\section{Appendix}

\subsection{Measurement}
Within the graphic arts world there are a great many measurement
standards.  Here are some of the more common ones.

\defin{inch}{2.54 cm, customary measure of length. Many printing
standards are based on the inch.}

\defin{point}{Unit of measure commonly used for fonts.
1 point = 1/72 of an inch}

\defin{pica}{12 points (6 picas = 1 inch)}

\defin{pixel}{The smallest unit of measure in raster graphics}

\subsection{File extensions}

\defin{GIF}{Graphics Interchange Format}
\defin{JPEG}{Joint Photographic Experts Group}
\defin{XCF}{eXperimental Computing Facility (Berkeley)}
\defin{XPM}{X Pix Map}
\defin{EPS}{Encapsulated PostScript}
\defin{PNG}{Portable Network Graphics}

\section{Computer Graphics Formats} Graphics on a computer are stored
in a variety of different ``file formats.'' A file format is a way of
structuring graphics information on a disk, so that it can be saved
and displayed or edited later. An editing or drawing program is often
limited in the number of file formats it can read and write, and
there are some industry standard file formats which are more common.
In general file formats are either Raster or Vector formats.

\subsection{Raster (Pixel) Graphics}
A raster graphic is made up of a large number of very small colored
dots, similar to a television screen or a monitor. This is the most
common computer graphics format, especially on the World Wide
Web. Raster graphics are good for photographs, for images that will
only be displayed on a computer screen, and for images in which print
quality is not of paramount concern. The majority of ``paint''
programs generate raster graphics. Also, scanners generate raster
images.

Raster graphics come compressed and uncompressed. Uncompressed raster
files, such as Windows bitmaps ({\tt .bmp}), or X-windows pixmaps
({\tt .xpm}), or netPBM pixmaps,
 are very large, but they display very quickly, with little
computation. This makes them good for such things as backgrounds.

In a bitmap a great many pixels are probably of the same
color. Instead of listing the definition of that color 4000 times
inside the image file, the computer can call that color ``2'', list
its definition once at the begining of the file, and thereafter refer
to it as ``2.'' This is the basic idea behind graphics file
compression. By using this simple technique file size can be greatly
reduced, and only minimal computation is required to display the file.
Examples of this ``loss-less'' compression are GIFs ({\tt .gif}),
TIFs ({\tt .tif}), and PNGs.

The most efficient way of storing a raster file is a complex technique
known as ``lossy'' compression, because some of the detail in the
image is lost during compression.  In theory (it usually works this
way) lossy compression only removes details to small for the human eye
to perceive. In practice lossy compression is great for photographs,
but not for line drawings, because it leaves artifacts along sharp edges.
 It is also best used for images which will
only be viewed on a computer, and not for images that will be
professionally reproduced. The best known lossy graphics format is
JPEG ({\tt .jpg/.jpe/.jpeg}) which is used for many graphics on the
Web. The JPEG 2000 ({\tt .jp2}) format was recently standardized, but 
as yet not many tools are available for it. It uses wavelet based compression
and offers higher quality than JPEG. 


\subsection{Vector (Line) Graphics}
Vector formats are different in that they store images as a group of
mathematically defined curves and shapes.  Vector formats retain their
quality when they are rotated, resized, and printed.  Vector graphics
are the correct format for images with sharp lines and text, and with
a relatively low number of colors, for example engineering or
mathematical diagrams, charts and graphs.


The other nice thing about vector graphics is that they are more
editable than raster graphics.  In raster graphics programs drawing a
line or a circle is in reality just setting the color of a bunch of pixels. In
vector graphics the program remembers drawing a line, and after drawing it
you can pick it up, resize it, stretch it, change its color, and a
great many other things not possible in raster graphics.
On a side note, most vector graphics formats will allow you to include
a raster graphic as just another element, so putting a photograph or a
graphic from the web into your diagram is feasible.
PostScript is the most famous vector graphics format. It does also
deal with raster graphic components. 

\subsection{Vector/Raster Conversion}
So, if graphics are in such distinct subsets, how do we move between them?

It is very possible to convert a vector image into a raster image,
through a process known as ``rasterizing.'' This is used to draw
vector images in the computer screen, and to print them. Many vector
graphics applications will have a ``rasterize'' or ``export''
function, where you can turn the vector graphic into your favorite
format for publication on the web. Just remember that this new file
will not have the same editability that the vector file did.

Conversion from raster to vector is much more difficult. There exist
some programs to asist, but in general this is a tedious ``by hand''
process. It is often easier to recreate the image from scratch.
The most popular technique is to trace over the image with a drawing
tablet and Adobe Illustrator. The SIPB office has a tablet on its 
iMac. 

\section{Cameras, Scanners and Drawing Tablets}

To get your pictures and videos off your digital camera and 
onto your Athena directories, there is the {\tt gphoto2}
in the {\tt outland} locker. {\tt gphoto2 -P -R} will download your
photos from the camera and onto the current working directory, 
and {\tt gphoto -D -R} will delete them fromt he camera's memory. 

The SIPB office has two flatbed scanners, a film scanner, 
and a drawing tablet, should you need them for graphical work.
Just drop by at the office, W20-575, any time it is open. 


\section{The New Media Center}

\footref{http://web.mit.edu/nmc/home.html}{Quoting its web page:} `` The New Media Center's lab is a
Macintosh cluster located in 26-139. This facility is dual purpose,
serving both as a classroom for hands on instruction and as a lab for
students who are required to use specific media based software for
academic or course-related projects. When a class is not using the
cluster, it is open to use by all MIT students, faculty, and
staff. NMC staff will also be present at scheduled hours to help those
with questions about programs, etc.''

The NMC has Macintosh computers with the standard gamut of 
commercial editing programs. Enjoy.

