All-Ink.com, a company of graphic design services and printing online, has decided to improve its online marketing strategy targeting small businesses with a new tool for creating custom Web sites.
"We asked, and survey our customers regularly and we know there is a growing demand for electronic products to help our clients and small business market," said Janet Holian, executive vice president and general manager of marketing for the company.
Also, Holin added: "We believe that a site is as important as a business card and our objective was to perform a design as easy and cost effective design and purchasing a All-Ink business card. For this reason we explore different ways to develop custom web sites best meeting the needs of our customers and based on our experience and knowledge by developing a robust online tool.
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Wednesday 14 October 2009
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Naturally, there are different ways to categorize all these technologies. One is by formatsize: narrow (or desktop) format is anything under 24 inches in width; wide (or large) for-mat is everything 24 inches wide or more (this is media size, not the size of the printer).Another way is by drum versus “plotter” configuration (based on the original CAD plot-ters used to produce computer-generated charts and graphics). What I’ve chosen to do,instead, is to group them by their logical (in my opinion) imaging characteristics. (Note:products, brands, and models current at the time of this writing.)
Digital Photo PrintUntil recently, and apart from the IRIS printing process, photographers who wanted actualphotographic output (reflective or backlit display) produced from their digital files had tomake an intermediate negative or transparency with a film recorder and then use aconventional enlarger to make the final print. But in 1994, a new type of printer was devel-oped that could print directly from a digital file without the need for the intermediatetransparency step. The photo processing industry has never looked back.
I break this category down into two groups: wide-format digital photo print and digitalphoto process .
Wide-Format Digital Photo PrintThis is top-of-the-line, continuous-tone photo output, and you’ll only find the priceydevices for doing this in photo labs, repro shops, service bureaus, and “imaging centers.”(See Chapter 10 for more about how to work with outside print providers.) I like the term“digital photo print;” others use words like “digital C-print” or “laser photo printing,”although not all devices use lasers.
How Does It Work?Either using three-color lasers (red, green, blue) or light-emitting diodes (LEDs), thesewide-format printers produce extremely high-resolution prints on conventional, light-sen-sitive, color photo paper that’s processed in the normal wet-chemistry photographic man-ner (although other processing “back ends” can be used). There is no screening, halftoning,or dithering of the image.
Italy-based Durst popularized this category of digital printers, and it now has several mod-els of the Lambda digital laser imager plus other variations including the Theta and theZeta printers, each with its own market niche. Using continuous roll feeding, the small-est (Lambda 76) can print a single image up to 31 inches by 164 feet, and the largest(Lambda 130/131, used at National Geographic Magazine’s headquarters) prints up to 50inches by 164 feet in one shot. Even larger sizes can be printed in sections or tiles. Tworesolution options (200 or 400 dpi) yield an apparent resolution of 4000 dpi. (see the“Apparent Resolution” explanation earlier in this chapter.) For color depth, the input is at24-bit, output is interpolated to 36-bit using RGB lasers to expose the photographic paper.There are approximately 800 Lambdas installed around the world.
The Océ LightJet 430 has a maximum output size of 50 × 120 inches, and the newer 500XL
model can go up to 76 inches wide (the older 5000 model prints to a maximum of 49 × 97
inches). The spatial/addressable resolution is either 200 dpi or 300 dpi with an apparent res-
olution of 4000 dpi. As with the Lambda, the input is 24-bit, interpolated to 36-bit output
color space (12-bit per RGB color). The LightJet uses three RGB lasers for exposure, and a
unique 270-degree internal drum platen for media handling (the media is held stationary
within the drum while a spinning mirror directs laser light to the photographic material).
Another high-end, large-format printer is the ZBE Chromira, which uses LED lights instead
of lasers. The print is processed in normal RA-4 chemistry through a separate processor.
There are two models and two sizes, 30 or 50 inches wide, with no limit on length. Yielding
300 ppi resolution (425 ppi “visual resolution” with ZBE’s proprietary Resolution
Enhancement Technology), this is another expensive piece of hardware (but less costly than
a LightJet or Lambda), so you’ll find one only at a photo lab or service bureau.
Digital Photo Process (Digital Minilab)
Digital photo printing isn’t limited to high-end, large-format devices. In fact, you may not
realize it, but most photo labs and photo minilabs today use the same technology to print
everything from Grandma’s snapshots to professional prints. These are the ubiquitous “dig-
ital minilabs” found at many photo retailers, drugstores, and big-box merchandisers like
Wal-Mart and Costco.
How Does It Work?
Digital minilabs made by Agfa, Noritsu, and Fuji are the standard at many photofinishing
labs and the new online processors described in Chapter 10. The Fuji Frontier (see Figure
2.18) was the first digital minilab used for the mass retail market. It’s a complete system that
takes input from conventional film, digital camera, digital media, or prints (with onboard
flatbed scanner) and outputs to digital media or prints via wet-chemistry processing. There
are several different models of the Fuji Frontier, and the largest output is 10 × 15-inch prints.
Digital Photo Print: For What and for Whom?
Photographers like the output from digital photo print/photo process because it looks like
a real photograph. In fact, it is a real photograph! Larry Berman, a photographer who is a
regular on the art show circuit, has most of his prints done on a Noritsu digital printer at
his local Costco. Berman pays only $2.99 for a 12 × 18 print that can also yield two 8 ×
10s. The costs for the wide-format variety (Lambda, LightJet, Chromira) are comparable
to wet-darkroom prints from a custom lab, but the digital versions will soon be replacing
the traditional ones as their materials become extinct.
The primary drawbacks with digital photo print are that paper choices are limited, and you
can’t do this yourself because the devices are much too expensive for self-printers to own.
