What is a Raster Image Processor (RIP)?

Ever wondered what a raster image processor or RIP does? And what does RIPping a file mean? Read on to learn more about the phases of a RIP, the engine at the heart of your Digital Front End (DFE).

The RIP converts text and image data from many file formats including PDF, TIFF™ or JPEG into a format that a printing device such as an inkjet printhead, toner marking engine or laser platesetter can understand. The process of RIPping a job requires several steps to be performed in order, regardless of the page description language (such as PDF) that it’s submitted in. Even image file formats such as TIFF, JPEG or PNG usually need to be RIPped, to convert them into the correct color space, at the right resolution and with the right halftone screening for the press.

Interpreting: The file to be RIPped is read and decoded into an internal database of graphical elements that must be placed on the output. Each may be an image, a character of text (including font, size, color etc), a fill or stroke etc. This database is referred to as a display list.

Compositing: The display list is pre-processed to apply any live transparency that may be in the job. This phase is only required for any graphics in formats that support live transparency, such as PDF; it’s not required for PostScript language jobs or for TIFF and JPEG images because those cannot include live transparency.

Rendering: The display list is processed to convert every graphical element into the appropriate pattern of pixels to form the output raster. The term ‘rendering’ is sometimes used specifically for this part of the overall processing, and sometimes to describe the whole of the RIPing process.

Output: The raster produced by the rendering process is sent to the marking engine in the output device, whether it’s exposing a plate, a drum for marking with toner, an inkjet head or any other technology.

Sometimes this step is completely decoupled from the RIP, perhaps because plate images are stored as TIFF files and then sent to a CTP platesetter later, or because a near-line or off-line RIP is used for a digital press. In other environments the output stage is tightly coupled with rendering, and the output raster is kept in memory instead of writing it to disk to increase speed.

RIPping often includes a number of additional processes; in the Harlequin RIP® for example:

  • In-RIP imposition is performed during interpretation
  • Color management (Harlequin ColorPro®) and calibration are applied during interpretation or compositing, depending on configuration and job content
  • Screening can be applied during rendering. Alternatively it can be done after the Harlequin RIP has delivered unscreened raster data; this is valuable if screening is being applied using Global Graphics’ ScreenPro™ and PrintFlat™ technologies, for example.

A DFE for a high-speed press will typically be using multiple RIPs running in parallel to ensure that they can deliver data fast enough. File formats that can hold multiple pages in a single file, such as PDF, are split so that some pages go to each RIP, load-balancing to ensure that all RIPs are kept busy. For very large presses huge single pages or images may also be split into multiple tiles and those tiles sent to different RIPs to maximize throughput.

The raster image processor pipeline. The Harlequin RIP includes native interpretation of PostScript, EPS, DCS, TIFF, JPEG, PNG and BMP as well as PDF, PDF/X and PDF/VT, so whatever workflows your target market uses, it gives accurate and predictable image output time after time.
The raster image processor pipeline. The Harlequin RIP includes native interpretation of PostScript, EPS, DCS, TIFF, JPEG, PNG and BMP as well as PDF, PDF/X and PDF/VT, so whatever workflows your target market uses, it gives accurate and predictable image output time after time.

Harlequin Host Renderer brochure

 

To find out more about the Harlequin RIP, download the latest brochure here.

 

This post was first published in June 2019.

Further reading:

1. Where is screening performed in the workflow

2. What is halftone screening?

3. Unlocking document potential


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What’s the difference between PDF/X-1a and PDF/X-4?

PDFX-1 PDFX-4

Which PDF/X should I use?

Somebody asked me recently what the difference is between PDF/X-1a (first published in 2001) and PDF/X-4 (published in 2010). I thought this might also be interesting to a wider audience.

Both are ISO standards that deliberately restrict some aspects of what you can put into a PDF file in order to make them more reliable for delivery of jobs for professional print. But the two standards address different needs/desires:

PDF/X-1a content must all have been transformed into CMYK (optionally plus spots) already, so it puts all of the responsibility for correct separation and transparency handling onto the creation side. When it hits Harlequin, all the RIP can do is to lock in the correct overprint settings and (optionally) pre-flight the intended print output condition, as encapsulated in the output intent.

