Sustainability of Digital Formats: Planning for Library of Congress Collections

File format for storing and interchanging CAM_RAW (camera raw) images, usually accompanied by JPEG secondary version(s) of the images. This format is a later version of the Adobe Digital Negative (DNG), Version 1.1 format. All DNG are considered by the specification as an extension of the TIFF 6.0 format and should follow all TIFF 6.0 formatting rules.

DNG 1.6 are closely related to Apple ProRAW files, as Apple and Adobe worked together to add new features and tags to this DNG version. According to Lux Camera's "Understanding ProRAW", the Digital Negative (DNG) Specification Version 1.6.0.0 specification and the Apple ProRAW public beta were released on the same day.

DNG 1.6 has new features compared to previous versions, which include:

• Sections on 64-bit format and Semantic Masks have been added to Chapter 2, “DNG Format Overview.”
• In Chapter 2, the section on “Camera Profiles” has been updated with information about triple-illuminant profiles.
• In Chapter 3, “Restrictions and Extensions to Existing TIFF Tags,” sections on NewSubFileType, Compression, and PhotometricInterpretation have been updated.
• Additional tags were added to Chapter 4, “DNG Tags” for Version 1.6.
• In Chapter 6, “Mapping Camera Color Space to CIE XYZ Space,” new information about triple-illuminant profiles has been added.
• In Chapter 7, “Opcode List Processing,” a section on new opcode WarpRectilinear2 has been added.

DNG 1.6 and later supports the concept of "Semantic Masks" which may be used for purposes such as image segmentation. Semantic information can be stored in IFDs with a NewSubFileType of 65540 (10004.H). The semantic information IFD should use a PhotometricInterpretation of 52527 (CD2F.H). A DNG 1.6 file can store more than one semantic mask.

Along with the addition of this field and concept are the tags SemanticName and SemanticInstanceID, to support the identification of specific masks: "Each semantic mask IFD should include a SemanticName tag with a unique string that identifies the purpose of the mask."

DNG 1.6 adds the new ProfileGainTableMap tag. The specification defines this tag as "contains spatially varying gain tables that can be applied while processing the image as a starting point for user adjustments."

DNG 1.6 and later supports new profile tags for specifying custom illuminant data. As per the specification, starting with DNG 1.6, the value may be set to 255 (Other) to indicate a custom illuminant.

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 DNG specification states that TIFF/EP stores the tags in IFD 0 (IFD stands for Image File Directories, in effect segments of a TIFF file), 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 converters is placed under private tags, in private IFDs, and/or a private MakerNote. (pp. 12-13)

Accessibility Features

Accessibility for still image content is often supported by the use of alt text when displayed on the web. The carriage of this data is typically not embedded in the file itself but rather in the HTML code. See See W3C's Text alternatives for non-text content for more information. Because DNG 1.6 behaves like a TIFF file, see TIFF_6: "TIFF files have moderate support for accessibility features through the use of embedded metadata in TIFF tags. For example, ImageDescription tag (code 270) describes the content of the image but this may not be accessible to screen readers.