Abstract
A problem to be faced very early when dealing with digital documents is how to represent them in a suitable machine-readable format. This chapter introduces the current widespread formats for digital document representation, divided by category according to the degree of structure they express. Formats that do not exhibit any high-level structure for their content are dealt with first: plain text and image formats (in turn divided into vector and raster ones). Then, the formats containing information on spatial placement of the document elements are introduced: PostScript and its evolution Portable Document Format, which represent the current standard in document exchange. Lastly, a selection of formats organized according to the content and function of the document components are presented, including Web formats and the official standard for text processing. Some insight in the file representation and organization aspects, and in the methods involved in producing and exploiting them, is given, aimed at ensuring a sufficient understanding and ability to recognize their pros and cons, without entering into subtle technical details.
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Notes
- 1.
As proved by Shannon [23], the number of bits required to specify sequences of length N, for large N, is equal to N⋅H (where H is the source entropy).
- 2.
In the rest of this section, both notations will be used interchangeably, as needed.
- 3.
Another coding standard in use during the ancient times of computing machinery, abandoned later on, was the EBCDIC (Extended Binary Coded Decimal Interchange Code), which started from the BCD (Binary Coded Decimal) binary representation of decimal digits, from 00002=0 to 10012=9, used by early computers for performing arithmetical operations, and extended it by putting before four additional bits. The decimal digits are characterized by an initial 11112 sequence, while the other configurations available allow defining various kinds of characters (alphabetic ones, punctuation marks, etc.).
- 4.
As a trivial example of how tricky UTF-16 can be: usual C string handling cannot be applied because it would consider as string terminators the many 00000000 byte configurations in UTF-16 codes.
- 5.
An indication that one of the main interests towards images is their transmission.
- 6.
An image compressed lossily, if repeatedly saved, will tend to lose quality, up to not being able to recognize its content anymore.
- 7.
The subsampling scheme is commonly expressed as an R:f:s code that refers to a conceptual region having height of 2 rows (pixels), where:
- R :
-
width of the conceptual region (horizontal sampling reference), usually 4;
- f :
-
number of chrominance samples in the first row of R pixels;
- s :
-
number of (additional) chrominance samples in the second row of R pixels.
- 8.
A variant of the LZ77, developed by J.-L. Gailly for the compression part (used in zip and gzip) and by M. Adler for the decompression part (used in gzip and unzip), and almost always exploited nowadays in ZIP compression. It can be optimized for specific types of data.
- 9.
An open-source project (http://pages.cs.wisc.edu/~ghost/) that does not directly handle PS and PDF formats, this way being able to handle some differences in the various versions or slangs of such formats. An associated viewer for PS files, called GSview, is also maintained in the project.
- 10.
In this section, PostScript code and operators will be denoted using a teletype font. Operators that can be used with several numbers of parameters are disambiguated by appending the number n of parameters in the form operator/n.
- 11.
The most common operations that modify the matrix are translation of the axes origin, rotation of the system of Cartesian axes by a given angle, scaling that independently changes the unit of measure of the axes, and concatenation that applies a linear transformation to the coordinate system.
- 12.
Some software applications that produce HTML documents exploit extensions not included in the official format definition, and hence part of their output represents semi-proprietary code that might not be properly displayed on some platforms.
- 13.
Note that HTML is an application, not a subset, of SGML.
- 14.
This representation is inspired by JAR files (Java ARchives), used by the Java programming language to save applications.
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Ferilli, S. (2011). Digital Formats. In: Automatic Digital Document Processing and Management. Advances in Pattern Recognition. Springer, London. https://doi.org/10.1007/978-0-85729-198-1_2
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