When you choose to serve an image in Photo CD format, powerful new things are possible that cannot be done with GIF and JPEG images. Perhaps most exciting is the fact that the images become interactive. You can zoom in on the image without it becoming pixelated. You must try it for yourself to believe it. Few people realize the world of detail that is captured in a standard 35mm film image. Photo CD can reveal all of it -- on the Web. Once zoomed, you can pan around the high resolution images. In addition to zooming, you can enlarge the images. This is analogous to enlarging a photograph in a real world dark-room. Now, Photo CD brings the digital equivalent to the Web.
Second, Photo CD brings color management to the Web for the first time. Unlike GIF and JPEG, Photo CD images are stored in a well-characterized color space called YCC. (Lawler) Based on international color standards, YCC promises to deliver the color of the original scene to your monitor, via color management software such as Kodak Color Management Software (KCMS). Essentially PhotoYCC is the color space that is used when storing Photo CD images. PhotoYCC is designed to retain all of the important scan information and permit the use of the stored image on any output device. In contrast, other scanning systems choose a more restricted color space (for example, various RGBs) that loses information. Once lost, information cannot be recovered. For the latest browsers with color management capability, the YCC data translates to the best possible color on your monitor or printer. In simple terms, you could visit a museum on the Web and see a Monet with the correct colors. Or visit a clothing store on the Web and know that a green shirt is really that shade of green.
All of this is possible, and easy to do. On the server side, images are simply referenced in pages using standard HTML. On the browser side, they simply appear in pages, along with functions that allow you to zoom, pan, crop, and rotate them. All of this is accomplished with standard Web technology, and a Java Applet. Let us describe in more detail the mechanism used in making Photo CD available on the Web.
26.3.2 Procedure
In this section we will describe how the images are prepared for the
World Wide Web. Let us outline the steps which are needed in
producing a Photo CD image on the Web. The steps are rather trivial,
but they do involve a third party (a Photo Shop) which holds the
machinery, an imaging device, to create the correct format.
(Kodak, Photo CD on the Web) Currently
this is rather expensive ($100K-$200K) for individuals to own.
Exposure and focus are automatic. Color balance is set through the selection of "film terms"-- special sets of data that interpret the color information collected by the PIW's scanner. Film terms take into account characteristics particular to various film types, and play a vital role in optimizing scan quality.
26.3.3 Technical Notes
Making Photo CD available for viewing on the Web involves the two ends
of the network, the server and the client. The technicality of
the Photo CD relies on the server.
(Kodak, Photo CD on the Web)
The soul of Photo CD on the Web is a set of CGI programs that are provided by Kodak. The webmaster for the server configures the programs and then they are simply dropped into the same directory as other CGI programs (usually the cgi-bin directory). Once that is done, the Photo CD image pacs to be served are copied to the directories indicated. At this point, they are ready to appear in pages. Standard HTML is used to do this, such as the 'img' tag. Standard URL's are constructed to point to the images via the cgi-bin syntax. For example, to place an image on a page, the simplest tag might be:
<img src="/cgi-bin/pcd/image.pcd">
When a page with Photo CD images in it is requested from a server, the CGI program pcd is invoked. The program does three things: extracts the requested image from the image pac, JPEG compresses it on the fly, and then sends the JPEG stream to the browser. It also has a smart caching mechanism: it stores the JPEG stream for a while in case you make a request for the same image again.
If just the name of the Photo CD file appears in the URL, then a default size version of the full image is served. However, a set of parameters can be included in the URL to define the sub-image desired, the Photo CD resolution level to extract it from, the rotation and the compression level to use. Thus, a single Photo CD image pac can appear on lots of different pages in different guises.
To make the image interactive, the HTML should make use of the Java applet when referencing the image. An interactive instance of the image might be:
<applet code=PcdApplet.class width=192 height=153>
<param name=src value="/cgi-bin/pcd/image.pcd"> </applet>
On the other hand, the client must have the capability to look at the images with browsers that can recognize the newest technology. There are three types of browsers that can display Photo CD images on the Web
In the latter two options, the browser simply edits the URL and issues another request, as needed. The image is then repainted in it's new form -- zoomed, or whatever was requested. One nice benefit of this architecture is that a new "page" is not created. Thus, ten requests to modify the image do not result in ten pages that you have to go "Back" through.
A good review on how to Construct a Photo CD URL on the Web is available.
Dan Haim
<haim@vt.edu>
Last modified: Sun Dec 15 17:18:35 EST 1996