On the other hand, PDF/X-4 supports quite a few things that PDF/X-1a does not, including:

  • Device-independent color spaces
  • Live PDF transparency
  • Optional content (layers)

That moves a lot more of the responsibility downstream into the RIP, because it can carry unseparated colors and transparency.

Back when the earlier PDF/X standards were designed transparency handling was a bit inconsistent between RIPs, and color management was an inaccessible black art to many print service providers, which is why PDF/X-1a was popular with many printers. That’s not been the case for a decade now, so PDF/X-4 will work just fine.

In other words, the choice is more down to where the participants in the exchange want the responsibility to sit than to anything technical any more.

In addition, PDF/X-4 is much more easily transitioned between different presses, and even between completely different print technologies, such as moving a job from offset or flexo to a digital press. And it can also be used much more easily for digital delivery alongside using it for print. For many people that’s enough to push the balance firmly in favour of PDF/X-4.

For further reading about PDF documents and standards:

  1. Full Speed Ahead: How to make variable data PDF files that won’t slow your digital press
  2. PDF Processing Steps – the next evolution in handling technical marks

About the author

Martin Bailey, CTO, Global Graphics Software
Martin Bailey, CTO, Global Graphics Software

Martin Bailey is Global Graphics’ Chief Technology Officer. He’s currently the primary UK expert to the ISO committees maintaining and developing PDF and PDF/VT and is the author of Full Speed Ahead: how to make variable data PDF files that won’t slow your digital press, a new guide offering advice to anyone with  a stake in variable data printing including graphic designers, print buyers, composition developers and users.

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What’s the best effective photographic image resolution for your variable data print jobs?

It goes without saying that the final quality of your printed piece is paramount. But when speed and time constraints are also critical, what can you do to ensure your files fly through the press and still reward you with the quality you expect? Optimizing the images in the piece is a good place to start, but if you’re creating a job with variable data, where there are thousands of pages to print, each with a different image, how do you know what a sensible effective resolution is for those images that will ensure your PDF file doesn’t trip up the print production workflow?

In his latest guide, Full Speed Ahead, how to make variable data PDF files that won’t slow your digital press, Martin Bailey, CTO at Global Graphics Software, advises not to ask the print workflow to do more work than necessary if that doesn’t change the look of the printed result. Images are commonly re-used within a VDP job, so being able to process each image only once and then re-use the result many times can significantly increase the throughput of the digital front end. On the other hand, some images are personal to every recipient and must therefore be processed for every single recipient, slowing the workflow down.

Martin offers the following tips for setting appropriate effective photographic image resolutions:

  1. Aim for 300 ppi, however the most appropriate image resolution for digital presses varies, depending on printing heads, media and screening used.
  2. Bear in mind image content; soft and dreamy images can be sometimes placed at a lower resolution.
  3. Don’t use a higher effective image resolution for photographic images than the output resolution as this is often not productive. The example in Fig 1 below illustrates how easy it is to use an image at several times the required resolution:

The same 12-megapixel image at 3 different sizes

Fig 1: The same 12-megapixel (4000 x 3000 px) image placed on the page at three different sizes. Source: Full Speed Ahead, how to make variable data PDF files that won’t slow your digital press.

When an image is placed onto a page the original resolution of that image is largely irrelevant; what matters is how many pixels there are per inch on the final printed page. As an example, if you have a photograph from a 12 MP compact camera it’ll probably be approximately 3000 pixels by 4000 pixels. If that’s placed on the page as 3 inches by 4 inches (7.5 x 10cm) the effective resolution is about 1000ppi (4000/4). That would usually be about three times as much as you need in each dimension.

A variety of tools are available for optimizing image resolution, and some composition tools can also do this automatically. To find out more about the best effective resolution for your images, and to pick up more tips for optimizing your images for variable data printing, download the guide:

Full Speed Ahead: how to make variable data PDF files that won't slow your digital press edited by Global Graphics Software

Full Speed Ahead – how to make variable data PDF files that won’t slow your digital press.

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Adjusting rendering of outlined text in Harlequin

By Martin Bailey, CTO, Global Graphics Software

In several sectors of the print market it is common practice to convert text to outlines upstream of a RIP, on the grounds that it’s then impossible for the wrong glyph to be printed. This is normal, for instance, in much of the label and packaging industry, especially when there is very robust regulation in place, such as in pharmaceuticals.

