Dynamic Images and the Web
Summary
This is a collection of information and links for those interested in creating dynamic
images.
What is a Dynamic Image
A dynamic image is one which is generated on-the-fly. The image will be update on
subsequent visits to the page or in response to direct user input or other external
factors. This differs from static images which change when the Web author explicitly
updates the site.
How do I create Dynamic Images ?
Images can either be grabbed from an application or created from data sets using
a graphics library. In-line images will be in CompuServe GIF format with JPEG and PNG as
possible other alternatives. External images can be in any format as long as the user has
a suitable viewer configured.
HTTP Headers
Image data can be returned directly to the user. In this case the correct MIME foramt
header must be generated. This will be of the form:
HTTP/1.0 200 ok
MIME-Version: 1.0
Date: Monday, 29-May-95 00:36:49 GMT
Pragma: no-cache
Content-Type: image/gif
... GIF DATA Follows
The content type should reflect the type of image data being sent. This enables the
browser to decide how to interpret the data and whether to invoke an external application.
There is a defined set of MIME image types, it's customary to prefix new types wth: x-.
Notice the blank line between the header and the data. Don't forget this!
Caching
As the images are dynamic we don't want them cached by a proxy or the user's browser.
We can either set the expiry date to the same as the data or use the Pragma:
no-cache field. One disadvantage of using links to images, rather than returning
the data directly, is that the header (and expiry dates) is generated by the server and
image data then may be cached.
GD Library
The original graphics library for creating CompuServe GIF files. The libary is written in 'C' and supplied
in source form. You will need to have a compiler and be able to code with the 'C' language
being the easiest choice. A number of other tools are based on this library.
The author had no trouble building gd library on Unix and Windows platforms with the
latter using MSVC.
GDIT
GD library Interpreter. A simple scripting language for the gd libarary. It has been
built on Sun OS, Solaris, AIX, HP-UX and NeXTSTEP platforms as well as NT and 95. Source
code is available for UNIX but there is an EXE for 95/NT. The home page has extensive
documentation (which may also be helpful if you are using the raw gd library) and some
nice demonstrations.
GDIT can take data sets from standard input and save images to
standard output. It's also nice for setting transparent colors on GIF files and adding
copyright notices.
Mozilla
Netscape Corp's Mozilla
Last Updated: July 25th, 1995.
© 1995 DB Harvey-George | 
