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Image Formats

Browsers generally display images in two different ways: (1) in-line, using the browser itself, and (2) externally, in a separate window, sometimes with the aid of a "Helper Application". The adjacent figure is an example of an in-line image; here is an example of an external image.

Image files come in a variety of formats, but most Web browsers support only two or three of the formats for in-line images. Thus, you may often need the capability to convert easily among the various formats.

Displaying In-Line Images

The popular browsers support only a limited number of graphics file formats for in-line images, with the most common being GIF and JPEG. For graphics created on a PC, files can often be saved in a BMP format, but sometimes the software will not let you save the work as GIF or JPEG. (This restriction was more common a few years ago than it is today.) You don't need to worry about the technical differences between BMP and JPEG and GIF formats; you only need to know that there are programs available that will convert from the BMP format to either GIF or JPEG.

Displaying External Images

Web browsers are more flexible in the formats they can display as external images because these are often displayed using separate "Helper Applications" that can be tailored to deal with a particular format. Provided a program exists (and runs on your computer) that can display the format, it is likely that the browser can be configured to display that format by launching the helper application.

For example, it is very common in scientific and technical settings to encounter images written in Postscript format. (Postscript files commonly have a .ps or .eps extension in their names.) A browser like Explorer cannot display Postscript directly, but it can be configured such that when it encounters a Postscript file it launches a helper application that displays the file in a separate window.

Common Helper Applications for Image Files

The following Table lists some image file types and the helper application that is commonly used to view it with Netscape on the Macintosh. (See the Appendix on configuring your system for a more complete list.)

Image Helper Applications
MIME TYPE APPLICATION ACTION EXTENSIONS
image/gif JPEGView Internal gif
image/jpeg JPEGView Internal jpg, jpeg, jpe
image/pict SimpleText Launch pict, pic
image/tiff JPEGView Launch tif, tiff
application/postscript Mac GSview Launch ps, eps

In this table, the "Mime Type" is a document classification, "Internal" for the action means the display is in-line and "Launch" means that an external window is launched. "Extensions" are the file extensions that the browser will recognize as indicating a file of this type. Thus this browser, upon encountering a file image.pict, would assume it to be a PICT image file and automatically launch SimpleText as an external viewer to display it.

Technical Issues

We shall concentrate in this Tutorial on giving a basic introduction to the use of images on the Web. This will be sufficient for putting simple images on your pages. If you are serious about optimizing the quality of your Web images, you may wish to consult more technical discussions of image processing.

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