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A Tutorial on Printing

James C. Owens

A. Analog and Binary PrintingPhotography is an analog or continous-tone process; the density (darkness)produced is a continuously increasing function of the amount of light falling on agiven area of the film or paper. This is ideal for accurately and smoothlyreproducing the wide range of light and shadow of pictorial images, but not forproducing the sharp edges of text because lens aberrations and light scatteringin the paper blur edges somewhat. Printing, on the other hand, is binary; theprocess can either deposit ink on the paper to give full darkness or deposit noink to leave the paper white. There are no intermediate shades, and theboundary between inked and non-inked regions is very narrow. For crisp,sharp, black text and line art, this is ideal, but if a pictorial image is to beprinted, a means for creating areas of intermediate density has to be found.

In 1875, Ives invented the needed process for printing pictorial images. Herealized that since the printing process gave only two states, inked and notinked, the only way to produce intermediate levels of gray was to modulate thefractional area of paper covered by ink. If the image were subdivided into smallregions by putting it in contact with a screen, each region small enough that itwas reasonably uniform and, more important, not readily visible to the eye, andif within each region he could deposit a spot of ink that covered enough of thearea to absorb just the right fraction of light, the printed image would look as ifit exactly reproduced the gray tones of the image. For example, if half the areaof a region is covered with ink, the region will reflect about half of the lightfalling on it; we call this a "50% gray" region. Ives called the process"halftoning" for this reason, because midtones were reproduced by "toning"(inking) about half of the area.

B. Important Concepts and DefinitionsIf the printing process is not intrinsically capable of reproducing all shades oflightness or color, the image must be subdivided into elementary areas, calledpicture elements or pixels, and the printing colorant deposited in such a waywithin each of them that its average reflectance approximately matches that ofthe original image. If the areas are small enough, the eye will not resolve themand the image will appear natural. No image detail smaller than the pixel can bereproduced.

The process of subdividing the image into pixels is called spatial sampling.The sampling frequency is expressed in pixels per inch (pixels/inch orpix/in); this is the correct specification for printer resolution.

Resolution, then, is a measure of the smallest object that can be replicated bythe printing process. For example, if we wished to print a pattern of alternatinglight and dark lines, each one of which is 1/100 inch wide, the printer must havea resolution of 1/100 inch, or 100 pixels/inch, to do so. A related concept, butnot identical, is addressability. It is common practice in low-resolutiondesktop printing to print in such a way that the smallest possible spots of inkoverlap significantly. We might, for example, print using ink spots 1/100 inchwide, but place them on centers only 1/200 inch apart so that they overlap by50%. Making addressability higher than resolution reduces artifacts such asstairstepping of diagonal lines, but it of course does not increase the visibility offine detail.

The process of controlling deposition to reproduce the apparent lightness of animage is called rendering. The form of rendering commonly used in binaryprinting is called halftoning, a process in which an appropriate fraction of eachpixel is covered with ink so that the average reflectivity of the pixel is correct.Halftone screen resolution is traditionally specified in lines per inch ratherthan pixels/inch because early screens were made by depositing parallel lineson a clear substrate. It was difficult to deposit two sets of crossed lines, so twoscreens, each having only lines, were crossed and the pair placed betweennegative and plate. For example, a "200-line screen" means that the halftoneresolution is 200 pixels/inch.

There is significant confusion and inconsistency of nomenclature in theliterature. We will always refer to the basic image elements, the smallestregions that actually represent image information, as pixels. If a halftone pixelconsists of a single continuous region of ink, whether it has been created by asingle droplet of one ink or a cluster of droplets of different colors of ink, we willcall it a dot. Dots, therefore, must appear in regular patterns having the samespacing as pixels. If the halftone pixel is built up from a regular array of smallerregions, each of which is independently covered with ink or not, we will call sucha region a subpixel. If a dispersed halftone pattern is used (as described in thetutorial section) without an obvious regular structure, we will call each areacovered with ink a spot. When appropriate for a particular printing process wewill use physical terms such as droplet to describe the smallest physicallyrealizable image element.

