There is a vast variety of data formats, some of which are relatively universal; others are specific for certain software programs or operating systems. The most widespread format may be the "tagged image file format" or TIFF (suffix .tif). There are many derivatives of this format which quite often causes the problem that a software is unable to open a file that was created with a different program. Other common formats are bitmap (*.bmp) and windows metafile (*.wmf). Postscript (*.ps) is a page description format with the necessary information for printers. Encapsulated postscript (*.eps) may contain CMYK, greyscale, vector or compression data and consists of a postscript code that is independent from the type of output device. Such files can be read by many programs but often not further processed.
Certain formats (*.gif, *.jpg, *.png) are used to compress images in order to reduce storage space and download time for use in the World Wide Web. These formats employ different algorithms to store sequences of pixels with repeating patterns as one piece of information instead of pixel by pixel as in the uncompressed formats.
The "graphic interchange format" (GIF) can render the entire colour range of 24 bit RGB images but one image may only contain 256 colours at the most (8 bit). Otherwise the compression occurs without information loss. It is most useful for graphics with sharp contours. A useful feature is that one colour can be set transparent. The format was invented for the online service CompuServe and there are patent issues to be considered.
The JPEG format ("joint photographers expert group") changes the colours of the individual pixels assuming that the eye is unable to resolve certain details anyway. The compressing factor is adjustable so that the user has direct influence on the quality of the image - and on the data size. Transparency of colours is not possible. Many consumer cameras offer this format as an option. The compression is very effective but because of the loss of information this file type is not useful for quantitative imaging but probably serves best for standard photography.
The "portable network graphic" (PNG) was developed especially for the use in websites as a patent free alternative to GIF. PNG supports not only truecolour and greyscale images (like JPEG) but also palettized images (similar to GIF). This means that an individual 8 bit palette is stored along with the image and each pixel refers to one of the colours in this palette. Thus not each pixel has to carry three-channel information and the overall amount of data is reduced. Shades of transparency can be achieved via storage of an additional alpha channel whereas GIF supports only binary transparency. The compression is without loss of information for truecolour images which however renders it less effective with respect to data reduction as compared to JPEG compression. This is especially the case for landscape photography, for example, while for graphics and rather uniform images the compression factor tends to be higher.
