AutoCAD includes four more xref-like features: the ability to attach raster images, DWF files, PDF files, and DGN files to drawings. I cover DWF and PDF files in the next section and refer you to the online help if you should find some DGN files on your hands. The image feature is useful for adding a raster logo to a drawing title block or placing a photographed map or scene behind a drawing. A raster, or bitmapped, image is one that's stored as a field of tiny points.
Most AutoCAD drawings are vector files. A vector drawing is a graphic file defined by storing geometrical definitions of a bunch of objects. Typical objects include a line (defined by its two endpoints) and a circle (defined by its center point and radius). Vector-based images are typically smaller (in terms of the disk space they occupy) and more flexible than raster images, but also are less capable of displaying visually rich images, such as photographs.
Raster images often come from digital cameras or from other programs, such as Photoshop. Raster images also can come into the computer from some kind of scanner that imports a blueline print, photograph, or other image.
Whether you're doing your own scanning or having a service bureau do it for you, you need to know that AutoCAD handles most of the popular image file formats, including: the Windows BMP format; the popular Web graphics formats GIF and JPEG; common print formats such as PCX, PNG, and TIFF; the less popular DIB, FLC, FLI, GP4, ...