Analog or continuous-tone printing uses a process capable of generating afull range of levels of each color (typically 256) at any location. Binary orhalftone printing uses a process capable of only two levels, ink or no ink, andhalftoning must be used to reproduce pictorial images. Gray-scale printing

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refers to intermediate systems, those capable of generating a few differentlevels of gray with each spot, as for example by using several inks of differingpigment concentration, but still requiring some degree of spatial halftoning toreproduce the full range of image density and color.

Raster printing simply refers to the pattern in which pixels are laid downduring printing, along parallel scan lines running across the page. In most inkjet printers, the print head shuttles back and forth across the paper, advancingonly a fraction of the head width at each pass, so that the image is laid downalong a number of parallel scan lines in several overlapping passes, a processcalled shingling. This procedure is very important in giving time for the ink todry, in avoiding artifacts, and in giving an opportunity for inoperative nozzles tobe replaced by working ones. In optical printers, a laser beam is rapidlyscanned across the page while the page itself moves slowly forward. Theresulting pattern of pixel laydown along parallel lines is called a raster, afterthe Latin rastrum, or rake.

C. Digital HalftonesIn binary or gray-scale digital printing, which require halftoning, each pixel mustbe created by filling in only part of the pixel area. How is this done?

The halftone pixel is subdivided into an array of NxN subpixels, and it is thesubpixels, not the full pixels, that are individually colored. For example, mostgraphic arts scanners divide each pixel into a 12x12 array of subpixels. We canthen expose (and deposit ink from) 0, 1, 2, 3,...up to all 144 of them, giving 145different average reflectance levels that the pixel can have. The obviouslyunfortunate consequence of this is that the exposing spot must be very small,only 1/12 the width of the pixel, and hence the writer resolution must be high.Writing a 200-line screen image requires a printer resolution of 200x12 = 2400subpixels per inch! This is why digital platesetters and halftone proofers forgraphic arts always have resolutions between 1800 and 3000 subpixels per inch- it is needed to approximately match the tone reproduction capability of digitalsilver halide prints printed at 200 pixels/inch. Matching the quality of 500pixel/inch, 256-level (8 bits per color) photographic prints would require aresolution of 16 x 500 = 8000 subpixels/inch. Of course, such high-resolutionwriters can make superb text and graphics, much sharper than photographicsystems can.

A second disadvantage of binary printing processes is that much more dataneeds to be transferred to the printer. Instead of 8 bits per pixel per color, thegraphic arts system must send 1 bit per subpixel; to achieve the same 256levels of density requires 256 bits per pixel per color, a factor of 256/8 = 32times as many. This is why digital graphic arts printers usually have a high-speed RIP (raster image processor) built into the printer and why they usuallyuse several laser beams to write several subpixels simultaneously.

We can understand at this point why, if a digital printer can write even a fewdifferent levels of density rather than just the two (0 and 1) of a binary system,fewer subpixels are needed to generate the necessary number of pixeldensities, and this gives significant advantages in reducing the writer resolutionrequired and in reducing the data rate. This is the reason why multiple densityinks, or, better yet, multiple drop size capability is so important in inkjetprinting.

D. Color Reproduction and Color HalftoningThe eye can distinguish tens of thousands of shades of color. Fortunately, weneed not have that many shades of ink; it has been known for centuries thatgood color reproduction can be accomplished by using only three primarycolors. For "additive" systems such as television monitors, which produce thethree colors independently from triads of phosphors (which you can see if youlook at the screen with a magnifying glass), the primaries are red, green, andblue, referred to as the RGB set. If all three are present, white is seen. For"subtractive" systems such as photography and printing, where the colorantsare partially or fully overlapped, the three primaries are:

cyan (white minus red = blue plus green)

magenta (white minus green = blue plus red)

yellow (white minus blue = green plus red)

which are called the CMY set. If all three are present on a print at full coverage,black is seen (or neutral gray if coverage is less than 100%).

Note #1 on printing: Printers often refer to these colors as "blue, red, andyellow," but this is not correct; the first two of these actual colors are criticallydifferent from blue and red.

Note #2 on printing: Although three primaries are adequate to reproduceneutrals and a range of colors, printers usually add a fourth ink, black (denotedby K, making the four-color set CMYK), for three practical reasons:

First, black ink is cheaper than colored ink, so if a neutral color is required theywill use black ink rather than equal amounts of C, M, and Y.

