who is concerned about a particular objective and/or characteristic can check the status of DNG here, but must check the status of alternatives elsewhere. (There is further reading at the articles on raw image formats, dcraw, and reverse engineering).
Objectives
These objectives are repeatedly emphasized in Adobe documents:
- Digital image preservation (sometimes known as "archiving") : to be suitable for the purpose of preserving digital images as an authentic resource for future generations. Assessment : The US Library of Congress states that DNG is a recommended alternative to other raw image formats: "Less desirable file formats: RAW; Suggested alternatives: DNG". The Digital Photography Best Practices and Workflow (dpBestflow) project, funded by the United States Library of Congress and run by the American Society of Media Photographers (ASMP), singles out DNG, and states "DNG files have proven to be significantly more useful than the proprietary raw files in our workflow".
- Easy and/or comprehensive exploitation by software developers : to enable software to be developed without the need for reverse engineering; and to avoid the need for frequent software upgrades and re-releases to cater for new cameras. Assessment : Software could support raw formats without DNG, by using reverse engineering and/or dcraw; DNG makes it easier, and many software products can handle, via DNG, images from cameras that they have no specific knowledge of. An unresolved restriction is that any edit/development settings stored in the DNG file by a software product are unlikely to be recognized by a product from a different company. (This problem is not specific to DNG).
- In-camera use by camera manufacturers : to be suitable for many camera manufacturers to use as a native or optional raw image format in many cameras. Assessment : About 12 camera manufacturers have used DNG in-camera. About 38 camera models have used DNG. Raw image formats for more than 230 camera models can be converted to DNG.
- Multi-vendor interoperability : to be suitable for workflows where different hardware and software components share raw image files and/or transmit and receive them.
Characteristics
All of the above objectives are facilitated or enabled by most of these characteristics:
- Freely-available specification : This can be downloaded from the Adobe website without negotiation or needing justification.
- Format based on open specifications and/or standards : DNG is compatible with TIFF/EP, and various open formats and/or standards are used, including Exif metadata, XMP metadata, IPTC metadata, CIE XYZ coordinates, ICC profiles, and JPEG.
- Self-contained file format : A DNG file contains the data (raw image data and metadata) needed to render an image without needing additional knowledge of the characteristics of the camera.
- Version control scheme : it has a version scheme built into it that allows the DNG specification, DNG writers, and DNG readers, to evolve at their own paces.
- Freely-available source-code-based software development kit (SDK) : there are 3 aspects - there is an SDK; it is source-code-based (as can be verified by examination); and it can be downloaded from the Adobe website without needing justification.
- Documented to have no known intellectual property encumbrances or license requirements : there is both a "Digital Negative (DNG) Specification Patent License" which says that anyone can exploit DNG, and a statement that there are no known intellectual property encumbrances or license requirements for DNG,
Technical summary
A DNG file always contains data for one main image, plus metadata, and optionally contains at least one JPEG preview. It normally has the extension "dng" or "DNG".
DNG conforms to TIFF/EP and is structured according to TIFF. DNG supports various formats of metadata, (including Exif metadata, XMP metadata, IPTC metadata), and specifies a set of mandated metadata.
DNG is both a raw image format and a format that supports "non-raw", or "processed", images. The latter (non-raw) format is known as "Linear DNG". All images that can be supported as raw images can also be supported as Linear DNG. Images from the Foveon X3 sensor or similar, hence especially Sigma cameras, can only be supported as Linear DNG.
DNG can contain raw image data from sensors with various configurations of color filter array (CFA). These include: conventional Bayer filters, using 3 colors and rectangular pixels; 4 color CFAs, for example the RGBE filter used in the Sony DSC-F828; rectangular (non-square) pixels, for example as used in the Nikon D1X; and offset sensors (for example with octagonal pixels) such as Super CCD sensors of various types, as used in various Fujifilm cameras. (Or combinations of these if necessary). DNG specifies metadata describing these individual parameters; this is one significant extension to TIFF/EP.
When used in a CinemaDNG movie clip, each frame is encoded using the above DNG image format. The clip's image stream can then be stored in one of two formats: either as video essence using frame-based wrapping in an MXF file, or as a sequence of DNG files in a specified file directory.
Timeline
This provides a mixture of the dates of significant events (such as "the first X") and various counts of usage at the anniversaries of the launch (each 27 September).
Counts of products and companies that use DNG in some way are provided primarily for illustration. They are approximate, and include products that are no longer sold. The purpose is mainly to demonstrate that such products and companies exist, and to show trends. Convertible raw image formats (camera models whose raw images can be converted to DNG) only include official support by Adobe DNG converters; not unofficial support by Adobe products (sometimes reaching about 30), nor support by other DNG converters.
- 2003, late : Adobe started work on the DNG specification.
- 2004, early : Adobe started talking to other companies about DNG.
- Launch , 2004, 27 September : Adobe launched DNG. Specification version 1.0.0.0 published. Convertible raw image formats: 60+
- 2005, February : Specification version 1.1.0.0 published.
- 2005, June : First digital back to write DNG, the Leica DMR (Digital Modul R) back for the R8 and R9.
- 2005, July : First camera manufacturer to provide a DNG converter for its own raw file formats - Hasselblad's FlexColor.
- 2005, July : First genuine digital SLR camera to write DNG, Hasselblad H2D.
- 1st anniversary , 2005, 27 September : Camera manufacturers: 4. Camera models: 7. Software products: 70+. Convertible raw image formats: 70+.
- 2005, October : First compact camera to write DNG, Ricoh GR Digital.
- 2006, July : First monochrome digital back to write DNG, MegaVision E Series MonoChrome.
- 2006, September : First rangefinder camera to write DNG, Leica M8.
- 2006, September : First camera to offer the user a choice of proprietary raw or DNG, Pentax K10D.
- 2nd anniversary , 2006, 27 September : Camera manufacturers: 8. Camera models: 9. Software products: 120+. Convertible raw image formats: 110+.
- 2007, May : First raw converter & photo-editor whose first raw-handling release only supported DNG, Pixelmator.
- 2007, July : First underwater camera to write DNG, Sea&Sea DX-1G. (Based on Ricoh Caplio GX100).
- 3rd anniversary , 2007, 27 September : Camera manufacturers: 10. Camera models: 13. Software products: 170+. Convertible raw imag
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