Administration Guide : PDF Image Replacement workflows : Background

The Open Prepress Interface (OPI) specifications were developed by the Aldus corporation to allow a prepress operator to place a low-resolution For Placement Only image (FPO) into a PostScript-generating page-layout program and have the low-resolution FPO replaced in the PostScript stream at print time.
PostScript uses an opaque imaging model, meaning that colors are solid. Any transparent elements and the elements they interact with must be combined into opaque objects through a process called transparency flattening. Today PDFs can be exported directly from InDesign files and Illustrator documents can be saved directly as PDFs, without the need to go through a PostScript generation process that requires transparency flattening. This means that transparent objects maintain their transparency and no longer have to be broken into many smaller images, potentially losing the visual intent of the objects as well as bloating the PDF to many times its original size.
With earlier versions of Xinet image-replacement software, FPOs that were placed in a layout application and had transparency applied to them could not be OPI replaced, because they were flattened by the layout application at the time it generated PostScript. During flattening, these images were divided into various pieces and merged with the images above or below them according to the transparency effects applied. The new merged images could no longer contain any reference to the original image on the server since they had become combinations of images and other effects (such as drop shadows), and as a result OPI replacement would fail. This would typically happen at one of two times: at the time the application produced its PostScript, or when the server converted the PDF to PostScript.
The Xinet PDF Image Replacement engine works directly in the PDF format, removing the conversion to PostScript, thus avoiding the flattening process, and allows conversions that are impractical in PostScript. The final result will be a PDF that includes the high-resolution images with transparency preserved and converted to the target image format (resolution, colorspace, sharpening, scaling etc.) as defined by the PDF Image Replacement queue.
With Xinet PDF Image Replacement workflows, OPI references in PDFs are no longer required to resample, color convert, or sharpen raster images included in the PDF. Local high-resolution images can also be included as you export the PDF, with all images still being properly converted according to the Image Replacement settings of the PDF Image Replacement queue. For InDesign users, Xinet provides plug-ins that make the process easy, by automatically handling PDF export options.