Data conversion

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Data conversion is the conversion of one form of computer data to another--the changing of bits from being in one format to a different one, usually for the purpose of application interoperability or of capability of using new features. At the simplest level, data conversion can be exemplified by conversion of a text file from one character encoding to another. More complex conversions are those of office file formats, and conversions of image and audio file formats are an endeavor that is beyond the ken of ordinary computer users.

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[edit] Information basics

Before any data conversion is carried out, the user or application programmer should keep a few basics of computing and information theory in mind. These include:

  • Information can easily be discarded by the computer, but adding information takes effort.
  • The computer can add information only in a rule-based fashion; most users want additions of information that can only be accomplished by humans.
  • Upsampling the data or converting to a more feature-rich format does not add information; it merely makes room for that addition, which usually a human must do.

For example, a truecolor image can easily be converted to grayscale, while the opposite conversion is a painstaking process. Converting a Unix text file to a Microsoft (DOS/Windows) text file involves adding information, namely a CR (hexadecimal 0D) byte before each LF (0A) byte, but that addition is easily done with a computer, since it is rule-based; whereas the addition of color information to a grayscale image cannot be done programmatically, since only a human knows which colors are needed for each section of the picture--there are no rules that can be used to automate that process. Converting a 24-bit PNG to a 48-bit one does not add information to it, it only pads existing RGB pixel values with zeroes, so that a pixel with a value of FF C3 56, for example, becomes FF00 C300 5600. The conversion makes it possible to change a pixel to have a value of, for instance, FF80 C340 56A0, but the conversion itself does not do that, only further manipulation of the image can. Converting an image or audio file in a lossy format (like JPEG or Vorbis) to a lossless (like PNG or FLAC) or uncompressed (like BMP or WAV) format only wastes space, since the same image with its loss of original information (the artifacts of lossy compression) becomes the target. A JPEG image can never be restored to the quality of the original lossless image from which it was made, no matter how much the user tries the "JPEG Artifact Removal" feature of his or her image manipulation program.

Because of these realities of computing and information theory, data conversion is more often than not a complex and error-prone process that requires the help of experts. It is safe to say that only the success of artificial intelligence could put data conversion companies out of a job.

[edit] Pivotal conversion

Data conversion can occur directly from one format to another, but many applications that convert between multiple formats use a pivotal encoding by way of which any source format is converted to its target. For example, it is possible to convert Cyrillic text from KOI8-R to Windows-1251 using a lookup table between the two encodings, but the modern approach is to convert the KOI8-R file to Unicode first and from that to Windows-1251. This is a more manageable approach: an application specializing in character encoding conversion would have to keep hundreds of lookup tables, for all the permutations of character encoding conversions available, while keeping lookup tables just for each character set to Unicode scales down the number to a few tens.

Pivotal conversion is similarly used in other areas. Office applications, when employed to convert between office file formats, use their internal, default file format as a pivot. For example, a word processor may convert an RTF file to a WordPerfect file by converting the RTF to OpenDocument and then that to WordPerfect format. An image conversion program does not convert a PCX image to PNG directly; instead, when loading the PCX image, it decodes it to a simple bitmap format for internal use in memory, and when commanded to convert to PNG, that memory image is converted to the target format. An audio converter that converts from FLAC to AAC decodes the source file to raw PCM data in memory first, and then performs the lossy AAC compression on that memory image to produce the target file.

[edit] Lossy and inexact data conversion

For any conversion to be carried out without loss of information, the target format must support the same features and data constructs present in the source file. Conversion of a word processing document to a plain text file necessarily involves loss of information, because plain text format does not support word processing constructs such as marking a word as boldface. For this reason, conversion from one format to another that has less features is rarely carried out, though it may be necessary for interoperability, eg converting a file from one version of Microsoft Word to an earlier version for the sake of those who do not have the latest version of Word installed.

Loss of information can be mitigated by approximation in the target format. There is no way of converting a character like ä to ASCII, since the ASCII standard lacks it, but the information may be retained by approximating the character as ae. Of course, this is not an optimal solution, and can impact operations like searching and copying; and if a language makes a distinction between ä and ae, then that approximation does involve loss of information.

Data conversion can also suffer from inexactitude, the result of converting between formats that are conceptually different. The WYSIWYG paradigm, extant in word processors and desktop publishing applications, versus the structural-descriptive paradigm, found in SGML, XML and many applications derived therefrom, like HTML and MathML, is one example. Using a WYSIWYG HTML editor conflates the two paradigms, and the result is HTML files with suboptimal, if not nonstandard, code. In the WYSIWYG paradigm a double linebreak signifies a new paragraph, as that is the visual cue for such a construct, but a WYSIWYG HTML editor will usually convert such a sequence to <BR><BR>, which is structurally no new paragraph at all. As another example, converting from PDF to an editable word processor format is a tough chore, because PDF records the textual information like engraving on stone, with each character given a fixed position and linebreaks hard-coded, whereas word processor formats accommodate text reflow. PDF does not know of a word space character--the space between two letters and the space between two words differ only in quantity. Therefore, a title with ample letter-spacing for effect will usually end up with spaces in the word processor file, for example INTRODUCTION with spacing of 1 em as I N T R O D U C T I O N on the word processor.

[edit] Open vs. secret specifications

Successful data conversion requires thorough knowledge of the workings of both source and target formats. In the case where the specification of a format is unknown, reverse engineering will be needed to carry out conversion. Reverse engineering can achieve close approximation of the original specifications, but errors and missing features can still result. The binary format of Microsoft Office documents (DOC, XLS, PPT and the rest) is undocumented, and anyone who seeks interoperability with those formats needs to reverse-engineer them. Such efforts have so far been fairly successful, so that most Microsoft Word files open without any ill-effect in the competing OpenOffice.org Writer, but the few that don't, usually very complex ones, utilizing more obscure features of the DOC file format, serve to show the limits of reverse-engineering.

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