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Raster Image Processing

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The raster image processor (RIP) is the core technology that does the computational work to convert the broad range of data we use to create a computer graphic into the one-bit data that drives a physical imaging device. A font file delivers PostScript language to the RIP that describes a series of points and vector curves between those points to outline, for example, the letter A. The RIP has a matrix grid at the resolution of the output device and computes which spots on the grid get turned on and which are turned off to create the shape of that letter on the output device. The spots on the grid can only be turned on or off — which is how binary data is encoded — either as 0 or 1. The grid then acts as a switch to turn a mechanical part of the imaging engine on or off.With computer-to-plate technology for lithographic printing plate production, a laser is used to expose an emulsion on a printing plate. Most plate-setters have a resolution of 2,000 to 3,000 lspi (laser spots per inch). The RIP calculates all the spots that must be turned ‘on’ to create the graphic that will be imaged on the printing plate. If the image fills a typical sheet-fed press, it is (30 inches x 3,000 lspi) x (40 inches x 3,000 lspi) = 1.08 trillion, which takes 10 gigabytes of computer memory to store and transfer. A printing plate for flexographic print production is created by turning a laser on and off at a slightly lower resolution. An inkjet printer uses the same RIP process to deliver the same one-bit data to each inkjet nozzle for each colour of ink in the printer. Most inkjet engines have a resolution between 600 and 1,200 spots per inch — so the matrix grid is smaller — but if it is an eight-colour printer, the data for all eight nozzles must be synchronized and delivered simultaneously. An electophotographic (Xerox) printer usually has a resolution similar to an inkjet printer and utilizes a similar RIP process to change a grid of electrostatic charges to positive or negative on an electrostatic drum that is the maximum media size the machine can image. Each colour in the printer has a separate raster image that charges the drum in the right spot to attract that colour of toner to that exact location. The data for each colour must be synchronized for simultaneous delivery. The data must refresh the charge on the drum after each print in order to pick up new toner. This basic understanding of RIP’s place in a computer graphic workflow is essential to understanding how to prepare files for, and manage, RIP resources. It is also essential in solving some of the common problems we see in various RIPs. When we compare the two mass production imaging technologies, lithography and flexography, to the personalized imaging technologies, electrophotography and inkjet, we can identify some core similarities. In lithography and flexography, a high-powered laser is used to alter a physical emulsion that is durable and finely grained enough to let the laser image a spot that is one three-thousandth of an inch without affecting the spot of equal size beside it. We can reliably image that spot in a serif of a glyph set in one point type or a hair on a face in a photo that is imaged with a 5 micron frequency modulated (FM) screening pattern. The mass production technology assures us that the first print will be identical to the millionth print. The raster grid of one-bit data that the RIP produces must be delivered to the imaging drum or the inkjet nozzle for every image that is produced with an inkjet printer or an electrophotographic engine. This is what allows us to make every image different and personalize it for the person we are delivering the image to. It also makes the process slower and less reliable for mass production. The RIP produces a lower resolution raster grid, so the detail in photos and letter shapes is not as precise. We can have a RIP discard data if we have too much detail for the raster grid it is producing. The RIP does not do a good job of interpolating more data to produce additional detail in a photo or graphic shape if that information is missing to begin with. It is useful to note, a typical photo resolution is 72 pixels per inch and the raster grid for a lithographic printing press that will print a book cover is 3,000 lspi. Our RIP needs much more data than the web page image contains. Most of the photos we are reproducing today are captured with electronic devices — digital cameras, phones, scanners, or hand-held devices. Graphics are not just black and white — they have tonal values from 0% (nothing) printing to 100% (solid) printing. If we want to render the tonal values in-between in half percent increments, we need 200 addresses to record the different values. Computer data is recorded in bits, two values (on and off), and bytes, which are eight bits strung together in one switch. The number of values a byte can record is 256 — the number of combinations of on and off that the eight bits in the byte can express. A computer records a byte of data for each primary colour (red, green, and blue — RGB) for each detail in a photo, as a pixel (picture element), which controls the phosphors on electronic imaging devices. A RIP must convert the eight-bit RGB values into the four primary printing ink colours (cyan magenta, yellow, and black — CMYK). There are two distinct steps here: (1) conversion from RGB to CMYK continuous tone data (24 bit RGB to 32 bit CMYK); and (2) continuous tone to one-bit screening algorithms. We have to be in the output colour space before we can apply the one-bit conversion. It converts the eight-bit tonal values into one-bit data by dividing the area into cells that can render different sizes and shapes of dots by turning spots on and off in the cell. A cell with a grid that is 10 laser spots wide by 10 laser spots deep can render different 100 dot sizes (10 x 10), from 1% to 99%, by turning on more and more of the laser spots to print. A plate-setter for lithographic platemaking is capable of firing the laser 2,000 to 3,000 times per inch. If the cells making up our printing dots are 10 spots square, we can make dot sizes that have a resolution of 200 to 300 halftone screened dots in one inch. A RIP has screening (dot cell creation) algorithms that convert the data delivered in RGB pixels at 300 pixels per inch into clusters of laser spots (dots) for each printing primary colour (CMYK).This description of how a RIP processes photographic data from a digital camera can help us understand why it is important to capture and deliver enough resolution to the RIP. It must develop a detailed representation of the photo in a halftone screened dot that utilizes all of the laser spots available. The basic rule is: Required PPI = 2 x lines per inch (LPI) at final size. So if you need to print something at 175 lines per inch, it must have a resolution of 350 pixels per inch at the final scaled size of the reproduction. Use this rule if you are not given explicit direction by your print service provider. You can use a default of 400 ppi for FM screening where lpi is not relevant. It is important to know that each time we view a computer graphic on our computer screen, it is imaging the screen through a RIP process. The RIP can change from one software program to another. This is why some PDF files look different when you open them in the Preview program supplied with an Apple operating system than they do when opened in Adobe Acrobat. The graphics are being processed through two different RIPs. The same thing can happen when the image is processed through two different printers. The challenge is to consistently predict what the printed image will look like by viewing it on the computer screen. We use the acronym WYSIWYG (what you see is what you get) to refer to imagery that will reproduce consistently on any output device. Designers have faced three significant challenges in trying to achieve WYSISYG since the advent of desktop publishing in the early 1980s. The first challenge was imaging typography with PostScript fonts. The second was colour managing computer screens and output devices with ICC profiles. The third and current challenge is in imaging transparent effects predictably from one output device to another. Font problems are still the most common cause of error in processing client documents for all imaging technologies. In the next topic, we look at that problem in depth before addressing the other two challenges in achieving WYSIWYG.