Sustainability of Digital Formats
 Planning for Library of Congress Collections

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Still Image >> Curator's View

The illustrative tables presented here are intended to suggest how a curator of still images would determine format preferences. The first table illustrates a planning matrix that would result from analyzing the significant or essential characteristics for subcategories of still images. The second table illustrates how this analysis of significant features would be combined with technical information about formats to produce a set of format-preference statements for the various content subcategories.

NOTE: This page was drafted in 2004 and has not been updated. Beginning in 2015, the Library of Congress has published its format preferences as a Recommended Formats Statement, updated annually.


Table 1: Significant characteristics of still image subcategories

  Description Clarity (resolution) [1] Color maintenance [2] Rendering expectations beyond normal [3] Special functionality required by custodians [4] Special functionality expected by end users Effects of technical protection [5]
I1 Pictorial expression of high value. Examples: Works by graphic artists, photographers, advertisers for whom the designated community has high interest in the artist's intent.

Images for which the artist's pictorial intent is less significant but color or tonality is significant. Examples: documentary photographs of nature, fashion, architecture; newspaper "file" photos; Landsat images (may be multispectral)
Very important Important Resolve or render fine detail Crop and scale, etc., without artifacting   Must not affect clarity or normal rendering
I2 Images for which spatial resolution is important, but color depth and precise color accuracy are not important; may include typography or line elements; may be created using vector ("draw") tools. Examples: maps, graphs, technical drawings, vector graphics delivered in non-editable form or saved as bit-maps. Very important, clean lines/edges desirable   Resolve or render fine detail Crop and scale, etc., without artifacting   Must not affect clarity or normal rendering
I3 Pictorial expression w/lower artistic value, such as: routine output of a portrait studio;images with significance as the expression of everyday life ("snapshots"); interesting-but-not-artistically valuable photographs collected as part of an oral history project [6] Important or less important depending on item         Must not affect normal rendering
I4 Images incidental to Web harvesting, including animations consisting of only a few frames [7] Less important         Not important
I5 Vector graphics that the designated community will wish have in dynamic form ("editable"). Examples: architectural CAD drawings, diagrams, designs. Retain precision of original   Through specialized software Retain functionality of original thru graphic design software [8] Retain functionality of original thru graphic design software Must not affect functionality for end users
I6 Virtual Reality imagery for static 3-D objects
[For future consideration.]
           

Notes:

1. For bitmapped images, clarity is typically represented by spatial resolution and tonal resolution (color depth). Spatial resolution relates to the number of pixels used in the two dimensions of an image; it is expressed either as absolute numbers or as ppi/dpi (pixels/dots per inch) in relation to dimensions on paper (original or intended). High-quality full-page magazine printing from a digital photograph requires images that range from 3,000x4,000 to 10,000x13,000 pixels.

2. The degree to which the color gamut represented in a given image can be managed and maintained through various outputs or migrations. This requires the use of formats with metadata that documents the color encoding used and provides additional data such as the color profile of the device used to create the image.  Additional elements may be needed in the case of multispectral images.

3. For still pictorial images, normal rendering includes onscreen viewing and rendering to print on a typical home or office printer. Normal rendering would also allow manipulation through software that allows image analysis, color correction, cropping, and scaling to permit fair use as "quotations." Normal rendering must not be limited to specific hardware models or devices and must be feasible for current users and future users and scholars.

4. To preserve digital content and provide service to users and designated communities now and decades hence, custodians must be able to replicate the content on new media, migrate and normalize it in the face of changing technology, and disseminate it to users at a resolution consistent with network bandwidth constraints.

5. Technical protection must not prevent custodians from taking appropriate steps to preserve the digital content and make it accessible to future generations. See Note 4.

6. Includes home photography and other images logically the same as I1, but with less artistic value for designated community.

7. Image files harvested from the Web through a program targeted specifically at capture of visual materials would be considered as I1, I2, or I3.