Every page description language defines “scan conversion” rules that specify which pixels should be marked when a graphic is painted onto a page; these build on the concept of “pixel touching”, specifying exactly when a vector shape counts as touching a pixel and therefore marking it.

When you’re using PDF (or PostScript, before that) the scan conversion rules are different for text specified using live fonts and for vector shapes. If you started with live text and then converted it to outlines then you have switched from using the text scan conversion rules to using the vector graphic rules. That has always meant that text converted to outlines tends to render slightly heavier than text using live fonts. And the smaller the text is, the more the weight difference becomes apparent.

FIG 1
FIG 1 – 2pt text in Times Roman showing various scan conversion rules.

In Fig 1 you can see this difference very clearly for very small Western text rendered at 2pt and 600dpi, still a common resolution for digital printers and presses. The top line shows text using live fonts, and the second line shows the PDF scan conversion rule for a vector fill. Note that at 2pt the RIP only has about 12 pixels for the height of an upper-case glyph.

In early 2018 we added a new scan conversion rule for vector fills alongside our pre-existing rules in the Harlequin RIP. The intention was to make it possible to emulate the much lighter output that Esko’s FlexRIPs produce. Unfortunately, it also tended to emulate the ability for very fine structures, especially fine horizontal strokes in small text, to disappear. You can see this in the third row of text in Fig 1.

This is obviously not an optimal solution, so we continued our development, and have now extended the original solution with what is called “dropout control”. This prevents very fine sections of a vector fill “dropping out” when they manage to fall on the page in such a way that they don’t cross the locations in the pixels that would trigger anything being marked. You can see the effect of this in the bottom line in Fig 1.

Light rendering with dropout control was delivered to our OEM partners in late 2018 under the name RenderAccurate.

Even this optimized output won’t exactly match the output of live fonts, because the fonts themselves often include hints to the rendering engine, designed to ensure maximum legibility and conformance to the font designer’s vision. These hints can, for instance, ensure that vertical stems are the same width in all glyphs, or that the curved base of a glyph will extend slightly below the baseline to make it visually balance with glyphs with flat bases that sit on the baseline. Those hints were discarded when the text was converted to an outline, and so can’t be used any more. But the new scan conversion algorithm certainly strikes a good balance between matching the weight of live fonts and maintaining legibility.

The effect is visible in very small text in Latin fonts, as shown in Fig 1, but the impact is often masked by the physical effects of printing. And Latin glyphs tend to be relatively simple, so that the human eye and brain are pretty good at filling in the missing segments without too much impact on legibility or comprehension.

On the other hand, Chinese, Japanese and Korean (CJK) fonts are often more complex, with the result that the effect is visible at larger point sizes. And the meaning can be obscured or altered much more easily if strokes are missing. Fig 2 illustrates the same effects on Japanese text at 3pt, rendered at 600dpi. At this size and resolution, the RIP has about 22 pixels for the height of each glyph.

FIG 2
FIG 2 – 3pt text in MS Mincho, showing, from top to bottom: live fonts; default rendering for outlined text; the new, lighter, outlined text; and lighter text with dropout control.

The glyphs shown in FIG 2 are complex compared to Western scripts, but any solution that will be used with CJK scripts must obviously also be proven with the most complex character shapes, such as the Kanji in FIG 3. Some of these have so many horizontal strokes that they simply cannot be rendered with fewer than 22 device pixels vertically and require more than that for reliable rendering. The sample in this figure is rendered at with around 27 pixels for the height of each glyph.

FIG 3
FIG 3 – More complex Kanji in KozGoPro-Regular, showing, from top to bottom: live fonts; default rendering for outlined text; and the new, lighter text with dropout control.

This article has deliberately used very small text sizes as examples, simply because the effects are easier to see. But the same issues arise at larger sizes as well, albeit more rarely.

On the other hand, it is precisely because the issue appears more rarely, and because the effects are less immediately noticeable, that makes the risk of dropping strokes so dangerous. It’s perfectly possible that an occasional missing stroke, perhaps in an unusually light font, may go unnoticed in process control. And that might result in a print that disappoints a brand owner, or even that fails a regulatory check, after the label has been applied or the carton converted and filled, or even after the product being shipped.

So, when a brand demands lighter rendering of pre-outlined fonts, make sure you’re safe by also using dropout control in your RIP!