A workflow protocol developed by Aldus Corporation used in electronic prepress to link desktop publishing systems and high-end CEPS. Essentially, high-resolution color images are stored on a central network server, to which all the workstations are connected. Low-resolution files are sent by the server to individual computers working on page layout. The low-res images are imported into the page (in a kind of FPO way), positioned, and comments sent back to the OPI server provide specific cropping, scaling, positioning, and color information about the image. The server's PostScript driver inserts the proper instructions into the PostScript code. When the page is ultimately output to an imagesetter connected to the network, the high-resolution image is swapped for the low-res one, and the indicated instructions as to cropping, etc., are executed.
OPI is useful for minimizing high-resolution-file travel on networks; their large file size can make traffic screech to a halt. And by utilizing only low-resolution viewfiles on workstations, processing speed is increased. The efficacy of OPI is contingent upon the use on the workstations of OPI-compatible software; many page layout programs are increasingly including support for OPI, although some OPI specifications for color separation haven't been effectively nailed down yet. Although OPI is often compared to DCS, the latter is strictly a color separation protocol, while the former is more of a workflow protocol.