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 | Adobe Digital Negative (DNG), Version 1.1 |
| Description | File format for storing and interchanging camera raw images. This subtype
of TIFF 6.0 wraps bitmaps produced by a variety of cameras. Special metadata
about image characteristics is recorded under tags that have been added
to the normal TIFF set; see Notes below. The special
metadata is created by applications like the Adobe Camera Raw plug-in
for PhotoShop, and is later used in various applications that interpret
the camera raw data to produce a variety of image outputs. As an extension of the TIFF 6.0 format, DNG is compatible with the TIFF/EP standard. The DNG specification states, "It is possible (but not required) for a DNG file to simultaneously comply with both the Digital Negative specification and the TIFF/EP standard." (p. 10) In Notes below, see the comment concerning what is sometimes called Linear DNG as compared to Raw DNG. This Web page currently reflects the specification for version 1.1.0.0 and it will be updated for any new versions up to 1.2. |
| Production phase | Middle-state format. |
| Relationship to other formats | |
| Is subtype of | TIFF_6, TIFF, Revision 6.0 |
| May contain | CAM_RAW, Camera Raw Formats |
| Has earlier version | Adobe Digital Negative (DNG), Version 1.0.0.0, not documented at this time |
| LC experience or existing holdings | See TIFF_6 |
| LC preference | None established. LC staff, however, have intermittently discussed the value of a preference for born-digital photographs in DNG form ("somewhat normalized"), as compared to camera raw files (CAM_RAW; "too raw") or uncompressed TIFF files (TIFF_UNC; "too cooked"). |
| Disclosure | Fully documented. Developed by Adobe System, Inc. |
| Documentation | Digital Negative (DNG) Specification, version 1.1.0.0 (February 2005). See also TIFF_6. |
| Adoption | Camera raw and DNG are much discussed in online photography forums. As of February 2007, the DNG hardware and software support page reported that eight camera manufacturers have introduced cameras that generate the DNG format natively and that about forty software applications support the DNG format. Adobe reports that the DNG converter continues to add support for proprietary camera formats, with more than one hundred camera models represented. Barry Pearson's DNG products Web page ("frozen" in 2005) lists 77 non-Adobe products that support DNG. |
| Licensing and patent claims | Adobe statement: "Adobe provides the DNG Specification to the public for the purpose of encouraging implementation of this file format in a compliant manner. This [Web page] is a patent license granted by Adobe to individuals and organizations that desire to develop, market and/or distribute hardware and software that reads and/or writes image files compliant with the DNG Specification." |
| Transparency | Wrapper is transparent; encoded image bitstream may require tools to render. |
| Self-documentation | See Tags for TIFF and Related Specifications.
Metadata may be embedded in a DNG file using tags from (1) TIFF_6,
(2) TIFF/EP or EXIF_2_2
(see also TIFF_UNC_EXIF), (3) IPTC (TIFF
tag 33723), and (4) XMP (TIFF tag 700). Regarding TIFF/EP and EXIF, the specification states that TIFF/EP stores the tags in IFD 0, while TIFF_UNC_EXIF stores them in a separate IFD. Either location is allowed but the EXIF location is preferred. Proprietary metadata that may be used by camera manufacturer's raw convertors is to placed under private tags, in private IFDs (Image File Directories), and/or a private MakerNote. (pp. 12-13) |
| External dependencies | None |
| Technical protection considerations | None |
| Normal rendering for still images | The format behaves like any TIFF file. Images encoded as raw will be rendered with a "normal" look by a reader that can interpret the special metadata in a DNG file. |
| Clarity (support for high image resolution) | Raw image data and DNG's special metadata support the output of images with high levels of clarity, depending upon the capabilies of the camera used to produce them. See Notes below. |
| Color maintenance | Excellent. Support provided for ICC profiles and for color space identification. |
| Support for graphic effects and typography | Not relevant in this camera-oriented format. |
| Functionality beyond normal image rendering | The format is intended for use with image manipulation tools that execute a variety of processes to transform the DNG into "pictures" suitable for different outputs. |
| Tag type | Value | Note |
| Filename Extension | dng | Recommended by the specification (p. 11) |
| Filename Extension | tif | The specification states that "readers should accept either the '.DNG' or '.TIF' extensions for compatibility with TIFF/EP." (p. 11) |
| Internet Media Type | image/tiff | This is suggested in an indirect way by some commentators; comments welcome |
| Magic numbers | None found | DNG may use the magic numbers associated with TIFF_6; comments welcome |
| General | Rationale for DNG. From the specification: "Seeking a greater
degree of flexibility and artistic control, professional photographers
increasingly opt to manipulate raw data from their digital cameras. Unlike
JPEG and TIFF formats which store images that have been processed by the
camera, camera raw files capture unprocessed or minimally processed data
directly from the camera sensor. Because they are analogous to film negatives
in a photographer's workflow, camera raw formats are often referred to
as 'digital negatives.' Camera raw formats offer both advantages and disadvantages.