Dye Sublimation
Dye sublimation (also known as “dye diffusion thermal transfer” and typically called “dye sub”)
is for high-quality photo and digital snapshot printing (and pre-press proofing). Dye-sub print-
ing has a loyal following among some photographers who prefer it to inkjet printing.
How Does It Work?
With dye sub a single-color ribbon containing dye is heated by a special heating head that
runs the width of the paper. This head has thousands of tiny elements that, when they
heat up, vaporize (“sublimate”) the dye at that location. The gaseous dye spot is then
absorbed into the surface of the paper. Since the paper receives separate cyan, magenta,
yellow, and sometimes black passes of the dye ribbons to make up the final image, the
resulting layering of color provides a smooth, seamless image. Photo dye-sub printers only
have 300 or so dpi resolution, but they can deliver continuous tone images because of this
layering and the way the dyes diffuse or “cloud” into the paper. Some dye subs add a pro-
tective layer (a clear UV laminate) as a fourth and final step after the single-color passes.
Fuji Pictrography
Many top photo labs and retouch studios, especially those involved with the fashion and beauty indus-
tries, use the Fuji Pictrography printer (models 3500 and 4500) for high-quality prints and proofs, also
known as Fujix prints. Pictrography uses a unique, single-pass, four-step process (see Figure 2.19). A sheet
of photosensitive “donor” paper is exposed to laser diodes (LD). A small amount of water is applied to
create the dye image on the donor paper with heat. The dye image is then transferred to the “receiving”
paper with a combination of heat and pressure. Finally, the receiving paper, with its transferred dyes, is
peeled off and separated from the used donor paper. This is not photographic paper, although Fuji claims
the equivalent image permanence. Only special Fuji paper can be used. Two resolutions (267 dpi and
400 dpi) are available with a maximum paper size of 12×18 inches (4500 model only).
Figure 2.19 The Fuji Pictrography and its unique four-step printing process.
Courtesy of Fuji Photo Film USA, Inc.
Electrophotography (Color Copy/Color Laser)
Also called “xerography” (“xeros” for dry, “graphos” for picture), electrophotography
involves the use of dry toners and laser printers or printer/copiers. (The liquid-toner
version or “digital offset” was described in the last chapter.)
How Does It Work?
Many color lasers use hair-thin lasers to etch a latent image onto four rotating drums, one
each for the four printing colors (see Figure 2.20). The drums attract electrically charged,
Electrophotography: For What and for Whom?
Traditionally used as proof printers by pre-press departments and production printing
operations, color laser printers are becoming more short-run printing presses in quick-
print shops as well as businesses. They are also used as primary color output devices in
graphic arts departments and design studios, and now, by artists—especially photogra-
phers. Indiana photographer Seth Rossman likes this type of output. “For photographers,
it’s an almost perfect medium. I use it in continuous-tone mode, which gives it more of a
dithered effect, so no dots.”
Electrophotographic printing is fast and reasonable, with 8x10 prints under $1.00 at many
retailers, and images can be printed on a small range of substrates including matte paper
and commercial printing stocks. “If you want top-quality photo prints from a color laser
printer,” says photographer Phillip Buzard, “the paper must be very smooth and very white.”
The main disadvantages of electrophotography are the limited maximum output size (usu-
ally 12x18 inches) and the high initial cost of the machines if you’re self-printing. The
image has a slightly raised surface when viewed at an angle, especially on glossy or cast-
coated stock, but the colors can be very bright and saturated. Depending on the type of
screening and resolution used, prints sometimes have a lined or halftone-dot look (see
Figure 2.17 earlier in this chapter).
Inkjet
For the most flexibility in terms of choices of printer brands and types, inks, papers, sizes,
and third-party hardware and software support, you can’t go wrong with inkjet. There are
photo printers, proof and comp printers, you name it. As far as quality goes, I’ve seen high-
resolution desktop, thermal and piezo inkjet prints on glossy and semi-gloss paper that
rival—even surpass—any traditional photographic print. In addition, certain inkjet print
combinations exceed all other standard, color-photo print processes in terms of projected
print longevity or permanence.
Simply described, inkjets use nozzles to spray millions of tiny droplets of ink onto a sur-
face, typically paper. While earlier devices had an obvious digital signature, the newer print-
ers are so much further along that many inkjet prints can now be considered continuous
tone for all practical purposes.
There are two main types of inkjet technologies: continuous flow and drop-on-demand,
which is further subdivided into thermal, piezoelectric, and solid ink (see Table 2.3). (We’ll
go into more detail about inkjet printing in Part II.)
Continuous Flow
Although this is the original technology that started the high-quality, digital-printing boom,
continuous flow has become much less popular over the years. The most famous example is
the IRIS printer, which is no longer manufactured although there are many of these printers
still in use. The IRIS has been replaced with the ITNH company’s IXIA, pronounced “zia.”
How Does It Work?
A single printhead moves along a rod above the paper that is wrapped around a rotating drum.
The printhead encloses four glass nozzles (one for each of the printing colors: cyan, magenta,yellow, and black) that are each connected to a bottle of translucent dye ink. In each head is
a tiny vibrating piezoelectric crystal that pushes out a million ink droplets per second. As the
ionized ink droplets exit the nozzle, some receive an electrostatic charge; some don’t. The
charged ink droplets are deflected away from the drum and recycled. But the uncharged
ones—our heroes—pass through the deflector and end up hitting the paper to form the image.
Although the IRIS/IXIA has a maximum resolution of only 300 dpi, its apparent resolution
is more like 1800–2000 dpi due to its variable dot size and overlapping dot densities.
Continuous Flow: For What and for Whom?
