Introduction to Computer Text File Formats
The specification of how a document can be displayed and printed is determined by the
format in which the file is stored. Several standards have been proposed by governments
and companies. This document will briefly describe some of the attributes of the most
popular formats. The reader should be aware that many more formats than discussed are in
existence and more are being proposed as standards quite frequently.
The lowest common denominator of text file formats is the American Standard Code for
Informtation Interchange (ASCII). There are
actually two forms of ASCII: standard and extended. Standard ASCII only contains
codes for 128 characters, (i.e. a 7 bit binary code). It is transportable across all
networks and capable of being accessed and manipulated on all computers. Extended ASCII is
a non-standard format containing codes for 256 characters, (i.e. 1 byte code). The first
(lower), 128 characters of extended ASCII are actually just the standard ASCII character
codes. The second (upper), 128 character codes are machine dependent. Each hardware vendor
defines the codes for their own platform. The characters for the codes differ from font
(character typeface), to font. They are typically used for various purposes such as:
specifing special graphic characters; extended device control (printer, modem, etc.); and
application specific file format codes.
Even transfering plain text files between different systems can present problems. The
most common problem is the way in which different systems represent the end of the line.
The Macintosh stores a carriage return character (ASCII code 13), at the end of each line,
whereas Unix uses a linefeed character (ASCII code 10), to denote the end of the line and
yet DOS does it a third way using both a linefeed and carriage return character at the end
of lines.
The PostScript
file format was developed by Adobe
Systems Inc. It was designed as a typesetting language for specifying high quality
page layouts. There are actually several forms of PostScript. Only two forms will be
briefly mentioned here: PostScript and encapsulated PostScript. The driving force behind
the development of the PostScript (sample file),
language was as a device driver language for laser printers. It has since developed into a
display language and exists in two levels offering two-dimensional and three-dimensional
drawing features. Encapsulated PostScript (EPS), is simply an extension of the
PostScript language to achieve machine independence. The aim was to allow documents to be
transportable across hardware platforms without any loss of document data, structure or
display information. This goal is only now being realized through the development of the
Adobe Acrobat system that will be discussed later in the course. EPS files are typically
much larger than corresponding PostScript files since they include font table information
and PostScript functions to aid portability. For example the sample document above required 30K (30,000 characters),
of storage for the PostScript version and over 900K (900,000 characters) for an EPS
version.
Rich Text Format (RTF), was developed by the Microsoft Corp. for many of the
same reasons as PostScript was developed. It was not however designed as a full-featured
typesetting language. As such it will not store enough layout information about a document
to ensure WYSIWYG platform portability. Most word processors support the RTF format
(usually in the Open or Save dialogs options are provided for specifying the document
type). RTF (sample file), unlike PostScript is a text format
only. Any documents containing graphics converted to RTF and then re-opened as an RTF
document will not contain the original document graphics.
HyperText
Markup Language (HTML), is
the format underlying the World-Wide Web (WWW), documents that WWW browser programs such
as Mosaic display and traverse. HTML is a document structure language. As such it does not
provide operations for a writer to specify exactly how a document should be displayed,
only the outline or structure may be indicated.
Any HTML document viewed with Mosaic, Netscape Explorer, or Internet Explorer can be
viewed in raw HTML form (i.e. with the HTML formatting code not interpreted). By
choosing View source from the File menu (View menu
if using Netscape) or choosing Save as ... from the File menu
and then setting the saved file type to HTML one can then look at the HTML document
structure codes. The HTML format provides operation codes for linking (connecting),
documents together. This is the basis for Hypertext and it forces a non-linear
method of information organization upon the author. A writer must continually think about
relationships that exist in their information and how best to present those relationships
to the readers. HTML is actually a subset of a more general and powerful document
specification language SGML. An extension presentation of HTML with regards to
writing HTML documents for publication on the Internet will be covered in the Network Hypermedia section of the course.
Further Exploration
Common Internet File Formats