\section{Multimedia content types}
\label{multimedia_content_types}
A multimedia document, such as this Web exhibition, consists of a number
of different content types, such as still images, movies, and 3D scenes.
The definitions given below 
of these and related concept are my own,
relevant to this context, and might not be universally accepted or applicable.

\subsection{Still images}
A {\em still image\/} (or just {\em image\/})
is a 2D array of pixels, which each has a color.

The color is usually specified in 
RGB (red, green, blue) colorspace
as 3 numbers from 0 to 255, 
i.e.\ there are $2^{24}$ = 16.8 million possible colors. 
Two important image classes exist: 
\begin{description}
\item [24 bit images:]
	Images which allow for any number of the 16.8 million (24 bit) colors 
	to be present simultaneously in a given image
\item [8 bit colormapped images:]
	Images which allow only for a maximum of 256 (8 bit) 
	colors to be present
	in a given image. These colors are drawn from the 24 bit palette,
	and the mapping between the up to 256 numbers and the 24 bit colors
	constitutes a {\em colormap\/}.
\end{description}
Some of the image file formats found on the Web are:
\begin{description}
\item	[GIF (Graphics Interchange Format):]
	GIF images are 8 bit colormapped, and are compressed
	using a loss free algorithm 
	(LZW).
	The compression scheme is well suited for images with large area of
	the same color, such as the images found in this Web exhibition.
\item	[JPEG (Joint Photo Experts Group):]
	JPEG images are 24 bit color, and are compressed 
	using a lossy algorithm.
	The compression scheme is well suited for continuous 
	tone natural images,
	such as scanned photographs. 
	I have chosen not to use this image format at all in this project.\\
	Actually, JPEG is a encoding scheme, not a file format.
	The file format used on the Web is called 
	JFIF (JPEG File Interchange Format).
        For more details on JPEG, see the
	\linkf{JPEG FAQ}
	{http://www.cis.ohio-state.edu/hypertext/faq/usenet/jpeg-faq/top.html}.
\item	[SGI / IRIS / RGB:]
	SGI is a native 
	\linkf{Silicon Graphics}
		{http://www.sgi.com/}
	image file format. Images can be of different types, 
	including 24 bit color.
	They can be uncompressed or run-length encoded (RLE).
	Files have extension {\tt .rgb}, hence one of the names.
	This format has been used a lot in this project as an intermediate
	image file format; this will be described in more detail below.
	On a \sgi{} computer, {\tt man 4 rgb} provides more information
	about this image file format.
\item	[PNG (Portable Network Graphics):]
	PNG is still in development, but is mentioned here due to it's
	promising specifications: it is lossless, well-compressed, and
	supports up to 48 bit color, thus being a likely replacer of GIF.
\end{description}
All of the above are described in further detail in
the Graphics File Formats FAQ 
\linkf{part 3}{http://www.cis.ohio-state.edu/hypertext/faq/usenet/graphics/fileformats-faq/part3/faq.html}
(see also part 
\linkf{1}{http://www.cis.ohio-state.edu/hypertext/faq/usenet/graphics/fileformats-faq/part1/faq.html},
\linkf{2}{http://www.cis.ohio-state.edu/hypertext/faq/usenet/graphics/fileformats-faq/part2/faq.html}, and
\linkf{4}{http://www.cis.ohio-state.edu/hypertext/faq/usenet/graphics/fileformats-faq/part4/faq.html}.)
Compression per se, including LZW and JPEG (but not RLE),
is described in further detail in the 
\link{Compression FAQ}
{http://www.cis.ohio-state.edu/hypertext/faq/usenet/compression-faq/top.html}.

\subsection{Movies}
A {\em movie\/} is a set of images, which are displayed sequentially.
This {\em video track\/} can have an accompanying {\em audio track\/},
but in this context I will not be dealing with the latter.

