Sustainability of Digital Formats
 Planning for Library of Congress Collections

Introduction | Sustainability Factors | Content Categories | Format Descriptions | Contact
Content Categories >> Still Image | Sound | Textual | Moving Image | Web Archive | Datasets | Geospatial | Generic

Still Image >> Preferences in Summary


Bitmapped digital visual images

General
Photographs and other graphics. Clarity characteristics (bitstream encoding) should be used as the primary consideration; choice of file formats as secondary. The Library prefers formats that support the inclusion of descriptive and technical/administrative metadata, favoring metadata in standardized forms, e.g., EXIF_2_2; DIG35 (which is incorporated in JPX_FF), JPEG 2000 Part 2 (Extensions) jpx File Format; and NISO Z39.87, Technical Metadata for Digital Still Images.

Bitstream encoding for visual images (relates to clarity)
Spatial resolution (affects zooming or enlargement potential)
• Higher spatial resolution preferred over lower spatial resolution. Resolution may be expressed as pixels per inch, e.g., 300 ppi, or by pixel dimensions.
• Specific pixel dimensions will vary from application to application.
• Commercial photofinishers, for example, generally recommend that customers seeking an 8x10-inch print provide images with at least 1200x1600 pixels (approximately the output of a 3 megapixel digital camera), noting that 2400x3000 pixels (7 megapixel camera) will produce improved results.
• Professional photographers, for example, may produce images for, say, magazine advertisements with pixel counts of 7000x5600 or higher. The digitization of historical photographs by the Library of Congress represents another professional example: in 2004, 35mm negatives were scanned at 5400x3800 pixels.
• Pixel settings as of the time of creation, e.g., as captured in a camera or saved in a paint program, preferred over rescaled or interpolated settings.
Characteristics related to tonality (for color and grayscale)
• Specified color space preferred over unspecified or unknown color space.
• Color space from initial creation preferred to transcoded color space.
• RGB or luminance-chrominance color space, e.g., YUV, YCC, YCrCb, preferred for images originating from scanners or cameras.
• For RGB and luminance-chrominance formats, greater bit depth (tonal resolution) preferred over lesser (e.g., 24-bit RGB preferred to 8-bit indexed color; 16-bit-per-channel data preferred to 8-bit.).
• For RGB formats, inclusion of creation-device ICC color profile or equivalent preferred to omission.
• For RGB formats, sRGB color space preferred when profiles or other color management tools have not been employed.
• Color-specifying color space, e.g., CMYK and CIELab, preferred for bitmapped images originating in paint or other graphic arts software.
• CMYK images that comply with Specifications for Web Offset Publications (SWOP) or Specifications for Newsprint Advertising Production (SNAP) preferred over non-compliant images.
• Scene-referred data: no preference has been established at this time.
• Multispectral (hyperspectral) images: no preference has been established at this time.
Characteristics related to compression (for color and grayscale)
• Uncompressed bitmapped images preferred over use of
• lossy compression.
• lossless compression schemes that are not fully disclosed or are subject to use-based license fees.
• Lower compression ratios preferred over higher for same compression scheme and resolution.
• Discrete Wavelet Transform (DWT) compression (e.g., JPEG 2000) preferred over Discrete Cosine Transform (DCT) compression (e.g., JPEG).
Special functionality
• Image layer encoding only recommended if layer-separation is essential to creator's intent or layers are to support offset printing (as in PDF/X) and curators wish to retain that functionality. In other cases, images flattened to one plane are preferred.
• The following elements are acceptable; documented use in publicly disclosed formats is preferred to undocumented use or use in formats for which the specifications have not been disclosed.
• Multipage
• Multiresolution and quality layers
• Regions of interest (ROI) encoded at higher quality than the background
• Camera RAW files: images in these varied, proprietary formats should be normalized as indicated in the less desirable file formats section below.
File format and characteristics
• Not copy-protected rather than copy-protected.
• Relatively complete descriptive and technical metadata rather than minimal.
• Acceptable file formats for color or grayscale bitmapped images, grouped in order of preference.
1.   TIFF_UNC (TIFF, Uncompressed Bitmap) in any color space supported by TIFF
TIFF_UNC_EXIF (TIFF Uncompressed File with Exif metadata) for images from digital cameras
TIFF/EP (TIFF/EP, for Electronic Photography) for images from digital cameras
JP2_J2K_C_LL (JPEG 2000 Part 1 Core Coding, Lossless Compression) in JPX_FF (JPEG 2000 Part 2 [Extensions] jpx File Format), conforming to capabilities of a baseline JPX reader
JP2_J2K_C_LL (JPEG 2000 Part 1 Core Coding, Lossless Compression) in JP2_FF (JPEG 2000 Part 1 [Core] jp2 File Format)
TIFF/IT (TIFF/IT, for Image Technology) for prepress images
2.   JP2_J2K_C_LSY (JPEG 2000 Part 1 Core Coding, Lossy Compression) in JPX_FF (JPEG 2000 Part 2 [Extensions] jpx File Format), conforming to capabilities of a baseline JPX reader
JP2_J2K_C_LSY (JPEG 2000 Part 1 Core Coding, Lossy Compression) in JP2_FF (JPEG 2000 Part 1 [Core] jp2 File Format)
3.   PDF/A (PDF for Long-term Preservation)
PDF/X (PDF for Graphics Exchange)
JPEG_DCT_BL (JPEG DCT Encoding, Baseline), lossy compression in JFIF (JFIF JPEG File Interchange Format)
4.   PNG (Portable Network Graphics Format)
5.   GIF (GIF Graphics Interchange Format, version 89a)
6.   BMP file (Microsoft Windows Bitmap Format)