One advantage is increased artistic control for the end user. The user
can precisely adjust a range of parameters, including white balance, tone
mapping, noise reduction, sharpening and others, to achieve a desired
look. One disadvantage is that unlike JPEG and TIFF files which are ready
for immediate use, camera raw files must be processed before they can
be used, typically through software provided by the camera manufacturer
or through a converter like the Adobe Camera Raw plug-in for Adobe Photoshop
software." (p. 9) Adobe provides a free Adobe DNG Converter that translates
raw files from many of today's popular cameras. Re: TIFF tags. The DNG specification lists some restrictions or extensions for the TIFF tags BitsPerSample, Compression, Photometric Interpretation, and Orientation (pp. 15-16). It also lists DNG-specific tags, including but not limited to: DNGversion, UniqueCameraModel, LocalizedCameraModel, CFAPlaneColor (CFA stands for Color Filter Array), CFALayout, LinearizationTable, BlackLevel, WhiteLevel, DefaultScale (for cameras with non-square pixels), BestQualityScale, DefaultCropOrigin, DefaultCropSize, CameraSerialNumber, LensInfo, RawDataUniqueID, OriginalRawFileName, OriginalRawFileData (to incorporate the raw file as created), AsShotICCProfile, and CurrentICCProfile (pp. 17-44). The specification also describes how raw camara sensor values are mapped to linear reference values (p. 45) and how camera color space is mapped to CIE XYZ color space (pp. 47-48). "Raw DNG" and "Linear DNG." The photographer Barry Pearson writes that DNG files are well understood to contain "mosaiced" sensor data (typically from a Bayer array), and he calls this format Raw DNG. But he reminds readers of another option, Linear DNG, said to be "a rarer variety of DNG, containing RGB image data (or not just RGB!) arranged in a rectilinear format. (Most practical examples of Linear DNG hold RGB image data, but it isn't a constraint, and 4 or more colours are allowed. For example, A Linear DNG obtained by converting the raw file from a Sony F828 has an extra channel, 'Aqua,' rather like Cyan). This RGB image data may have come from demosaiced raw image data, or from another source such as TIFF or JPEG or something else. (There is little difference between 'demosaiced' and 'never mosaiced')." Jeff Schewe, writing in a 2005 issue of PhotoshopNews.com reported that Adobe's DNG convertor application "allows you to either keep the DNG in the un-demosaiced form or to demosaic the file and convert to a linear file. Generally, you'll want to keep the file in its un-demosaiced form as there is a space savings. Linear DNGs have already been converted to RGB files and therefore are larger." The article as presented includes an inserted editor's note: "[Digital imaging expert] Bruce Fraser sent the following comment regarding Linear DNG: 'The only reason to use Linear DNG is to feed the file to a DNG reader that can't understand the particular flavor of DNG that DNG Converter creates for the specific camera, or to act as an interchange format—e.g., lens correx with DxO can write out a linear DNG that ACR can read. This is typically NOT an option people want, not because of size considerations, but because it's no longer really raw—it's half-baked. All the operations that take place during demosaicing are set in stone and can't be redone.'" |
| History |
URLs
• Digital Negative (DNG) Specification, version 1.1.0.0, February 2005 (http://www.adobe.com/products/dng/pdfs/dng_spec.pdf).
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URLs
• Adobe DNG page (http://www.adobe.com/products/dng/).
• OpenRAW Organization (http://www.openraw.org/)
• Discussion of DNG and raw at the OpenRaw site (http://www.openraw.org/node/1482)
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Page including links to two Adobe documents by Bruce Fraser: "Understanding digital raw capture" and "Linear gamma." (http://www.adobe.com/products/photoshop/cameraraw.html).
• Linear DNG by Barry Pearson (http://www.barrypearson.co.uk/articles/dng/linear.htm)
• DNG Workflow by Jeff Schewe in PhotoshopNews.com (http://www.photoshopnews.com/2005/05/23/dng-workflow-part-i/)
• DNG products listed by Barry Pearson (http://www.barrypearson.co.uk/articles/dng/products_y1.htm)
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