Image Formats

Unfortunately, image files come in a variety of formats, so Web browsers must either be able to deal with many formats, or you must have the capability to convert easily among these formats.

Browsers generally display images in two different ways: (1) in-line, using the browser itself, and (2) externally, in a separate window, often 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.

Displaying In-Line Images

The popular browsers support only a limited number of graphics file formats for inline images: XBM, GIF, and JPEG. The first is a format used almost exclusively on Unix workstations, and virtually never seen in the Mac/PC world. The last two formats, however, are used on a wide variety of computer systems, including Macs and PCs.

For graphics created on a Macintosh, files can often be saved in a PICT format, but not GIF or JPEG. You don't need to worry about the technical differences between PICT and JPEG and GIF formats; You only need to know that there are programs available that will convert from the PICT format to either GIF or JPEG. So if you are using graphics software on a Macintosh to create artwork that you want to put on a Web page, you will probably also need access to one of these format conversion programs. More on this later.

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 that can display the format and runs on your computer, 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 Netscape 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.

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