File formats
The table below outlines many of the common raster image formats currently in use.
| Format | Properties/Technologies | Description | Recommendations |
|---|---|---|---|
| .tif / .tiff | Uncompressed Baseline TIFF v.6 | A de facto standard and widely used in an uncompressed form for storing archival versions of images. The TIFF format is flexible and can supports a number of options such as LZW[2] compression, multiple pages and metadata embedding and data creators should be clear on what type they are producing. The TIFF format supports embedding of EXIF metadata as well as GeoTIFF metadata for georeferencing. | Generally the uncompressed baseline v.6 is seen as being the only suitable TIFF for preservation purposes although extensions such as GeoTIFF and TIFF/EP (both containing specialized metadata) are equally acceptable and based on this standard. Aside from the previously noted problems of compression, the LZW algorithm used within TIFF is based on proprietary software and files implementing this are considered unsuitable for long term preservation. |
| .png | Portable Network Graphics, an ISO standard and supported by the W3C[3] | Intended as a replacement for the GIF format, PNG offers 32-bit colour depth, lossless compression, support for an Alpha channel (transparency) amongst a host of other features. PNG does not, however, support standard EXIF metadata and, as the format was primarily designed for internet use, it does not support colour spaces other than RGB. | PNG has become the format of choice for lossless presentation. Although TIFF is the preferred format for long term storage of images, the PNG format offers a number of features and should be used in preference over the GIF format where possible e.g. where lossless compression is required. While the format offers certain advantages over JPG (e.g. less visible artefacts) it is not recommended for use with digital photographs. |
| .jpg / .jpeg | An ISO standard format developed by the Joint Photographic Expert Group | The JPG format was primarily designed for photographic or painted images with smooth varying tones. The format offers 32-bit colour depth and extremely efficient lossy compression algorithms. JPEG also allows EXIF and IPTC metadata to be embedded within the file. | Although JPG files are usually drastically smaller than their TIF or PNG equivalents, the use of lossy compression makes them unsuitable for long-term storage. Where compression is required, data creators are advised to use the JPEG2000 format with lossless compression. |
| .jp2 / .jpx | JPEG2000, an ISO standard and an intended replacement for the JPG format. | The uptake of JPEG2000 has been relatively slow since its release in December 2000 with browser support still lacking. The format offers higher performance and lossless compression ratios aimed at minimising file size and compression artefacts. The format also differs from standard JPG in that it uses XML (the JPX format) to store metadata within the file and can include within this IPTC, GML and Dublin Core metadata elements[4] | The format is now widely being investigated as a viable format for preservation due to its use of lossless compression and scalability. Various institutions and organisations such as the Wellcome Library (Buckley 2009), the University of Connecticut (Lowe & Bennett 2009) and the Digital Preservation Coalition (Buckley 2008) have produced reports in the last few years that suggest JPEG2000 will become increasingly popular as a preservation format. |
| .gif | Graphics Interchange Format, a proprietary format developed by Compuserve | Widely used on the Web for both still and animated images, the GIF format offers lossless compression but with a limited palette (8 bit / 256 colours) and limited options for embedding metadata. | Although it has now been superseded by more up-to-date formats (e.g. PNG), the GIF format is still widely in use. It is recommended that either the PNG format is used if compression is required or TIFF for long-term storage. |
| .bmp | Bit-Mapped Graphics Format created and owned by Microsoft | Commonly used in many older Windows applications, the BMP format is similar to GIF in that it is ideally suited for simple graphics. The format does optionally incorporate compression although this is not as efficient as that of GIF. BMP has limited options for embedding metadata. | Not recommended as either a working format or a format for long-term storage. |
| .psd | Adobe Photoshop document file, a proprietary format owned by Adobe. Primarily used for creating or editing images. | The PSD format is highly flexible and is often touted as an ‘industry standard’ imaging solution. The format supports masks and layering, transparency, text and a number of other features making it an ideal format in which to create and edit images, it also supports EXIF, IPTC and XMP metadata. PSD has limited compression support and thus files tend to be large. | The PSD format is an ideal format in which to create and edit files but its proprietary and closed nature means that there is limited third-party support and it is unsuitable for long-term storage. Whilst copies of a file could be stored in the PSD format so that future editing (if desired) would be possible, migration of images to open uncompressed formats – i.e. TIFF- is advised for long-term accessibility. |
| .cpt | Corel Photo-Paint image, a proprietary format owned by Corel. Primarily used for creating or editing images. | CPT is the native format of Corel’s Photo-Paint software and, as the main competitor to Adobe Photoshop, provides a similarly wide range of functionality. | As with the PSD format, CPT is ideally suited to the creation and editing of images and, while copies may be kept in this format for possible future editing, the format is highly specific to Corel software and copies of images should be stored in uncompressed formats such as TIFF. |
| .dng | Adobe Digital Negative format, an open format developed by Adobe. | Based on (and compliant with) the TIFF/EP format, DNG is an openly documented archival format for the storage of raw files generated by digital cameras. Adobe developed DNG in the hope that it will become a single raw processing solution that enables a more efficient workflow when handling raw files from multiple camera models and manufacturers (A free converter to convert existing RAW image formats to DNG can be downloaded from Adobe’s website[5]). The DNG format is able to read all image tagging (EXIF & IPTC) from the original RAW file and store these in the DNG image and, in addition, supports other metadata to be embedded via XMP[6]. | DNG is suitable for the long-term storage of image data. |
| raw (various extensions) | Raw bitmap files are usually vendor specific and proprietary in nature. | Raw files are unprocessed bitmap files in a number of different formats created directly by a digital camera (and occasionally by digital scanners). Due to the lack of standardisation it is almost impossible to characterise a raw file, many files are uncompressed but other use both lossy or lossless compression (and some allow users to select). The lack of standardisation also means that many files require specific software applications to open the files. | Raw formats are unsuitable for long-term storage and data creators are advised to convert such files, where possible to standard formats such as TIFF or DNG. |
[2] The Lempel-Ziv-Welch lossless data compression algorithm.
[3] http://www.libpng.org/pub/png/
[4] https://jpeg.org/jpeg2000/index.html