• Acceptable file formats for bitonal bitmapped images, in order of preference
1.   TIFF_G4 (TIFF Bitmap with Group 4 Compression)
2.   J2K_COMP (JPEG 2000 Part 6, Compound Images) with G4 or JBIG2 compression in JPM_FF (JPEG 2000 Part 6 [Compound] jpm File Format)

• Less desirable file formats
File format Suggested alternatives
RAW TIFF_UNC (TIFF, Uncompressed Bitmap)
JP2_J2K_C_LL (JPEG 2000 Part 1 Core Coding, Lossless Compression) in JP2_FF (JPEG 2000 Part 1 [Core] jp2 File Format)
DNG (Adobe Digital Negative)
PhotoShop PSD TIFF_UNC (TIFF, Uncompressed Bitmap)
TIFF/IT (TIFF/IT, for Image Technology)
PDF/A (PDF for Long-term Preservation)
PDF/X (PDF for Graphics Exchange)
PCD format (ImagePak derived from Kodak PhotoCD), from highest resolution version TIFF_UNC (TIFF, Uncompressed Bitmap)
JP2_J2K_C_LL (JPEG 2000 Part 1 Core Coding, Lossless Compression) in JP2_FF (JPEG 2000 Part 1 [Core] jp2 File Format)
Encapsulated PostScript EPS TIFF_UNC (TIFF, Uncompressed Bitmap)
TIFF/IT (TIFF/IT, for Image Technology)
PDF/A (PDF for Long-term Preservation)
PDF/X (PDF for Graphics Exchange)
FlashPix, from highest resolution version TIFF_UNC (TIFF, Uncompressed Bitmap)
JP2_J2K_C_LL (JPEG 2000 Part 1 Core Coding, Lossless Compression) in JP2_FF (JPEG 2000 Part 1 [Core] jp2 File Format)

Vector-based digital visual images

General
Diagrams, graphs, and drawings created in draw software. The bitstream encoding in a variety of formats is based on similar principles and approaches. The primary preference considerations, therefore, have to do with the openness or standardization of a format (disclosure), and/or with how widely adopted it is.

For certain classes of works, e.g., documentary architectural drawings, the Library has indicated that its current preference is not to retain vector data and but to receive these works saved as bitmapped images (see section 1 above).

File formats and characteristics
• Not copy-protected rather than copy-protected.
• Relatively complete descriptive and technical metadata rather than minimal
• Acceptable formats in order of preference.
SVG_1_1 (Scalable Vector Graphics, Version 1.1) or SVG_1_2 (Scalable Vector Graphics, Version 1.2)
• AutoCad Drawing Interchange Format (DXF)
• Formats for which the compilers plan additional investigation.
• Initial Graphics Exchange Specification (IGES)
• Computer Graphics Metafile (CGM and WebCGM)
• Less desirable formats
Format Suggested alternatives
Adobe Illustrator (AI) SVG_1_1 (Scalable Vector Graphics, Version 1.1)
SVG_1_2 (Scalable Vector Graphics, Version 1.2)
Saved as bitmap (see above for preferred formats)
CorelDraw (CDR) Ditto
Corel Exchange (CMX) Ditto
Micrografx Draw (DRW) Ditto
Windows Metafile (WMF) Ditto
Standard for the Exchange of Product Model Data (STEP) SVG_1_1 (Scalable Vector Graphics, Version 1.1)
SVG_1_2 (Scalable Vector Graphics, Version 1.2)


Back to top

Last Updated: 02/10/2014