The main advantage the IRIS/IXIA is the wide range of media accepted plus the high
image quality and the ability to produce deep, rich blacks. When printed on textured fine-
art paper, these prints have a beautiful velvety look, but the slow print speed (30–60 min-
utes per print) plus the time-consuming maintenance and manual paper mounting have
reduced demand for these expensive ($45,000) drum machines.
Drop-on-Demand
This is where most of the inkjet action is. The reason it’s called drop-on-demand is because
only the ink droplets that are needed to form the image are produced, one at a time, in
contrast to continuous-flow where most of the ink that’s sprayed is not used. The three
main categories of drop-on-demand, inkjet printing are: thermal, piezo, and solid ink.
Thermal
How Does It Work? This process, which was invented in 1981 by Canon (“Bubble Jet
Printer”), is based on the heating of a resister inside the printhead chamber (see Figure
2.21). As the resister heats up, a vapor bubble surrounded by ink is formed, and the
increase in pressure pushes an ink droplet out of the nozzle in a printhead. After the bub-
ble collapses, more ink is drawn in from the ink reservoir, and the cycle repeats.
Thermal Inkjet: For What and for Whom? The largest number of inkjet printers sold
in the world today fall into this category. They’re affordable and widely available with up
to excellent image quality that rivals photographic prints.
Piezoelectric
How Does It Work? When certain kinds of crystals are subjected to an electric field, they
undergo mechanical stress, i.e., they expand or contract. This is called the “piezoelectric
effect,” and it’s the key to this popular brand of digital printing, called “piezo” for short
(and not to be confused with “piezography,” which is described in Chapter 11). When the
crystalline material deflects inside the confined chamber of the printhead, the pressure
increases, and a tiny ink droplet shoots out toward the paper (see Figure 2.23). The return-
ing deflection refills the chamber with more ink.
Both the wide-format and desktop models of piezo printers come only in plotter versions
with the printhead assembly going back and forth over the paper to create the image. Piezo
printheads are typically single units with all colors included; they are a permanent part of
the machine and usually need no replacing.
Examples of piezoelectric inkjet printers include— Wide-format: Epson Stylus Pro 4000,
7600 and 9600; Roland Hi-Fi JET Pro-II, Mimaki JV4, and Mutoh Falcon II. Desktop:
Epson Stylus C84, Stylus Photo R800 and 2200.
Piezo Inkjet: For What and for Whom? In the desktop category, there’s only one piezo
player, and that’s Epson. With six- to seven-color inks in dye and pigment versions, these
are the printers that have historically owned a significant share of the photographer-
artist, self-printing inkjet market. Other manufacturers join Epson in the wide-format
category. As with thermal, piezo inkjet printers are widely available and produce up to
excellent image quality.
Solid Ink
How Does It Work? Formerly called “phase change,” solid ink technology is the inkjet
oddball. The Xerox Phaser 8400 (Xerox is the only real player in this category) is a true
piezoelectric inkjet, but there are several surprises. First, the pigmented colors come in the
form of solid blocks of resin-based inks, although the ink still ends up as a liquid after
heating (hence the term “phase change”). These printers also have the affectionate nick-
name “crayon printers,” from the resemblance of the ink sticks to children’s crayons.
And instead of a smaller, reciprocating printhead assembly, there is a single printhead that
extends nearly the width of the paper with 88 nozzles in each of four rows. The same piezo
substance we’ve already learned about shoots the ink droplets out as before, but in another
twist, the ink doesn’t go onto the paper; instead, the ink goes onto a turning offset drum
that is kept warm so the ink doesn’t solidify. The drum then transfers (in a single pass) the
still-molten ink to the paper under pressure to form the image.
Solid Ink: For What and for Whom? With ink that sits on top of the paper creating a
definite relief effect, the colors are brilliant and sharp since the ink drops don’t spread or
bleed. However, even at 2400 dpi, “near-photographic” might better describe the image
quality. Solid ink inkjet is fast, it prints on a variety of media, and it yields highly satu-
rated images that some photographers, designers, and illustrators love. Disadvantages
include limited output size (letter/legal) and relatively poor image permanence (Xerox
claims only “a year or more” with office lighting, “over several years” with dark storage).
With all this new, accumulated information about pixels, hardware, and printing tech-
nology under our belts, let’s move our attention to what it takes to create and process a
digital image.
Printing software allows you to access and interface with your printer. Before you can print from a drawing, painting, image-editing, or page-layout program, the printer software program must be correctly installed onto the computer, usually from the CD that comes with your printer. (Photo-direct printers that take media cards don’t require computers, and the printer software can be accessed directly from the printer itself).
Every print device requires a particular “printer driver” for the specific operating system of the computer. (Note that it’s your computer’s operating system that you match to the printer, not the software application.) You must have the right driver for your printer in order to support all the printer’s features (paper selection, quality level, and so on) and to tell the print engine how to correctly render the image’s digital data. If you change your operating system, you may need to install an updated printer driver, which you can normally download from the printer-manufacturer’s website.
When you select “print” from your application’s File menu, what you get is a series of menu screens and dialog boxes for that particular printer driver. If you have a PostScript printing device, you need to use a PostScript driver and select it.
There are three common ways to produce continuous-tone images such as photographs with any printing method, whether analog or digital: with halftone screening, contone imaging, or alternative screening (dithering). All three have roles in the digital printing process, and each printer manufacturer uses its own method and guards it closely. This is the real Secret Sauce of digital printing.
Halftone Screening
Since the late-19th century, continuous-tone (or “contone”) images have been rendered by the process of “halftoning.” Since smooth transitions of grays or colors are impossible to print with analog or even digital devices (remember, all computers and digital printers use binary information that is either on or off, one or zero), images that use halftoning have to be broken down into tiny little dots or spots (I use the two words interchangeably). The darker portions of the image have larger spots with less space between them; the lighter areas have smaller spots with more space to reveal the paper underneath.At the right viewing distance, our brains then merge all the spots together to give us the impression that what we’re seeing is one smooth image. (Hold the page with the apple farther and farther away from you to see.) It’s just a trick—an optical illusion.
