Introduction to Raster Image Processing
Find out how RIPs operate and what benefits they offer.
The raster image processor (RIP) is the brain behind the digital imaging process, regardless of the type of image or output device used. Here, Coudray explores the functions and features of RIPs that screen printers need to know.
The conversion of digital graphic information into a form that can be written (printed) by an output device is the function of the raster image processor (RIP). Those involved in digital prepress are familiar with the name and the concept of a RIP. But the extent of their understanding usually goes about as far as the ability to select an output device and send a file to it.
The RIP is at the heart of the digital imaging process and does all the work. The ultimate result is a printed or imaged page, panel, sheet of film, or screen. Whether we are sending an image to a laser printer or a sophisticated computer-to-screen (CTS) imaging system, the RIP delivers an array of functions. Unfortunately, many screen printers do not take advantage of the capabilities that are available to them through their RIPs.
How RIPs work
<P>The process of rasterizing an image involves converting Postscript or raster information from the original file into individual spots or dots that will be output by the imaging device.
The spot size is equal to the resolution of the output device. The RIP sends the information to the device through control software known as a driver, which is specific to each individual output device (we need the appropriate driver for each device the RIP drives). Until relatively recently, we would have needed a separate RIP for each output device we were running. But this is changing to reflect more efficient imaging workflows in which we will RIP a file once, and output it to multiple devices. For example, we could process an image for producing film positives and use the same processed image to produce an inkjet proof.
Two primary types of RIPs are available: hardware and software. A hardware RIP is a dedicated piece of computer equipment with an application specific chip-set designed to optimize the processing of graphic files. Such RIPs are usually bundled with specific output devices, most typically an imagesetter. But this is an older configuration and is generally found in first and second generation RIPs. They are usually closed architecture, meaning that we can't access the software directly, other than through routine maintenance functions.
The more modern approach is the software RIP. Here the imaging software is loaded onto a dedicated server. The server could be any high-end Mac, Windows NT or 2000, or UNIX box. Often the machines will have multiple processors to divide and expedite the billions of calculations necessary to rasterize an image.
The architecture is typically open, which means we can add functionality to the software as we need more capability. This will become more apparent later when we discuss all of the things that the RIP software can offer. With a software RIP, we can often purchase the RIP separate from the imaging device. The RIP just processes the image and then directs the digital flow to whatever device(s) we have specified.
RIPs include features to support both image and workflow processing, such as job queuing, open prepress interface (OPI), image caching, time processing, and production logging. The job queue functions allow for multiple users to send images to the RIP simultaneously. The jobs are spooled and held in a print queue for imaging.
The RIP operator has the ability to allow jobs to arrive in the queue, to print in the order they arrive, or to delay or expedite jobs within the queue as necessary. These capabilities can be very useful when only one separation needs to be rerun due to an error or a problem on press. Very large or complex jobs can be scheduled to RIP on the second shift, or even at night when intensive processing will not interfere with smaller jobs needed during the day.
OPI-type RIPs allow for placement of low resolution images for position only within the digital document. The displayed graphic has only enough resolution to be viewed properly on the computer monitor.
At the time of imaging, the OPI function replaces the low-res image with the proper high-res version of the image for halftone screening or large-format output. This is particularly beneficial when there are many images in a document. The OPI capability allows for smaller working documents and less processor-intensive video display.Mark Coudray
<P>However, OPI is losing favor due to the emergence of DCS and the DCS 2.0 file format. DCS was invented by Quark and is now almost universally accepted as a method of high-resolution substitution. To use DCS, you simply save the graphic file as DCS file format. DCS is a special type of EPS file and behaves in much the same way. It is very easy to place or import an image into a layout or illustration program using this format. For screen printers, DCS 2.0 works very well for documents that contain both process-color and spot-color elements.
This replacement of high-resolution images in working graphic documents has a natural and logical evolutionary trend that leads to variable data capabilities. As processors become faster and faster, the primary image is RIPed and cached, and specific or variable information is then merged with the primary image to create the final output. This is called variable-data printing, and it is an emerging technology that offers very interesting opportunities.
To make variable-data printing possible, the RIP must be able to work from image caches. This involves ripping several images and holding them in storage. Then, as jobs are printed, the specific data or variable data is merged with the stored images according to user-defined parameters. The result is specific output that matches the user needs.
An example might be signage that directs certain groups to certain activities. The basic poster would be the same, but the variable graphics relating to the groups and the activities would change with each sign.
Other image-related functions that are provided by RIPs include nesting and imposition. Nesting is the ability to fit or arrange multiple images across the smallest possible area of the output media for most efficient use of the media. An example for textile printers would be to nest three or four pocket logo designs across the width of the media instead placing only one image in the center.
Imposition is more intensive and generally applies to multi-page documents that require post bindery processing. For screen printers, imposition is more related to step-and-repeat work. It is very important for decal, tag, and label printers. Imposition can be very intensive and often is often supported through separate software that runs in conjunction with the RIP, although it may also be part of the RIP itself.
The next three image-related functions are color separation, trapping, and halftone screening. These functions are all specific to the RIP employed and are generally transparent to the user. What this means is that the user will define how they want the image separated, trapped, and screened, and the RIP makes all the decisions about how to achieve the desired characteristics.
User-defined parameters are stored as preferences, scripts, or routines in the RIP. When it comes time to send a job for output, the operator simply chooses the desired output device based on how the image will be used. The separation, trapping, and halftone screening are all done on the fly as the image is output.
The trend is clearly to incorporate this technology at the RIP level. In the past, these three functions were handled by dedicated applications that worked in conjunction with the RIP. As workflows become increasingly automated and technical labor becomes less available, however, the separation, trapping, and screening will almost surely become a standard component of the RIP.
Other RIP features are designed to allow for the smooth and efficient processing of large volumes of images. They provide accounting information and productivity information so that the maximum amount of work can be done on any given shift.
One of the big issues with digital files is the time it takes to process them. If ever there was a good example of the "time is money" principle, digital imaging is it.
Every operator who has ever output a large image is familiar with the job that takes hours to process before it crashes. In order to minimize the chances of this happening, it is necessary to preflight the images before sending them on for processing. Often this is done with standalone software, but there are an increasing number of preflight features being built into today's RIP software.
Besides finding missing and conflicting fonts and graphics, the preflight software looks for things like proper file format and image resolution based on how the file will be output. This is necessary to avoid over and underprocessing of data. With over processing, the images take too much time to process, data is discarded as redundant, and there is no visual advantage when the final print is delivered. With under-processed images, low resolution results in serious pixelization and artifacting (especially with JPEG images.)
In addition to the preflight functions, there are the time-reporting capabilities that log how long it took to RIP and image each job. This is especially useful for management in determining job estimates and if it will be necessary to charge additional fees for particularly difficult jobs. Long processing time is becoming an increasingly common problem with complex vector images that incorporate mesh gradients and transparent Postscript images. These two elements can bring almost any RIP to its knees.
The management productivity logs featured in most RIP software keep track of overall job performance and quantity over the day. They are used in production planning and job costing to help in profit analysis. They are useful in determining traffic trends and can be helpful in identifying processing patterns throughout the course of the day.
RIP functions will become more and more important to all of us as the use of in-house imaging increases. As the trend toward rapid job turnaround continues to pressure us, the efficient and integrated processing of digital images will be a key component of maintaining a profitable shop. A complete understanding of the capabilities of the RIP within this workflow is crucial for our success.