Second, if a color is required which needs unequal amounts of C, M, and Y, theycan replace the common amount (which would give a neutral) with K and onlyuse the differential amounts of the two remaining colored inks. This is called"gray component replacement" or "undercolor removal" depending on how it isdone.

Third, for black text, using only one ink gives a sharper image becausemisregistration of the three colored inks is precluded.

We cannot reproduce the entire gamut of visible colors, such as the purespectral colors seen in a rainbow, for example, with any set of three realprimaries, but if the primaries are well chosen a reasonable range can beprinted using the four "process colors" CMYK. If a larger gamut or special colorsare required, printers add more inks and call the process "hi-fi color." Orangeand green are often added to extend the printable gamut in directions where itis limited, and also special highly saturated colors may be added for company

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logos and trademarks.

Ink Overlap and Layering EffectsIn photography, the three emulsion layers and hence the three colorantsformed during development are fully superimposed; light must pass through allthree layers to reach the eye. Hence the dyes must not only control the threeprimary colors independently, with as little overlap and interaction as possible,but the dyes must be transparent. If the colorant layers were heavilypigmented and therefore opaque because all the light not absorbed werescattered, no colors could be seen except that of the top layer.

The same problem of overlap exists in printing. It is desirable to use pigmentedinks for permanence, for their sharp-cutting spectral absorption, which givesgood color saturation, and for their high optical absorption and hence relativelylow cost compared with dyes. Unfortunately, they may scatter a significant partof the light.

Printers using conventional halftone patterns partially avoid the problem byrotating the four halftone screen patterns with respect to each other (angles of-15, 0, 30, and 45 degrees are a common choice) so that the centers of the C,M, Y, and K pixels do not coincide. The principal reason for screen rotation is toavoid the appearance of moire patterns, the coarse banding patterns thatappear if the halftone screens do not have exactly the same spacing. Animportant secondary reason is to avoid depositing all four inks on top of eachother. For low densities and hence low percentage coverage, the spots of eachcolor do not overlap but form circular rings called "rosettes." For high densities,of course, when the spots are large, they will overlap, and at maximum density,full coverage, the inks will overlap completely just as in photographic prints.Using heavily pigmented inks would give rise to a significant problem inproducing strongly saturated colors that require nearly full coverage by two ormore of the inks.

In practice, the black ink can be quite opaque, while the CMY inks must bereasonably transparent. They can be pigmented, but the pigment particles mustbe very small so that there is relatively little scattering of the light passingthrough each ink layer.

We conclude that it is generally desirable in systems using pigmented inks thatthe spots of each color be deposited so that they overlap as little as possible,although they must overlap at high densities. The details of ink depositionstrategy and the resultant appearances of color and visibility of spot patternsare complex and must be carefully worked out for each type of ink, substrate,and printing application.

E. Digital Marking Technologies for Desktop Printing1. ProcessesMany novel photochemical systems have been invented for copying or printingmonochrome or color images without wet chemical processing. Those havingthe highest quality for pictorial images have, not surprisingly, been based onsilver halide, such as laser and LED printing onto Polaroid instant film or 3M DrySilver material, and Fuji's Pictrostat and Pictrography systems. They were all,however, relatively slow and expensive. A number of photochemical systemsnot based on silver halide have also appeared; the most recent of these to bewidely known have been the Mead Cycolor and the Fuji Thermal Autochromesystems.

If we restrict ourselves to only those processes suitable for desktop digitalprinting at rates of about one page per minute or more, there are only three:

Electrophotography and closely related processes such as electrography,ionography, and magnetography, in which dry toners are deposited imagewiseonto paper and fused is the oldest and most widespread. Although multileveland even continuous-tone processes have been demonstrated, process controlproblems have limited almost all practical systems to binary operation. Someprinters have used suspensions of very fine toner particles in dielectric liquids togive high resolution and high density with thin layers of toner, such as theIndigo printer. Other related processes such as photoelectrophoretic migrationimaging and Elcography have been demonstrated. Overall, dry tonerconventional electrophotography has proven to be a versatile, reliable processcapable of very good binary imaging, both monochrome and color, for text. Itcan be quite good for pictorial images as well, but would not be credited withphotographic quality because its resolution is too low.

