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Raster Images#

Edited by Kieron Niven#

Section 1: Introduction to Raster Images#

  • 1.1 Raster images in Archaeology
  • 1.2 Current Issues and Concerns

Section 1. Introduction to Raster Images#

1.1 What are Raster Images?#

Raster Images are commonplace in archaeological archives and, while they can be the product or component of a number of different processes, they essentially consist of the same basic elements i.e. an image composed of a matrix of pixels with a fixed size/resolution. In archaeology, raster images can be created from a wide variety of processes ranging from original data capture such as digital photographs, scans and drawings through to outputs or 'products' such as plotted geophysical survey data and images from GIS layouts.

This guide aims to cover the common types of raster image created as components of archaeological research, these include:

  • Digital photographs
  • Aerial photographs
  • Document scans
  • Screenshots and output from vector applications
  • Original images such as illustrations, posters, etc.

Although some technical description will be given in this guide, in depth discussion of aspects of raster images such as resolution, colour space and bit depth won't be covered and are more than adequately explained in existing guides, specifically JISC Digital Media's Advice on Still Images[1]. Likewise, vector images are covered in more detail in the Vector Images guide. For both raster and vector images, as common components of larger project workflows, links are also made to technique focused guides such as CAD, GIS and VIrtual Reality.

1.2 Current Issues and Concerns#

One of the most obvious issues with digital raster images is the wide range of formats available in which images can be created and stored. Raster image formats can vary massively in terms of individual features and capabilities and can cover the whole gamut of format types from proprietary, software specific formats through to open standards. A key component of this assortment of file types is also the range of individual features and capabilities each file format possesses - such as compression (lossless or lossy), colour depth, support for transparency, embedded metadata - and it is important that an appropriate file format is chosen for the image being created both during the data creation stage and for long-term storage. In addition, in certain project workflows, images may change formats at different points in the project depending on how they are being used. In these scenarios it is also important to be aware of what range of functionality and metadata each format supports and what could potentially be lost during each format migration.

[1] http://www.jiscdigitalmedia.ac.uk/stillimages/

Section 2: Creating Raster Images#

  • 2.1 General Considerations

Section 2: Creating Raster Images#

2.1 General Considerations#

Although raster images can vary massively in terms of source and use, there are a number of features which remain constant and should be considered when creating and using such files. As with many files types, it is practically impossible to specify a precise setting for these elements and these should be considered within the context of the wider project and used at a level which is fit for purpose.

  • Resolution - essentially describes the level of detail within an image expressed as a pixel count (e.g. pixels per inch (ppi) but also dots or samples per inch (ddi/spi))[1]. The higher the resolution, the more detail is captured in an image and, consequently, the larger the file size. Image resolution is an important consideration for all raster images
  • Bit depth -
  • Colour space -
  • Digital photographs (raw or jpg, maintain orig. resolution, issues of embedde metadata) CAPTURE
  • Aerial photographs (covered in AS guide, world file, metadata) CAPTURE
  • Document scans (often made into pdfs (and downsampled, OCRed (can be repeated))) CAPTURE
  • Screenshots and output from vector applications (e.g. CAD and GIS layouts, Geophys plots, VR screenshots) (resolution)
  • Original images such as illustrations, posters, etc. (original resolution, loss of editability when exported e.g. layers etc.)
  • project planning and requirements - As highlighted in Section 1 of this guide, depending on the type of project, the creation of raster images can be a starting point or final product depending on the relevant project aim and workflow. of some projects and largely tied up with the source of the raster image, may form part of a wider lifecycle.
    • - transparency, compression, resolution, colour space and bit depth, layered and multipage files - how this works with different file types i.e. different functionaily of PNG compared to TIFF.

Colour space - dependant on purpose of image, need a policy. Generally RGB would, I'd have thought, be the most suitable as it can hold significantly more different colours than CMYK.. When dealing with material that is essentially "pre-press", however, preserving CMYK colour mode is probably an important factor to consider.

Most jpegs originating from digi cameras etc will be RGB, so assuming they come relatively unmolested they shouldn't be an issue. Files that might need more thought would be rasterised output from .ai files etc that have been produced for print and may be in native CMYK mode. Converting these to RGB may lead to loss of original colour information (although not necessarily to loss of colour - if you see what I mean).