By knowing all this you can affect the coarseness or smoothness of printed images in a number of ways. With digital printing, depending on the capabilities of the device and the software used to drive it, you can vary the number of spots, the size of the spot, the closeness of the spots to each other, and the arrangement of the individual color spots that make up the final image.
While old-school halftoning utilized the process of photographing images through glass or film screens (hence the terming “screening”), most of the halftones these days are made digitally. These amplitude-modulated (AM) screening halftones are created on digital devices that place dots that are either round, elliptical, or rectangular on a grid-like cell made up of little squares. Each halftone dot is actually made up of clusters of printer dots. The more printer dots in a cell, the bigger the halftone dot, and the darker that cell appears. Also, the more cell squares (the bigger the grid), the more shades of gray or color available.
For example, a two-by-two cell can yield five possible tones (the paper is one) as follows
1. no dots, all you see is the paper
2. one dot, 25% tone
3. two dots, 50% tone
4. three dots, 75% tone
5. four dots, 100% tone (solid, no paper showing)
Commercial digital printing systems, imagesetters, and some binary, digital desktop print- ers such as color and B&W lasers use digital halftoning as part or all of their image-ren- dering methods.
Contone Imaging
Digital continuous-tone or contone imaging, most clearly seen in digital photoprinting and dye sublimation devices, works differently. Image pixels are still involved, but instead of using halftoning as a middleman to break the various tones in an image apart, contone devices translate the pixel information directly through the printer to the paper. As the image is being rendered, the printer is, in essence, asking each image pixel, “which color and how much of it?” Therefore, the more pixels or the higher the bit depth, the better the image. Because the printed image is made up of overlapping dyes of each primary color with no spaces between them, the color transitions are very smooth and the resulting images are very photorealistic.
Alternative Screening (Dithering)
Certain branches of digital printing, specifically inkjet and electrophotography, now use a relatively new screening type:frequency modulated (FM) screening or stochastic screening to produce near- or at-continuous-tone images where the dots are smaller and more irregular than halftone dots. Perfectly shaped, regularly spaced halftone dots are replaced with more randomly shaped, irregularly placed ones. If you know what a commercial mezzotint screen looks like, you’re not too far off.
HP, for example, combines halftoning with what it calls PhotoREt Color Layering Technology on many of its desktop inkjets. PhotoREt layers the color dots on top of each other and dithers them with error diffusion, which is a common dithering method (others include ordered-matrix dithering and threshold dithering). Error diffusion means that the error in creating a specific color—say green, which has to be made up of the only colors the printer has available, primarily for green: yellow and cyan—is spread to the adjacent dots. If one is too green, the next one over is made to be less green. And so on. If you stand back and look at the print, it all balances out, and what you see is “green.” (Note that there is no green ink in 99.9 percent of all inkjet printers; Canon’s i9900 is the lone desktop exception at the time of this writing. All the green—or any of the other colors of the rainbow—must come from a visual blending of primary colors that the printers do have.)
Epson employs its own proprietary algorithms (an algorithm is the mathematical set of instructions the printer software uses to control and precisely place the ink droplets) for what it calls AcuPhoto Halftoning, actually a type of error-diffusion-type dithering.
Canon uses what it calls Precision Color Distribution Technology for its dot layering technique to ensure uniform color. Moving away from inkjets, the Xerox Phaser 7750 color laser printer uses a combination of digital halftoning and a special dithering pattern to render the image.
Why is all this talk about dithers and halftones important? Because the type of screen rendering will partially determine the “look” of an image when printed using that particular screening or halftoning technology. This is a big part of what makes up a print’s “digital signature.”
When you get experienced enough, you will be able to spot the differences between the specific types of digital output. And you can make your purchase or service choices accordingly.
The bottom line is that when you’re at the upper end of digital printing quality, including inkjet, you’ve pretty much entered the world of continuous-tone imaging. The dots touch with no space between them, and the four or six (or more) colors are layered next to or on top of each other to blend together and form a smooth image. The dividing line between continuous-tone and screened images, at least with high-quality, 8-bit digital printing, is disappearing.
The ability of the human eye to distinguish fine detail is called visual acuity, and it is directly related to distance. As you move farther away from the visual source, you reach a point where you no longer see the detail, and everything merges together. This can be determined scientifically by using alternating black-and-white lines of a specified width and then measuring the angle made from the eye to these lines at the maximum resolvable distance. It has been shown that the visual acuity of a normal eye with 20/20 vision is somewhere between 30 seconds of arc (when lighting is “ideal”) and one minute of arc (when the lighting is “ordinary”). This is the maximum visual resolution possible for most humans.
From this information, all kinds of interesting formulas [c = 2 × d × tan(RADIAN ANGLE SYMBOL ÷ 2)] and conclusions can be drawn (see Table 2.2). One is that at any given viewing distance, you gain nothing by having higher resolution than the maximum resolving power of the eye because no finer details can be perceived. This is the upper limit, so there’s no point going beyond that.
However, things are not so simple. These resolving power charts are based on high contrasting, black-and-white lines or letters (see illustration above and think of the chart at your eye doctor’s office). The images that most of us print are anything but that. We have complex patterns of dots or device pixels, overlapping dots, and all the rest. So how does Table 2.2’s “details per inch” relate to the dots per inch of inkjet printing? It is generally believed that printer resolution (dpi) must exceed maximum visual resolution (“depi”) by a significant amount, on the order of double, triple, or more.