Thermal transfer printing, in which dye is heated and transferred to a receiversheet, is a true continuous-tone process capable of excellent pictorial images.There are two types of thermal transfer printers. In the first, a linear array ofresistors (similar to the writing bar in old fax machines) writes an entire rasterline of pixels at once as the donor and receiver are pulled over it. This is calledthe D2T2 (Dye Diffusion Thermal Transfer) process, introduced commerciallyabout ten years ago. The Kodak printers, in particular, have excellent controlsystems and give images that are as good as, and in some respects betterthan, conventional photographic prints. Text is good but not excellent becauseresolution is only 300 pixels/inch and there is some thermal spreading of thepixels. Unfortunately thermal transfer printers are relatively slow (about 90seconds per A4 print) since four passes over the bar are required to depositthree colors and a protective covering layer, and relatively expensive (over$5000).

The second type of thermal transfer printer is laser thermal printing; an arrayof high-power (about 1 Watt) diode laser beams are focused to very small spotsto transfer dye at high resolution for halftone proofing, as in the KODAKApproval System. In this process the dye is actually sublimed or ablated fromthe donor onto the receiver rather than merely softened and diffused into it.Hence we have the odd situation of a process capable of continuous-toneimaging being used to simulate a binary printing process.

Inkjet printing is the third technology, and one that has made remarkableprogress in the last fifteen years, building on the continuous-jet inventions ofHertz and Sweet, the drop-on-demand thermal "bubble jet" printers of Hewlett-Packard and Canon, and the piezoelectric heads of Epson and Tektronix. Less

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than ten years ago the expensive printers by Iris and Stork were showing thatinkjet could produce very high quality, but desktop printers were still primitive.The resolution of the inexpensive printers was below 200 pixels/inch and imagequality, even for text, was poor. Now, however, the best desktop inkjet printerssurpass electrophotography for pictorial image quality and are much lower inhardware cost. They lag significantly only in speed and in media cost. Thetechnology is continuing to develop so rapidly that comparisons of printers areof only transient value. Both improvements in existing head technologies andnew approaches to drop ejection, such as the methods suggested bySilverbrook, are under intense study.

2. Characteristic Problems of Low-Resolution Binary PrintingElectrophotographic and inkjet desktop printers share several characteristicsand problems because they are both primarily binary printers. Laserelectrophotographic printers generally produce binary toner spots of fixed size,and inkjet printers generally produce single-volume droplets and hence binaryink spots of fixed size. Such binary printers having 200 to 300 pixels/inch, whichwas the maximum resolution available before the HP of 1991, were widelyutilized as office printers but had problems with text and graphics and evenmore serious problems with images. Using these spots as subpixels meant thathalftone pixels were very large and visible. The resulting problems included:

Text sharpness

"Jaggies" in text and in fine lines at shallow angles to the raster

Highly visible halftone patterns

Poor color reproduction and color transitions

Banding, streaking, and nonuniformity

Because increasing the resolution of the marking engines appeared to be verydifficult, a great deal of work was done (and still is being done) in an effort touse image processing to reduce the visibility of these artifacts. One type ofwork addressed jaggies (reconstruction artifacts), the other addressedhalftoning.

JaggiesThe only real solution to jaggies, which are visible offsets when an edge crossesfrom one row of pixels to another, is higher resolution, but increasing thenumber of gray levels can help, and these efforts were called "antialiasingalgorithms" even though the problem was not aliasing but reconstruction. Thebest known of the solutions was Hewlett-Packard's original ResolutionEnhancement Technology (Ret), which a method of smoothing the jaggies oftext edges but recognizing that certain patterns of pixels generated by fontsoftware could be replaced by other patterns, individually designed, that lookedsmoother. More important, HP added the capability of reducing the laser powerin their printers to write a smaller spot which looked less dense. Although theactual addressability of their printers was not increased, the lighter spot couldbe used to make text and also near-horizontal and near-vertical lines looksmoother by partly filling in the most obvious pixel steps.

