The footer, sometimes called the trailer, is a data structure similar to a header and is often an addition to the original header, but appended to the end of a file. A footer is usually added when the file format is upgraded to accommodate new types of data and it is no longer convenient to add or change information in the header. It is mainly a result of a desire to maintain backward compatibility with previous versions of the format. An example of this is the TGA format, later revisions of which contain a footer that enables applications to identify the different versions of its format and to access special features available only in the later version of the format.
Because by definition it appears after the image data, which is usually of variable length, a footer is never found at a fixed offset from the beginning of an image file unless the image data is always the same size. It is, however, usually located at a specified offset from the end of an image file. Like headers, footers are usually a fixed size. The offset value of the footer may also be present in the header information, provided there was reserved space or padding available in the header. Also like a header, a footer may contain an identification field or magic number which can be used by a rendering application to differentiate it from other data structures in the file.
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