Plus, as digital imaging writer and publisher Wayne Cosshall explains it, there are other issues like presentation. If you print on fine art or textured paper, you could get away with a lower resolution because the paper’s texture will create its own detail and somewhat fool the eye. Also, if you frame a print behind glass that lowers the contrast of the print a little, so again, you can get away with less print resolution.
The formula numbers give you a place to start, but your own experience and your own style of printing and displaying will determine which printer resolutions will work best for you.
This entire concept of viewing distance and the eye’s maximum resolving power was brought home to me in dramatic fashion when I visited well-known documentary and fine-art photographer Joel Meyerowitz at his studio in New York City. Meyerowitz had just started experimenting with in-house inkjet printing, and he wanted to see how it compared to traditional C-prints, which he was used to getting from the top photo labs in New York.
He and I both analyzed two 11 × 14-inch prints made of the same image he had photographed in Tuscany (see Figure 2.10). Using a loupe (magnifier), I could see the difference between them.
The cloudy smoothness of the C-print and the discrete dots of the HP Designjet 130 print. At first I was discouraged, but then Meyerowitz had me put the loupe away and view both prints from a normal viewing distance. Voilá! The inkjet print was beautiful and actually superior. The colors were better differentiated and richer, and there was an overall sharpness that surpassed the traditional lab print. “The inkjet print is more alive,” Meyerowitz enthused. “It’s just plain better, and I’ve been looking at color prints for more than 30 years.”
The theory worked: When viewed at a normal distance, the inkjet dots had merged into one continuous-tone image.
Another aspect of printer resolution commonly overlooked is the relationship between viewing distance and visual acuity.
Viewing Distance: It matters how close you or your viewers are to your prints. Consider the ubiquitous billboard that could be printed at a high resolution but never is. If you’ve ever seen a billboard up close, you know that the dots are huge. Yet, billboards are perfectly readable at the distance from which they are meant to be viewed—across the street or driving down the road.
The key point here is that you don’t need more printer resolution than you need. Normal people will stand back to view a large image, and they will get up close to a small one. This means that larger or fewer dots are more acceptable on big prints destined to be viewed from further back.
If you’re wondering how to estimate standard viewing distances, photographer Joe Butts gives this formula: 1.5 × the diagonal dimension of the art piece. To calculate the diagonal, it’s a + b =c . For example, to figure the viewing distance for an 8 × 10 print: 8 squared plus 10 squared is 64 plus 100 equals 164 inches. The square root of 164 is 12.806 or rounding it off, 12.8 inches. Multiplying by 1.5, the viewing distance would be 19.2 inches (see Table 2.2). Similarly, the normal viewing distance for a large 40 × 60-inch print is
about 9 feet. You won’t see many dots from there!
Viewing distance, however, is only one-half the story.
Any type or text that’s part of a bitmapped image is no different than the rest of that image, and it will print with the same resolution of the image file. (Note: while Photoshop versions 6 and later support clean, vector type, you can’t print it that way without first going through a PostScript printer or interpreter, or a file conversion to PDF format and printing from Adobe Acrobat. Although other factors such as paper surface quality and the kind of printing technology used can definitely have an impact, it’s the printer’s resolution—addressable, not apparent—that mainly determines the quality of the printed, bitmapped type. A high dpi (dots per inch) will generally yield higher quality type with smoother edges while a low dpi produces type with ragged edges (see Figure 2.9).
If you’re printing from a drawing or page-layout program, the rules change somewhat. Adobe Illustrator and InDesign (version 1.5 and later) don’t require a freestanding PostScript interpreter for good-looking type. Other programs like Quark XPress need PostScript font support from a utility program like Adobe Type Manager (ATM) if your operating system doesn’t already have PostScript font support built in. In any of these cases, if you’re printing through an inkjet’s native printer driver, the type quality will still vary with the resolution of the printer. However, as soon as you bring in a PostScript interpreter, things improve significantly.
This seems to be the single most confusing word in all of the digital imaging world. And it doesn’t help that there are different terms and definitions for camera resolution, scanner resolution, monitor resolution, file resolution, and printer resolution. Since this is a book about printing, let’s concentrate on the last two:file and printer resolution .
File or Image Resolution
In basic terms, the resolution of a digital, bitmapped image is determined by how many pixels there are. This is called spatial resolution. If you have a scanned image and can count 100 pixels across (or down) one inch of the image (remember, bitmapped images actually have no physical size until they are rendered into a tangible form; at that point, you can measure them), then the resolution is 100 pixels per inch or 100 ppi. Technically, it’s pixels per inch (ppi) when you’re talking about image files, monitors, and cameras. But it’s dots per inch (dpi) when it comes out of a printer because, if it’s an inkjet, the printer’s software translates the pixels into tiny little marks or dots on the paper (see “Dots, Drops & Spots” box).
An image’s resolution will, in part, determine its quality or the degree of detail and definition. The more pixels you have in a certain amount of space, the smaller the pixels, and the higher the quality of the image. The same image with a resolution of 300 ppi looks much different—and better—than one of 50 ppi at the same relative output size.
However, there’s a downside to more pixels. The higher the ppi and/or the greater the bit depth, the more space the files take up, the slower they are to edit and work with, and the harder they are to print since extra pixels are simply discarded by the printer or can cause it to choke, stall, or even crash. The goal is to have a file that’s just big enough for the job, but not so big that it causes extra headaches.
So what is the best file or image resolution for digital printing? There is no standard rule of thumb for all digital devices as there is with commercial offset lithography. There, it’s well accepted that the ppi-to-lpi ratio (lpi is the “screen frequency”), which is also called the “halftone factor,” should be somewhere between 1.5 and 2.0. In other words, if you have an image that will be printed as a poster by a commercial print shop, the normal screen frequency would be 150 lpi. Multiply that by 1.5, and you get 225 ppi. Substitute 2.0, and you get 300 ppi. So your best image resolution in this example of commercial offset printing is usually between 225–300 ppi at final print size.