Multi layered/animated images
As with various other imaging solutions, Photoshop also supports layered images. Depending on the content, handling a layered image can be achieved in either of two ways:

  • Create a composite image of all the layers (TIF)
  • Create a separate image for each layer (TIF), applying suitable naming conventions ie:
    • original_image_name_layer1.tif
    • original_image_name_layer2.tif
    • original_image_name_layer3.tif

Animated images, such as animated-GIFs, can be dealt with in a similar fashion to layered images. However, the use of Animation Shop (comes bundled with the latest version of Paint-Shop-Pro) is recommended. To save each frame individually, open the gif image in Animation Shop. You will be presented with a filmstrip containing individual frames. Right click on the frame of choice and choose 'save as'.

      • http://www.jiscdigitalmedia.ac.uk/crossmedia/advice/file-formats-and-compression/
    • documenting data creation and processing - metadata exif, iptc?, xmp, Raw metadata
  • sources of data - digital photos, scans,
    • file types (whilst creating, working with, processing data)
  • 2.2 File Formats
.tif / .tiffUncompressed Baseline TIFF v.6This is the De facto standard and is widely used. Creators should be aware that there are various types of TIFF available, including compressed versions, which are not suitable as preservation formats. The uncompressed baseline v.6 is the only suitable TIFF for preservation purposes. Also note that the compression used by the TIFF (LZW) is based on proprietary software and is unsuitable for long term preservation. To ensure the image is an uncompressed baseline v.6. TIF file, please make sure no compression is selected when the file is saved. For any uncertainties, step 1 of the conversion procedure as outlined below will convert any existing images to uncompressed baseline TIF.
.pngPortable Network GraphicsPNG is supported by the World Wide Web Consortium (W3C) and is expected to be released as ISO/IEC International Standard 15948. Offering 32-bit colour depth, lossless compression and support for an Alpha channel (transparency), PNG has therefore become the format of choice for lossless presentation.
.jpgJoint Photographic Expert GroupThe JPG file format, while also offering 32-bit colour depth, is based on extremely efficient compression algorithms. This can result in drastically smaller file sizes when compared to their TIF or PNG equivalents. However, JPG uses a lossy compression technique and use should therefore be restricted to situations where file size is a concern.
.jp2 / .jpxJPEG2000JPEG 2000 has been an international standard since December 2000. However, there has been a relatively slow take up of the format in the industry and support remains patchy. A plug-in is usually required to view JPEG2000 images. At time of writing we would be wary of accepting JPEG2000 as an archival format. However, the likelihood is that this will change in the near future. Info on JPEG2000 from TASI - http://www.tasi.ac.uk/advice/creating/newfile.html#nf3 Also an interesting article in D-Lib in 2008 about using JPEG2000 for preservation http://www.dlib.org/dlib/july08/buonora/07buonora.html. They reckon it could be fit for purpose. New development in Sept 2009 - JPEG2000 is being used as a preservation format by the Wellcome Library - see the announcement and report - http://wellcomelibrary.blogspot.com/2009/09/wellcome-library-to-use-jpeg2000-image.html and http://library.wellcome.ac.uk/assets/wtx056572.pdf. Worth noting though that they chose this format with the knowledge in mind that they could redigitise the originals if there were any problems with this format in the future!
.gifGraphics Interchange Format (Compuserve)CompuServe's proprietary GIF format offers 8-bit lossless image compression and has been widely used throughout the World Wide Web. However, it has now been superseded by more up-to-date formats such as PNG.
.bmpBit-Mapped Graphics Format (Microsoft)
.pcdPhotoCD .pcd
.psdPhotoshop (Adobe)Adobe Photoshop is often touted as an 'industry standard' imaging solution. As such the ADS will no doubt encounter certain depositors submitting images in Photoshop's native format (PSD). Although we can accept PSD, it is better for all if the depositor could export the images from Photoshop to an archival format such as TIF before depositing.
Raw format
Adobe .dng The Adobe Digital Negative (DNG), is a new (February, 2005), publicly available archival format for the raw files generated by digital cameras. Although DNG is a propriety format from Adobe, it is publicly documented and the specification is readily available. The free Adobe DNG Converter to convert existing RAW image formats to DNG can be downloaded from Adobe's website. Adobe hopes that DNG will become a single raw processing solution that enables a more efficient workflow when handling raw files from multiple camera models and manufacturers.

