Raster image processor
A raster image processor is a component used in a printing system which produces a raster image also known as a bitmap. Such a bitmap is used by a later stage of the printing system to produce the printed output. The input may be a page description in a high-level page description language such as PostScript, PDF, or XPS, including raster and/or vector graphics. The input can also be or include bitmaps of higher or lower resolution than the output device, which the RIP resizes using an image scaling algorithm.
Originally a RIP was a rack of electronic hardware which received the page description via some interface and generated a "hardware bitmap output" which was used to enable or disable each pixel on a real-time output device such as a laser printer, an optical film recorder, computer to film, or computer to plate.
A RIP can be implemented as a software module on a general-purpose computer, or as a firmware program executed on a microprocessor inside a printer. For high-end typesetting, standalone hardware RIPs are sometimes used. Ghostscript, GhostPCL, and ColorBurst's Overdrive are examples of software RIPs. Every PostScript printer contains a RIP in its firmware. The RIP chip in a laser printer sends its raster image output to the laser.
Earlier RIPs retained backward compatibility with phototypesetters/photosetters, so they supported the older languages. So, for example, Linotype RIPs supported CORA.
Stages of RIP
- Interpretation: This is the stage where the supported PDLs are translated into a private internal representation of each page. Most RIPs process pages serially, one page at a time, so the current machine state is only for the current page. After a page has been output, the page state is discarded to prepare for the next page.
- Rendering: A process through which the private internal representation is turned into a continuous-tone bitmap. In practical RIPs, interpretation and rendering are frequently done together. Simple languages were designed to work on minimal hardware, so tend to "directly drive" the renderer.
- Screening: In order to print, the continuous-tone image is converted into a halftone. Two screening methods or types are amplitude modulation screening and stochastic or frequency modulation screening. In AM screening, dot size varies depending on object density—tonal values; dots are placed in a fixed grid. In FM screening, dot size remains constant and dots are placed in random order to create darker or lighter areas of the image; dot placement is precisely controlled by sophisticated mathematical algorithms.