In addition to two types of digital images (bitmaps and vectors), there are also a variety of bitmap and vector file types.  Bitmap files, at the most basic level, are just as described - a map or plot of each individual pixel.  However, image files can (and often do) contain other information - depending on their use.  For instance, the author or creator of the image might be included, as well as the date and copyright information.  In addition, because bitmaps tend to be large, various methods of compression or methods to reduce file size have been developed.

To designate extra information and compression methods, different file types have been developed.  File types designate a particular variety of bitmap or vector.  Windows and Unix operating systems idenpsdy the file type by adding a period followed by three letters to the end of the filename, as in “circle.gif” or “circle.jpg.”  This is called a file extension.  On the Macintosh (before OS X) file extensions are not used to designate file types.  A file designating the file type and the application that created it is attached to the image file instead.  However, Web servers and browsers also use file type extensions, so it’s advisable that Macintosh users do the same for image files generated for the web.

Web browsers currently support three image file types.  They are .gif, or Graphics Interchange Format (GIF), .jpg or Joint Photographic Experts Group  (JPEG), and .png or Portable Network Graphics (PNG).  Each of these formats is a bitmap file.  Currently, browsers do not support the display of vector-based images without a plugin.

Each of the file types for the Internet, and the appropriate use, are discussed in a later chapter.

Photoshop  can import a variety of bitmap and vector formats.  Importing external image files is covered later in the book.  Photoshop’s native file format is called Photoshop Document.  The extension is .psd.