The Tag Image File Format

The purpose of TIFF is to store raster image data, and to this end it provides a rich set of features and predefined image formats. Though TIFF is a complex format, it can easily be used for simple purposes because the set of required fields (opposed to the possible fields) is small.

A high priority of the developers was to give TIFF an open structure, so that future extensions can easily be added. This is a fundamental difference from most other image formats, and one of the main reasons why we chose it to be the target of our extension.

Structure

A TIFF field is a logical entity consisting of TIFF tag and its value. This logical concept is implemented as an IFD Entry, plus the actual value if it doesn't fit into the value/offset part, the last 4 bytes of the IFD Entry.

There is a number of predefined image types in the 6.0 spec, such as e.g. Bilevel, Grayscale, RGB Full Color, CMYK Images, and CIE L*a*b* Images. For each image type, there is a number of fields that are required by the specification; additional ones, like e.g. Artist, ImageDescription, GrayResponseCurve, Copyright aso. can be used as needed.

The key feature of TIFF with respect to our extension is the possibility to add new tags that describe new properties without interfering with the specification of all other formats. One can and, has to, request numbers for these private tags from the TIFF administrator, the Aldus (now Adobe) Developers Desk.

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