8. For images composed using vector graphics software, guidelines will be necessary (to be developed by custodians and user communities) as to when the functionality inherent in a vector representation is an essential characteristic and when the composition should be preserved as a bit-mapped graphic. Files in the proprietary native formats for commercial software used by professional graphics designers may include combinations of vector graphics and bit-maps, as well as a layering structure. Such files are expected to be reduced to bit-mapped images before being added to the Library of Congress collection.

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Table 2: Format preferences for still image subcategories

  Description Preferred formats [1] Acceptable formats
  Encoding type File type, subtype Encoding type File type, subtype
I1 Pictorial expression of high value. Examples: Works by graphic artists, photographers, advertisers. Designated user community has high interest in the artist's intent. Library selects digital form instead of hard copy.

Images for which the artist's pictorial intent is less significant but color or tonality is significant. Examples: documentary photographs of nature, fashion, architecture; newspaper "file" photos; Landsat images
Bit-mapped, rich color, uncompressed TIFF_UNC (RGB color, 24-bit or greater) [6] Lossless compression TIFF_LZW
JP2_J2K_C_LL in JPX_FF [2]
JP2_J2K_C_LL in JP2_FF
Same, produced by a digital camera TIFF/EP
TIFF_UNC_EXIF or TIFF_UNC (RGB color, 24-bit or greater)
Same, prepared to produce printing plates, e.g., for a magazine TIFF/IT
PDF/X
TIFF_UNC
Encoding for multispectral data No preference established at this time
I2 Images for which spatial resolution is important, but color depth and precise color accuracy are not important; may include typography or line elements; may be created using vector ("draw") tools. Examples: maps, graphs, technical drawings, vector graphics delivered in non-editable form or saved as bit-maps. Bit-mapped, color, uncompressed TIFF_UNC High-quality lossy or indexed color JP2_J2K_C_LSY in JPX_FF or JP2_FF
GIF
BMP
Vector delivery PDF/A
PDF/X
   
I3 Pictorial expression of lower artistic value, e.g., routine output of a storefront portrait studio; images with significance as the expression of everyday life ("snapshots"); interesting-but-not-artistically valuable photographs collected as part of an oral history project High-quality lossy [3]
or
Low-quality lossy [5]
JP2_J2K_C_LSY in JPX_FF or JP2_FF [4]
JPEG_DCT in JFIF or JPEG_EXIF [5]
GIF
Indexed color GIF
I4 Images incidental to Web harvesting, including animations consisting of only a few frames As available      
I5 Vector graphics that the designated community wants in dynamic form ("editable"). Examples: architectural CAD drawings, diagrams, designs. Vector • AutoCad [?]    
I6 Virtual Reality imagery for static 3-D objects
[For future consideration.]
       

Notes:

1. Other formats exist and may be added as preferred or acceptable in the future, particularly if adopted widely. This document lists two ISO-related variants of the TIFF format (TIFF/IT and TIFF/EP), and a variant of PDF (PDF-X) designed to send "camera-ready art" to a printer. These formats are listed here in a provisional sense; the compilers are working to determine how widely adopted these formats may be.

2. The JPX level of the JPEG2000 standard supports more effective color management than the level 1 format (JP2). JPEG2000 offers many options for choices of quality level, and storage order for the encoded image data (codestream). Future investigation is needed to determine whether particular options should be encouraged or avoided when the objective is responsible long-term custody.

3. The degree of compression and specific encoding algorithms applied to images produce files at varying levels of quality. American Memory guidelines do not address this specific question but the project's experience with photographs suggests that JPEG compression ratios on the order of 5:1 provide very high quality when reformatting these continuous tone originals, while wavelet-based compression applied to maps suggests that this type of compression algorithm (also used in JPEG2000) can provide high quality, even when sharply defined lines or text fonts are present.

4. JPX_FF is preferred to JP2_FF because of its more explicit support for embedded metadata.

5. An example of a low-quality compressed image would be a photograph provided for the amateur by a photofinisher.

6. There is a general preference for sRGB color space over RGB color space when profiles or other color management tools have not been employed.

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Last Updated: Wednesday, 06-Apr-2016 11:17:16 EDT