Other formats


See also OpenRaw as another alternative - DNG above is still essentially a proprietry format. http://www.openraw.org/

If we receive formats other than those listed above we should contact the depositor and ask if they can supply the data in a format we support. If not, we need to inform them of our current practice. This is that we endeavour to transform the file(s) into an archive format if the software we have to hand can do this in a quick and automated fashion. If this is not the case we will archive the file(s) as is, but will be unable to migrate it to newer versions of that format.

[1] JISC Digital Media Glossary entry for 'Resolution' http://www.jiscdigitalmedia.ac.uk/glossary/#r

Section 3: Archiving Raster Images#

    • 3.1deciding what to archive
      • Selection and retention
      • preservation intervention points / file and data lifecycles (specific to guide, will also be covered generically)
    • 3.2 deciding how to archive
      • archiving strategies (migration (to new format, to 'basic' format), emulation, refreshment)?
      • significant properties
      • file types
    • 3.3 Metadata and Documentation
      • project level
      • file level - We normally require captions to go with images. Sometimes the filenames used are long and descriptive enough to provide a caption, but often this won't be the case and the depositor will need to be approached to get captions. If future users don't know what the image is of then they won't be a lot of use really! As mentioned above, it is important to retain any embedded metadata such as EXIF information. In Paint Shop Pro 'SHIFT+I' displays image information - choose EXIF and tick all boxes to view all embedded metadata. Fortunately, Paint Shop Pro, Photoshop and Image Magick all retain this information when converting files to .tif format.
      • standards specific to #
    • 3.4 Structuring your archive

  • Copyright
    • specific copyright considerations for each guide.

Embedded Meta Data#

The process of embedding meta data within images, such as geo-tagging TIF files or producing EXIF information via a digital cameras, has become common practice. Care must be taken when converting or reproducing source images that this meta data be retained and much of the software we often use to convert JPEG to TIFF for preservation do not appear to preserve the EXIF metadata of the original JPEG (eg Image Magick, IrfanView). A quick test shows that Photoshop will preserve the EXIF tags.

There has been much discussion regarding when EXIF metadata would, or would not, be a significant property of a deposited image. The general consensus has been that there are obviously times (such as the VENUS examples) where EXIF metadata is inherently significant, but that EXIF metadata could be considered valuable whenever it could be seen to be valid and accurate, particularly if it has been created at the point of origin i.e. by camera software rather than later image editing software (which can often remove EXIF tags). If embedded EXIF metadata is seen to be valuable, it should be preserved.

Although it is possible to preserve JPEG EXIF within the TIFF tag structure it is better held in a separate file, avoiding the risk of loss or corruption during later migration and making the metadata more easily accessible. Extraction of EXIF fields is relatively straight forward, with a number of free tools available.

It is suggested that EXIF metadata be extracted and stored in a simple XML structure, with a text file of metadata per image. #image_filename#.meta has been suggested as a standard naming scheme, with these XML files to be stored in a "documentation" folder along side the preserved images. Other embedded metadata, such as IPTC or XMP, might also be extracted into this structure. Adding this functionality to the ADS Toolkit GUI would be beneficial in speed of working and standardisation of procedure. The ability for this to be ingested as a file-level metadata stream by Fedora has also been discussed in CATS meetings.

The issue of the possible impact of the additional files on the charging policy was discussed. The very small size of these files, with respect to the preserved images they accompany was pointed out, and is was generally agreed that these are files created for our benefit in managing the archive (as the metadata could be preserved within the image file itself).

--Jon Bateman, 31-Aug-2006

We need to look at our policy re. RAW files. Probably worth taking these as we can no doubt get viewers for the various types but I still think that we should archive as tiff with the metadata separate.

See also AHDS 'Digital images archiving study' at http://ahds.ac.uk/about/projects/archiving-studies/. Is there anything in this study that is useful to us?

Also worth watching out for Windows Media Photo - Microsoft's rival to the JPG

The InSPECT project (http://www.significantproperties.org.uk) is defining sig props of raster images - will be some useful stuff we can pull out of this report when it is finalised in Spring 2009

New DPC technology watch report on JPEG 2000. Needs a read and linking from this document I reckon: http://www.dpconline.org/docs/reports/dpctw08-01.pdf

The Koninklijke Bibliotheek, National Library of the Netherlands, has published a report on possible alternative file formats for storing master images from mass digitisation projects. Uncompressed TIFFs, the KB’s preferred format so far, take up far too much storage capacity to be a viable storage strategy for the long term. The report is available from the KB website.