Halftoning AlgorithmsHundreds of papers have been written on the development of new digitalhalftoning methods having less visible patterns than conventional growing-dothalftones, which were quite unacceptable. An excellent review has been given inRobert Ulichney's book, Digital Halftoning (MIT Press, Cambridge, MA, 1987). Inall cases the idea was to distribute the subpixels so that the pixel area wascovered by a regular or random pattern of separated subpixels. Some of themost noteworthy approaches were:

Bayer patterns: minimizing power spectral density at low spatial frequencies

Fixed threshold scatter: specifying different but fixed subpixel patterns foreach step in filling the pixel

Stochastic scatter: subpixels are randomly located in different pixels havingthe same fill

FM scatter: subpixel groupings of random size as well as random location inthe pixel

Blue noise: adding high-frequency noise to the thresholds of fixed patterns

Error diffusion: using the error between the desired pixel density and thatachievable to modifythe number of subpixels filling in adjacent pixels

All of these helped, and all had their own characteristic problems and artifacts.Error diffusion is especially interesting because it has the effect of expandingthe size of pixels in uniform or nearly uniform areas in order to achieve finercontrol of density and color at the expense of resolution. In other words, areasof detail reproduce edges well but density and color are inaccurate; broad,uniform areas of little detail can show density, color, and gradients well.Unfortunately error diffusion had its own artifacts, visible wiggly lines ofsubpixels called "snakes," but further work combining some of the approacheslisted have helped. In the end, though, higher resolution and/or modulation ofsubpixels to give more gray levels are the only solutions to the problems.

A Note on Reverse Engineering: When error diffusion halftoning or one of thestochastic halftoning methods is used, the original image data is still specifiedby pixels per inch, but the rendered and printed image may not show theperiodicity of the pixels. Hence microscopic examination of a print may notallow determination of the actual pixel size.

F. Inkjet Printing1. Technologies and CompaniesMany approaches to inkjet printing have been explored, and a considerablenumber have been introduced as products. We summarize the most importantin the following table.

Continuous Jet

Sweet Method (deflected drops write)

Binary deflection: Scitex

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Multiple deflection: Videojet, Linx, Imaje

Hertz Method (undeflected drops write)

Iris, Stork

Drop on Demand

Thermal

Piezoelectric

Sqeeze: Siemens

Bend: Tektronix, Sharp, Epson

Push: Dataproducts, Epson

Shear: Spectra, Xaar, Nu-Kote, Brother, Topaz

Other

Electrostatic: IBM, ESI, Minolta, Kodak

Acoustic: Xerox

Surface Tension

2. Resolution and variable dot technologyAs we have discussed, for text and fine lines, resolution is very important. Avalue of 500 pixels/inch or more is generally considered to give very good text,although the conventional printing industry would more than double thatnumber. Variable dot technology to give at least a few levels of gray cansignificantly improve the appearance of low-resolution text by filling in jaggies.

Figure 1. A typical plot of perceived text quality versus number of graylevels per pixel

For photographic images, resolution is less important if the imaging process isanalog, but even more important than for text if the process is binary. As waspointed out in Section C of this tutorial, a resolution of over 2000 subpixels/inchis required for good photographic quality for a binary process. The dependenceof image quality on resolution and number of bits follows the same generalshape as in the plot above, but the requirements are higher. With 64 levels ofgray, a resolution of 500 pixels/inch is probably enough. Even a few levels ofgray per dot significantly reduces the subpixel resolution required.

SummaryWe have discussed several key issues in inkjet printer design:

Printhead technology

Rendering methods

Color reproduction

Ink/media interactions

All of them are currently under extensive study in many laboratories, anddevelopments are occurring very rapidly. Some of the interesting questionsinclude:

What is the maximum practical number of nozzles?

Could full-width heads be made, and would they be practical?

Could ink drying be fast enough?

Could nozzle reliability be high enough?

What is the minimum droplet size?

What is the maximum droplet rate?

What is the best method for obtaining variable dots?

Can full-color, full-size A4 pictorial images, each one different, possibly bemade at a rate of several per minute?

ReferencesPrint Unchained: Fifty Years of Digital Printing, 1950-2000 and Behond; EdwardWebster. Published by DRA of Vermont, Inc., West Dover, VT (available fromIS&T)

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