However, with most high-quality digital processes, there is no “lpi” in the same sense as with offset. In the early days of inkjets, some people used the 1/3 Rule: Take the highest resolution of the printer and divide by 3. For example, an older Epson inkjet printer with a 720 maximum resolution would require a 240 ppi file for optimal results (the “Magic Resolution Number”). But then Epson printhead-based printers started coming out with 1440, then 2880, and now 5760 resolutions. One-third of 5760 is 1920 ppi, an absurdly high and unnecessary image resolution. Some photographers and artists still swear by the 240-ppi formula for even the latest models of desktop printers, claiming, correctly, that, for desktop Epsons, the “native driver resolution” is still 720, so the 1/3 Rule remains in effect. (According to Epson data, the “input resolution”—the resolution that data is rasterized at—is 720 “dpi” for desktops and 360 “dpi” for wide formats.) However, Epson now recommends 300–360 ppi at the size you intend to print as their current Magic Number; if you get below 240 you may start to see a difference in image quality, and conversely, you won’t see any improvement with bitmapped images by going over 360 ppi.
Hewlett-Packard (HP) has an “internal render resolution” of either 600 dpi or 1200 dpi, depending on the quality setting, and they recommend 150–200 ppi (or even up to 300 dpi) at final size for their inkjet printers. (HP likes to call it “pixels per printed inch” or PPPI.) They claim that scientists doing satellite photo reproduction for the government on their printers typically find that 125 ppi is adequate. In my own experience, 200 ppi is a good image resolution target for most HP inkjet printers.
Canon, also with a native printhead resolution of 600 dpi on many of its inkjets, says that an image must be greater than 180 ppi “to avoid pixelation that shows as staggering in contrast points.” They go on to recommend 200 ppi as the target with 300 ppi as the maximum needed for their inkjets.
For continuous-tone printers that don’t use halftoning or dithering (explained below), try to have your image resolution match the printer resolution. Most dye sublimation printers are around 300 dpi, so make your final image also 300 ppi.
Brand Manufacturer’s PPI Recommendation at Final Size
Canon 200–300 ppi
Epson 300–360 ppi
HP 150–200–300 ppi
Same for LightJets and Lambdas, which are, respectively, 300 dpi and 400 dpi at their maximum settings; an image resolution of 300 ppi should work well for them, too.
Chances are that if you are anywhere between 240 to 360 ppi in terms of image resolution at final print size, you’re going to be fine with most digital print devices, although the best answer is to either test several resolutions with the intended output device and evaluate the resulting prints, or ask a printmaker for recommendations if you’re using an outside printing service.
Measuring Image Resolution
Here are the most common measurement methods:
¦ By pixel array or dimension: Some people just say, “Here’s a 1600×1200 image” (pixels is understood). Once you’re familiar with certain files sizes, you’ll automatically know what a 1600 × 1200-pixel image (or any other size) will do.
¦ By total number of pixels: Multiply the number of horizontal pixels by the vertical ones, and you’ve got the total number of pixels or the pixel dimensions. A 1600×1200 image totals out at 1,920,000 pixels or about 2 megapixels.
¦ By pixels per inch and image
Pixel dimensions are one method of size: As long as you know both measuring image resolution.
uncompressed, 24-bit, RGB, color 300-ppi image set to an output size of 4 × 5 inches is just over a 5-megabyte (MB) file.
¦ By file size: Take the total number of pixels (pixel dimensions), multiply that by 3 (total RGB color bit depth—24 divided by 8), and you’ve got the file size in bytes (one byte is eight bits).
Divide that by one million, and you have the approximate final file size in megabytes. Example: 1600×1200 pixels = 1,920,000 pixels. 1,920,000×3 = 5,760,000 bytes or 5.76 MB. Pretty close.
¦ By single-side measure: Film-recorder users typically refer to the width of the image in pixels. A standard “4K file” is one that measures 4,096 pixels horizontally (as already stated, the reason it’s not 4,000 pixels is because of the way the binary system works). Because most film recorder output ends up as standard 35mm transparency film, the other dimension (2,730 pixels) is understood to be in the correct proportion to the first and isn’t mentioned.
Printer Resolution
Pull on your tall boots because we’re now going to be wading in deep!
How capable is the printing device of reproducing the information in an image? You may have the highest-resolution image imaginable, but if the printer isn’t able to output all the fine details you’ve worked so hard on, you’ve wasted your time. There are two main types of printer resolutions to be concerned about: addressable and apparent.
Addressable Resolution
Digital printers have to translate all those nebulous image pixels we learned about into real dots of ink or spots of dyes. The number of different positions on the paper where the printer is able to place the little dots per unit area is its addressable resolution. Think of it
Commercial LPI vs. DPI
Spatial resolution is a measure of how finely the image information is grouped to be reproduced or rendered by the output device. With the digital imagesetters used in commercial printing, this is where the line screen (or screen frequency) comes into play.
Using the typical 150 lines per inch (lpi) as the assumption, the printing dots are arranged in rows that are placed 1/150” apart. The spatial resolution is then 150 lpi. Now output the same image at 85 lpi, and you’ve lowered the spatial resolution (and reduced the detail of the image).
How does lines-per-inch (lpi) relate to dots-per-inch (dpi)? A 150 lines-per-inch image will probably be output on a commercial imagesetter at
2,400 dots per inch. The addressable resolution of this device is, then, 2400 dpi; the spatial resolution is 150 lpi. The 2400 dots are used to print the 150 lines.
Clear as mud, right?
