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Choosing The Right File Format/Raster Images

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Images (Raster Graphics)

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Most of your images are probably raster graphics, as vector graphics are less common. Although there are hundreds of formats to choose from two of the most popular are GIF (Graphics Interchange Format) and TIFF (Tagged Image File Format). Unfortunately these have been caught up in legal issues since 1994 over a Unisys patent on the LZW compression algorithm which both formats use. These patents have now expired although there is still an IBM patent valid until August 2006.

Thus the GIF and TIFF formats do not currently meet the requirement of being 'free from legal restriction in their use'.

The lossless PNG (Portable Network Graphic) format replaces GIF and has many advantages in quality, size and options (but lacks animation). Most importantly the PNG format is patent free and has been a W3C Recommendation and ISO Standard since 2003.

Another popular format for raster graphics is the lossy JPEG/JPG (Joint Photographic Experts Group) format, which is specially suited to storing photographic images.

...the JPEG committee have always tried to ensure in their standardisation work that the 'baseline' part of their standards should be implementable without payment of either royalty fees (volume related) or license fees (non-volume related)." JPEG Committee

...there are many patents associated with some optional features of JPEG, namely arithmetic coding and hierarchical storage. For this reason, these optional features should not be used for long-term storage of valuable images. W3C

Remember that JPEG is a lossy format, meaning that each time the image is modified and resaved there is some irretrievable data loss. Therefore it should be avoided for archival use unless shortage of space requires that lossy compression is used.

(Windows users should read Microsoft Security Bulletin MS04-028 and this press release)

Recommendation

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  • Use the PNG format.
  • Use the JPEG format, but avoid optional features.
  • If your software supports it, you can save in TIFF format without compression.

Comparison

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  •  
    A photo showing multiple levels of JPEG compression. As the photo goes right, the quality and file size both increase.
  •  
    Lossless, JPEG, and Webp compression compared.
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    A mad scientist cartoon, saved as a SVG.
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    A PNG with a transparent background.
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    Gifs can display animations, but have limited colors available, and take a great amount of space.

References

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