Raster image processor
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A raster image processor (RIP) is a component used in a printing system which produces a bitmap. The bitmap is then sent to a printing device for output. The input may be a page description in a high-level page description language such as PostScript, Portable Document Format, XPS or another bitmap of higher or lower resolution than the output device. In the latter case, the RIP applies either smoothing or interpolation algorithms to the input bitmap to generate the output bitmap.
Raster image processing is the process and the means of turning vector digital information such as a PostScript file into a high-resolution raster image.
Originally RIPs were a rack of electronic hardware which received the page description via some interface (eg RS232) and generated a "hardware bitmap output" which was used to enable or disable each pixel on a real-time output device such as an optical film scanner.
A RIP can be implemented either as a software component of an operating system or as a firmware program executed on a microprocessor inside a printer, though for high-end typesetting, standalone hardware RIPs are sometimes used. Ghostscript and GhostPCL are examples of software RIPs. Every PostScript printer contains a RIP in its firmware.
Earlier RIPs retained backward compatibility with photosetters so they supported the older languages. So, for example Linotype RIPs supported CORA (RIP30).
[edit] Stages of RIP
- Interpretation: This is the stage where the supported PDLs (Page description languages) are translated into a private internal representation of each page. Most RIPs process pages serially so the current machine state is only for the current page; i.e one page at once. Once a page has been output the page state is discarded to ready it for the next page.
- Rendering: A process through which the private internal representation is turned into a continuous tone bitmap. Note that in practical RIPs interpretation and rendering are frequently done together. Simple languages (mostly the most ancient) were designed to work on minimal hardware so tend to "directly drive" the renderer.
- Screening: In order to print, a continuous-tone bitmap is converted into a halftone (pattern of dots). Two screening methods or types are Amplitude Modification (AM) screening and stochastic or Frequency Modulation (FM) 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.
A RIP chip is used in laser printers to communicate raster images to a laser.
