Creating Digital Resources
for the Visual Arts:
Standards and Good Practice

Section 3. Creating Digital Images

3.3 Selecting image file formats

The file format chosen depends, in part, on:

• The nature of the object/event being captured
• Intended use of image/stage in creation process

Nature of the object/event

The nature of the object/event being captured may well affect the choice of file format, e.g. in the case of digitising greyscale images such as black and white, or tinted prints, using a 24bit (full-colour) format would be unnecessary. Similarly, when capturing a real-time event with a digital camera, format choice could be constrained by the camera specifications. (See Section 3.6.)

Image end-use/creation stage

The intended use of an image, i.e. its stage in the production process, will have a profound affect on file formats chosen. These are discussed more fully below in relation to:

• Primary acquisition
• Manipulation
• Archiving
• Delivery surrogates

Whatever the stage, formats chosen for images to be used in information-based resources should minimise, or at least postpone for as long as possible, any data loss.

Primary acquisition

This is the first and most important step in the process. The primary acquisition file format should:

• Be lossless so as to maintain the highest possible fidelity to the original object.
• Retain the best colour/greyscale information possible – at least 24bit colour/8bit grey

• Retain any acquisition device gamma/colour calibration information

Suggested primary acquisition formats: TIFF; PNG; SPIFF

PNG and SPIFF meet a lot of the requirements for a modern raster image format; amongst other features both:

• Are open non-proprietary formats
• Support metadata as text (some fields predefined)
• Support lossless compression (SPIFF supports lossy as well)

Manipulation/editing (working within imaging software)

Image manipulation/editing software should read and be able to edit standard image formats. However, if a substantial amount of editing is required on any individual image then proprietary formats of graphics programs, e.g. Photoshop .psd, can be very useful. These proprietary formats offer functionality not available for use with standard formats.

Temporarily changing or copying files to proprietary formats within imaging software means extra information specific to editing can be held. This allows manipulation to be saved and continued between editing sessions.

If proprietary formats are used for editing, then once final editing has been reached for an 'archival master' or 'delivery image', the edited image file should be saved or copied into the appropriate, non-proprietary format.

Archiving

The 'archival master' should be at the highest practicable resolution, in keeping with the storage facilities available (preferably as per acquisition resolution and format). With mass storage becoming less costly all the time and with more types to choose from (e.g. CD-ROM; DAT tape; DVD; removable disks i.e. JAZ and ZIP; optical discs), there are options to suit most budgets (for a fuller discussion of storage media see Section 7).

The archival format should:

• Be lossless so as to maintain the highest fidelity
• Retain the best colour/greyscale information possible – at least 24bit colour/8bit grey
• Be a standard format readable by most image editors/viewers
• Preferably hold metadata associated with the image. e.g. cataloguing, copyright information – the data will then move with the image, making retrieval easier and lost or broken databases less problematic

Suggested archival formats: TIFF; PNG; SPIFF

When archiving considerations are paramount or digital images are being used as a method of preservation of original objects, it may be appropriate to maintain both a copy of the 'primary acquisition' file and the 'archival-master'. In effect, two types of 'archived' image are kept: an 'archival-original' (unedited primary acquisition file) and an 'archival-master' (edited primary acquisition file).

Delivery formats are dependent upon end-user needs and the dissemination technologies involved. There are numerous factors to consider, including:

• Type of destination device, i.e. the end-needs of the users, e.g. on-screen, print, slide
• The capabilities of the receiving device(s) – colour capabilities, resolution and formats supported
• Delivery medium – networked or portable storage device
• Image use after electronic delivery – print output etc.
• The nature of the image, e.g. photo-realistic or presentation

One distinction to make is between that of delivery for:

• print publication
• electronic publication
  

Suggested print publishing formats

Consult with those doing the printing: this becomes even more important if it is a commercial printer. Commercial printers often want something along the lines of a TIFF/EPS file in CMYK format and it may be necessary to have a printer profile.

On-line delivery formats:

• Where bandwidth or storage space are a problem, then consider JPEG (at least for preview files), PNG (8bit) or GIF. For JPEG the degree of compression used will depend on the amount of acceptable degradation

In the future an appropriate use of technology may enable duplicates of the 'archival-master' file to be delivered by performing an 'on-the-fly' conversion to deliver a file that is in the resolution, colour depth and format that end-users require.

For non-networked electronic publishing:

• If bandwidth is not an issue then consider using TIFF or PNG (24/48bit)
• For electronic presentation-type images the choice is generally simpler – use PNG (8bit) or GIF

If SPIFF support becomes more widespread then it can be used for photo-realistic and presentation types of image under most conditions.

Screen display:

It is also worth noting that the type of monitor which a digital image is viewed on will affect the way it appears. Differences in monitors which can affect screen display include:

• Screen gamma values
• Monitor set up, i.e. position, brightness, contrast, colour temperature etc.
• Type of screen: VDU, TFT panel or projection system?
• Colour depth and resolution e.g. 256 colour or 'true-colour'

It is most important when deciding upon delivery formats to choose the ones that best suit the circumstances of the end-user.

Suggested formats for image creation for high quality information-based resources is summarised as follows

*These are as at the date of publication. As noted above, the file format scene is a continually advancing one and recommendations are subject to change. Please check the web-version of this guide for latest recommendations, or contact TASI/VADS directly. These suggestions are a guide, not gospel; individual image creation projects may well have idiosyncracies that are better suited to other formats. If necessary, contact a digital image consultancy to discuss individual needs. (Appendix 1 carries a list of useful organisations.)