10 lines per inch 150 lines per inch
As each dot or spot having its own address on the paper, and all this is measured in dots per inch (dpi). (Imaging scientists actually have other ways of talking about resolution, too, but I’ll leave the arcane terms and definitions to them.)
Do you know the story of the blind men and the elephant? Six blind men encountered an elephant for the first time. Each touched a separate part of the beast and was then asked to describe the whole animal. They did so but in very different ways. The elephant was either like a snake, a wall, a spear, a fan, a tree, or a rope depending on which blind man spoke.
And so it is with “addressability” and dots per inch. Those numbers you see listed on every print device’s spec sheet and in every advertisement only give you part of the picture. And each print-device manufacturer talks about it differently.
Take inkjet printers. The Epson Stylus Pro 4000 printer’s maximum resolution is listed as 2880 × 1440 dpi (Note: virtually all digital-printing devices have multiple modes that allow for more than one resolution setting; naturally, only the maximum is advertised.
The smaller the resolution numbers, the faster the printing, but the lower the image quality). The maximum resolution on the HP Designjet 130 is 2400 × 1200 dpi. For the Canon i9900, it’s 4800 × 2400 dpi.
So what do these numbers mean? The 2880 (or 2400 or 4800) refers to the horizontal axis and is the maximum number of dots the printer can cram into one inch across the paper, or in the direction of the printhead’s travel (see Figure 2.7). The other number (720, 1200, or 1440) is the maximum number of dots the printer can place in one inch down the paper (in the direction of the paper feed).Keep in mind that these are not separate little dots standing all alone; they are frequently overlapping or overprinting on top of each other.
Why are the horizontal numbers usually higher? Because it’s a lot easier to position the printhead precisely than it is to position the paper precisely. As software developer Robert Krawitz explains it, “The printhead typically doesn’t actually lay down a dot every 1/2880th of an inch in one horizontal pass. What happens is that different nozzles on the printhead pass over the same line or row to fill it in. It might require up to eight passes to print all of the intermediate dot positions and complete the row. This interleaving of dots is sometimes referred to [in the case of Epson] as ‘weaving.’“
Offsetting or “weaving” is one factor affecting an inkjet printer’s addressable resolution. (Note: the dot sizes and positions are representative only; actual printing dots are more variable.)
The idea is the same for the other inkjet brands, although each has its own way to arrive at the maximum resolution numbers. HPs do things like “color layering” to change both horizontal and vertical resolutions. Canons combine “dot layering” with other factors including small ink droplets, small nozzle structure, and a small nozzle pitch (the distance between nozzles on the printhead) to reach high dpi numbers.
What does all this mean? Honestly, not that much. Is 2880 × 1400 really 36 percent higher—if you simply multiply the two numbers together—than 2400 × 1200 dpi resolution? I’ve seen outputs from many printers with these stated maximum resolutions, and I would be hard-pressed to say one is that much better than the other.
The theory is that higher printer resolutions produce finer details and smoother tonal gradations. This is true up to a point, but you eventually reach a position of diminishing returns. The negatives of high dpi—slower printing speeds and increased ink usage eventually outweigh the positives, especially if you can’t really see the differences. (For more about this, see “Viewing Distance & Visual Acuity” below.)
When it comes right down to it, the dpi resolution numbers on a spec sheet are irrelevant.
They only tell a very small part of the story, just like the blind men’s elephant. There are many factors that go into what really counts—the image quality a particular printing device is capable of producing. Factors like printer resolution, the number of ink colors, the size of the ink droplets, the precise positioning of the dots, how the inkjet nozzles are arranged and fire, the order of the colors, the direction of printing, and the screening or dithering pattern of the image pixels—they all come into play. My advice: Don’t put too much stock in the dpi numbers alone, and don’t use them to compare printers of different types or brands. Instead, use dots-per-inch resolution only to weigh different models of the same brand. Then, at least you’re talking the same language.
If all this talk of dots, drops, and spots is making your head hurt, it’s time to sort all this out. I asked inkjet expert Dr. Ray Work, an internationally recognized authority on the subject, to help me clarify the differences from an inkjet printing point of view.
Dots: A dot is the mark on the paper or other inkjet receptive material resulting from the printing of one or more drops of ink. It is the smallest component of an inkjet-printed image.
Drops: A drop (or droplet) is that small amount of ink that’s ejected from the orifice in the inkjet print head that lands on the paper and forms a mark or dot.
Spots: With printing, a spot is the same as a dot.
When inkjet printers translate pixels into printed dots, it’s not a 1:1 conversion. Each pixel typically requires lots of dots depending on its color and value.
In addition, inkjet printers can place multiple drops per dot. Some HP printers can generate up to 32 ink drops for every dot yielding over 1.2 million colors per dot.
And there’s more. Inkjet printers can eject drops from their printheads one at a time and place them at different positions on the paper or on the same position. They can eject one or more drops on the same position to form one dot. They can eject drops of different sizes, which results in different size dots. They can eject bursts of drops that combine in flight prior to landing on the paper to form a single dot.
All of these amazing options are in play with the inkjet printers on the market today. (Learn more about inkjet printers in the “Comparing Digital Printing Technologies” section.)
Apparent Resolution
Continuous-tone printers such as digital photo printers and dye sublimation devices (explained in the “Comparing Digital Printing Technologies” section) are unique in that their spatial and addressable resolutions are the same. That is, each image pixel ends up being a “device pixel” at the printer end. There is no halftoning, dithering, or screening involved; the full pixel information in terms of color and tone/value is output directly to paper. Contone printers are playing a different game on the digital ball field.
Since these types of printers can only list relatively lowly 200 ppi, 300 ppi, or at the most, 400 ppi as their addressable resolutions, the manufacturers have come up with a marketing term—”apparent resolution”—to put them on equal footing with all the inkjets that are claiming much higher numbers.
