Multimedia Glossary: D

DARPA

Defense Advanced Research Projects Agency

DARPA (originally known as ARPA) is a branch of the American Defense Department that funds advanced research. The Internet originated as an ARPA project.

Data

MPEG-7

Date is audio-visual information described regardless of storage, docing, display, transmission, medium or technology. This includes graphics, still images, video, film, music, speech, sounds, text and any other relevant AV medium

Datagram

Datagram is also known as Data Packet or Network Message

Datagram is a unit of information that, apart from the data, also contains address information, which tells the network where the data must be sent and who the sender is. The datagram is thus like an addressed envelope with its contents.

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DDL

MPEG-7

Description Definition Language

The DDL is a language that allows the creation of new Description Schemes. Description Schemes can be modified and extended using the DDL.

DARPA

See DARPA

Description Definition Language

MPEG-7

See DDL

Descriptor

MPEG-7

See D

Descriptor Value

MPEG-7

A Descriptor Value is an instantiation of a Descriptor for a given data set, or subset thereof.

Description Scheme

MPEG-7

See DS.

Direct color

When 256 or less colors are used to be rendered on a computer's screen, a Color map / color palette / indexed color is typically used, but when more colors need to be described such a palette becomes too large. The Direct color method is used. In this method a specific value is assigned to a color hue. For computer screens RGB color model is used which directly encodes relative brightnesses of red, green, and blue to specify a color.

• 16-bit (Hi-Color) uses 16 bits to create colors, with a total of 65'536 colors.
  • This is sometimes called 15-bit color, as red, green, and blue components often take up 5 bits each (2⁵ = 32 levels)
  • The extra bit is either left empty, or sometimes used for one of the colors. When used, green is the most commonly used color.
• 24-bit color values can represent 16'777'216 distinct colors. Each component is 8 bits deep (2⁸ = 256 levels).
• 32-bit color consists of 24-bit color plus an additional 8 bits used either as empty padding space or to represent an alpha channel

Also see Color map / color palette / indexed color

Dithering

Dithering is the process by which a large number of colors are forced into a smaller color palette. This is typically used when a CRT screen may not be able to render manyh different colors. The closest color hues to the intended color hues are rendered. The loss of colors from the large intended palette results in a fuzzy image with fewer color hues.

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DNS

Domain Name System

The domain name is a human language name for a website at which web documents are kept. The domain name is the human friendly website address. This human fridnely name (such as daisy), is mapped to an IP address (such as 149.98.45.241) -- computers do not directly work with names, but with numbers. When a web user types in an address such as www.daisy.com, a nameserver looks up the IP address (in numbers) and sends the requests there. DNS's are very important in the internet environment. A web address typically consists of a domain name that needs to be resolved to an IP address (i.e. the numerical address, such as 215.145.09.189). The server IP and domain does not necessarily have to be linked to a unique physical machine -- the mapping may be to a unique virtual machine, which means that many different domain names and IP addresses may reside on the same physical server machine.

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Document Style Semantics and Specification Language

See DSSSL

Document Type Definition

See DTD

DOM

Document Object Model

The DOM is an API (Application Programming Interface) for markup documents such as XML and HTML. A document is considered to consist of objects. A paragraph would be a typical object, although not all objects may be visible to the browser. The relationships between document objects can be represented in a tree diagram. Objects are thus related just as human family members would be: there are parents, children and siblings. By manipulating this tree, the objects can be manipulated. The manipulation typically occurs by means of a scripting language, such as JavaScript. It is thus possible, for example, to change the order in which objects appear in a document. To make the scripting of such manipulation easier, scripting languages have built-in and pre-defined processes to operate on DOM.
Note that there are various flavors of DOM. For example, the HTML and XML DOM's are not exactly the same, and neither is the JavaScript DOM exactly the same as these.

Domain Name System

See DNS.

DS

MPEG-7

Description Scheme

A DS specifies the structure and semantics of the relationships between its components, which may be noth Descriptors and Description Schemes.
A Descriptor and a Description Scheme differs as follows:
A Descriptor contains only basic data types, as provided by the Description Definition Language; a Descriptor does not refer to another Descriptor, or sub-Description Scheme.

DSSSL

Document Style Semantics and Specification Language

DSSSL is an ISO standard for SGML documents' style sheet specifications. Just as SGML is the granny of HTML and XML, DSSSL is the granny of CSS. The conventions of CSS look similar to DSSSL statements. DSSSL is more powerful than CSS because it supports style programming, while CSS contains just a list of properties.

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DTD

This abbreviation could be confusing as it is sometimes incorrectly used for two different but related phrases: DTD and DTDecl.

Document Type Definition (DTD)

The Document Type Definition defines a particular application of SGML, such as HTML or XML. It states which element, attribute and entity types and other components are permitted in the particular document. These statements are written as markup declarations and specify among other things inheritence (e.g. that an element called A can or cannot be a child of an element called B).

In XML only documents with DTDs are valid documents.

Example
Here is an HTML 4.1 example of the DTD fragment for a paragraph (i.e. the definition of the HTML 4 paragraph). It states that a paragraph is an inline element.

<!ELEMENT P - O (%inline;)* -- paragraph -->

The HTML DTDs do not comply 100% to the syntax of SGML DTDs (as indicated by the O above - for Optional). This has been rectified in XHTML. Here is the XHTML DTD fragment for a paragraph.

<!ELEMENT p %Inline;>

Document Type Declaration (DTDecl)

You should rather not use the abbreviation DTD for the Document Type Declaration.

The Document Type Declaration calls on the DTD (either internal or external) to be used with its related document. The Document Type Declaration thus contains the DTD; it declares which document definition is to be used with the document.

The DTDecl consists of two parts: the FPI and the URI where the FPI can be found.

Example
Here is an example of declaring the document type in an HTML document. the first line is the FPI, the second line the URI where the DTD can be found.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN"
"http://www.w3.org/TR/REC-html40/strict.dtd">

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