Some of the movie file formats found on the Web are:
\begin{description}
\item	[MPEG (Moving Picture Expert Group):]
	There are several types of MPEG: MPEG-1, MPEG-2, and MPEG-4.
	It seems to me, that the MPEG most often found on the Web is
	MPEG-1, and usually with only a video track.
	MPEG seems to be by far the most popular movie format on the Web,
	and therefore it is one of the formats I offer the movies of
	this project in.\\
	MPEG uses 3 different compression techniques at once, with one of
	them being similar to JPEG. The compression is lossy and efficient.\\
	For more information about MPEG, see the
	\linkf{MPEG FAQ}
		{http://www.powerweb.de/mpeg/mpegfaq/},
	the
	\linkf{Moving Picture Expert Group Information Page}
		{http://www.vol.it/MPEG/},
	and the the first two sections of the article by
	\linkf{Patel, Smith, \& Rowe (1993)}
	  {ftp://mm-ftp.cs.berkeley.edu/pub/multimedia/papers/MM93.ps.Z}.\\
	For the record: MPEG is also known as ISO/IEC JTC1 SC29 WG11 ;-)
\item	[QuickTime:]
	QuickTime seems to be an Apple product; they host a
	\linkf{Web site}
	  {http://quicktime.apple.com/} about it.
	However, viewers and encoders for other platforms are available.
	QuickTime can store among other things movies, using different
	compression schemes. For this project, I have chosen
	``Animation'', which is an RLE scheme. This compression I presume is
	loss free. 
	For this reason, I also offer my movies in that format.\\
	For more information about QuickTime, see the
	\linkf{QuickTime FAQ}
	  {http://www.QuickTimeFAQ.org/}.
\item	[Animated GIF:]
	The GIF89 standard supports multiple images in the same file, and 
	an animation of these. \netscape{} has a {\em built-in\/} viewer not
	only for GIFs but also for animated GIFs, so therefore animated
	GIF has a big advantage. 
	Besides, Netscape will show the movie while
	it is being loaded. 
	For the these reasons, I have also made my
	movies of this project as animated GIF.
	On the downside, it is not possible to single step
	the frames, play the movie again, etc.\\
\end{description}


\subsection{3D Scenes}
A {\em 3D scene\/} is a collection of ``material'' objects, 
such as surfaces (constructed by a number of polygons), points and lines,
living in 3D space. The two 3D scene formats I have used in this project
are:
\begin{description}
\item	[Inventor:]
	Inventor is two things: 
	\begin{itemize}
	\item 	An object-oriented toolkit for developing interactive, 
		3D graphics.
	\item	A file format for exchanging 3D data among applications.
	\end{itemize}
	I will only be using Inventor in the latter sense, i.e.\ as a
	file format, or {\em programming language\/}.\\
	A technical note:
	Inventor V1.0 is called {\em IRIS Inventor\/}, whereas
	Inventor V2.0 is called {\em Open Inventor\/}. I will only be
	using Inventor V1.0, and I will refer to that as Inventor.\\
	What I call a {\em 3D scene\/} is in Inventor terminology called 
	a {\em scene\/}.\\
	Information about Inventor can be found at the
	\linkf{Open Inventor page}
	{http://www.sgi.com/Technology/Inventor.html}.
\item	[VRML (Virtual Reality Modeling Language):]
	VRML is {\em the\/} standard for 3D scenes on the Web.
	It is based on Inventor. It has a subset of the Inventor commands,
	and some additional commands which are not found in Inventor;
	this will be exemplified later.\\
	What I call a {\em 3D scene\/} is in VRML terminology called 
	a {\em world\/}.\\
	When I refer to VRML, I mean VRML V1.0;
	the specifications are available
	\linkf{here}
	{http://vrml.wired.com/vrml.tech/vrml10-3.html}.\\
	For more information about VRML, see the 
	\linkf{VRML Forum}
	{http://vrml.wired.com/}.
\end{description}
Inventor and VRML are explained further
\hyperref
{later}
{in section }
{}
{Inventor_VRML}.