Using the Océ LightJet 430 photo laser printer as an example, here’s how it works. The LightJet accepts 24-bit, RGB color data. We know that each color is 8-bit, which represents 256 possible values per pixel. The equivalent commercial halftone printing device would need a 16 × 16 cell to equal that same 256 levels (16 × 16=256). (If you don’t know what a halftone cell is, don’t worry; you’ll learn about it soon. Just stick with me for now.) So if you take 300 ppi (one of the LightJet’s two resolution settings) and multiply that by 16 (16 cell units per pixel), you get 4,800. That’s 4,800 “dots per inch of apparent resolution.” They’re not really dots in the same way that inkjets have dots, but that’s what the makers of these devices have come up with as a way to do battle with the army of inkjet printers covering the land. Unfortunately these “virtual dots” are of no use in forming sharp-edged vector elements, so dye subs and photo printers are at a disadvantage in printing fine text.
Some inkjets themselves have used “apparent resolution” to compete in the marketplace.
The now-discontinued-but-still-in-use, drum-based, wide-format inkjet printers IRIS and
ColorSpan’s Giclée PrintMakerFA have addressable resolutions of 300 dpi (the IRIS was replaced by the IXIA, which is still being sold). However, they both claim 1800–2000 dpi “apparent resolution,” based on either variable-drop technology, the ability to layer color
dots, or additional ink colors, or all three.
Pixels are the basic elements that make up a bitmap image. Pixels actually have no shape or form until they are viewed, printed, or otherwise “rendered.” Instead, they are little points that contain information in the form of binary digits or “bits” (ones and zeros—a “0” represents something, a “1” represents nothing or empty space). Bits are the smallest unit of digital information.
A 1-bit image is the lowliest of all bitmaps. There are only two digits to work with—a 1 and a 0, which means that each picture element is either on or off, black or white (I’m keeping this to a simple one-color example to start with). But a 2-bit image is much more detailed. Now you have four possibilities or values for each pixel: 00, 01, 10, 11 (black, white, and two shades of gray). Keep going, and you see that three bits yields eight values, four bits 16, eight bits 256, and so on (see Figure 2.3). In mathematical terms, this is called the power of two: 22 equals four choices (2 × 2), 28 is 256 choices (2 × 2 × 2 × 2 × 2 × 2 × 2 × 2). Generally speaking, a one-color digital image needs to be at least 8-bit (256 tones)
to be “photorealistic” or “continuous-tone” in appearance.
Digital Equivalents
8 bits=1 byte
1024* bytes=1 kilobyte (KB)
1024 kilobytes=1 megabyte (MB)
1024 megabytes=1 gigabyte (GB)
1024 gigabytes=1 terabyte (TB)
*it’s 1024 and not 1000 because of the way the binary system works with its powers of two—in this case, 2 .
So far, we’ve only talked about bits in terms of black, white, or gray. Since most people work in color, you now have to apply the same thinking to each color component of the image. So, in a 24-bit (8 bits per color) RGB image, there are 256 possible values of Red,256 of Green, and 256 of Blue, for a grand total of—are you ready?—16,777,216 possible values, tones, or colors for each pixel (see Figure 2.4). A CMYK color image is described as 32-bit, or one 8-bit channel for each of the four printing colors: cyan, magenta, yellow, and black or “K.” There is no more color information with CMYK; it’s just allocated differently than RGB.
Whether an image has one, two, four, eight, or even more bits of information per pixel per
color determines its bit depth. The higher the bit depth, the more detailed and realistic the image. (You don’t have to stop at 8 bits. Current input technology allows for up to 16 bits of information per channel—see Chapter 3 for the pluses and minuses of going “high-bit”.)
First things first. Ninety-five percent of all the images that photographers and artists end up printing digitally are binary images, also called raster images, also called pixel-based images, also called bitmaps. Confused yet? The term bitmap itself sends some people running for shelter. One reason is because Adobe Photoshop, considered the top image- editing software program, has a mode option called “bitmap” that converts an image into the crudest (1-bit per pixel) form. That’s unfortunate because there’s a lot more to bitmaps than that. In fact, bitmaps are the key to the Chamber of Secrets of digital printing.
To put it simply, a bitmapped image is a collection of pixels (picture elements) arranged on a rectangular grid (it’s a map of a bunch of bits). Each pixel can be described or “quantized” in terms of its color and its intensity or value. The more pixels there are and/or the more the depth of information per pixel, the more binary digits (the little ones and zeros that the computer understands) there are, and the more detailed the image (see “Pixels and Bit Depth” for more about this).
That other five percent of digitally printed images are called vector-based or object-oriented. Instead of a bunch of pixels arranged on a grid, vector graphics are made up of mathematical formulas that describe each object in an image in terms of its outline shape, line weight, fill, and exactly where it is on the page. Logos, type, and any hard-edged, flat-colored art are perfect for the vector format. And that’s why vector art often comes from drawing programs like Adobe Illustrator, Macromedia Freehand, or CorelDRAW. To further complicate matters, a bitmap image can be placed within a vector file, and inversely, some bitmap files contain certain vector information.
The problem with vector art is that since it doesn’t actually exist except as a formula, there needs to be a way to interpret it and bring it down to earth and onto the printed page. And the primary way to do that is through the computer language of Adobe PostScript, which complicates the digital printing process . Alternatively, you can convert the vector graphic into a bitmap through the process of “rasterizing,” and you’re back in bitmap business. (A “raster” is a grid-like organization of image elements.)
There are three things you need to know about bitmaps to fully understand the nuances of printing digital images: pixels and bit depth , resolution, and halftoning and dithering.
Let’s take them one at a time.