Halftones, Contones, and Dithers
From Online Printing
Half-tones, Connotes, and Dithers There are three common ways to produce continuous-tone images such as photographs with any printing method, whether analogue or digital: with half-tone screening, connote 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. Half-tone Screening Since the late-19th century, continuous-tone (or connote) images have been rendered by the process of half toning Since smooth transitions of frays or colours are impossible to print with analogue 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 half toning 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 (see 2.11).
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 colour spots that make up the final image. While old-school half toning utilized the process of photographing images through glass or film screens (hence the terming screening), most of the half-tones these days are made digitally. These amplitude-modulated (AM) screening half-tones are created on digital devices that place dots that are either round, elliptical, or rectangular on a gr d-like cell made up of little squares. Each half-tone dot is actually made up of clusters of printer dots. The more printer dots in a cell, the bigger the half-tone dot, and the darker that cell appears. Also, the more cell squares (the bigger the grid), the more shades of argy or colour available. For example, a two-by-two cell can yield five possible tones (the paper is one) as follows (see 2.12): 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) This is a simple example. Expand the cell to be, say, 16 squares across, and you now have a lot of possible tones that can be printed (see 2.13). 2.13 A 16×16 half-tone cell (centre with grid lines) with the half-tone dot growing from the centre out. 2.12 A 2×2 half-tone cell can produce five tones.
Commercial digital printing systems, image setters, and some binary, digital desktop printers such as colour and B&Wu lasers use digital half toning as part or all of their image-rendering methods. Connote Imaging Digital continuous-tone or connote imaging, most clearly seen in digital photo printing and dye sublimation devices, works differently. Image pixels are still involved, but instead of using half toning as a middleman to break the various tones in an image apart, connote 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 colour 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 colour with no spaces between them, the colour transitions are very smooth and the resulting images are very photo realistic (see 2.14). 2.14 Connote imaging, in this case with a Durst Lambda digital laser image, produces photo realistic images with overlapping dye colours Alternative Screening (Dithering) Certain branches of digital printing, specifically ink jet and electro photography, now use a relatively new screening type:frequency modulated (FM) screening or stochastic screening to produce nearer at-continuous-tone images where the dots are smaller and more irregular than half to né dots. Perfectly shaped, regularly spaced half-tone dots are replaced with more randomly shaped, irregularly placed ones. If you know what a commercial mezzo tint screen looks like, you’re not too far off (see 2.15).
This is where dithering comes in. In the 2.15 A simulation comparing dictionary, dithering means nervously half-tone screening (top) with frequency modulated screening (bottom), 30s excited or confused. Dithering is simply magnification. an alternative to half toning and is the Eyes Martin Allergens; panels process of breaking down a continuous Watcher Computer Technology tone image into a bunch of tiny, confused, excited little spots in a stochastic or random arrange cement Dithering, sometimes in combination with half toning, has been successfully implemented by ink jet and colour laser printers to output a full range of tones and image detail. HP, for example, combines half toning with what it calls Photorealism Colour Layering Technology on many of its desktop ink jets Photorealism layers the colour 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 colour say green, which has to be made up of the only colours 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 ink jet printers; Canon’s 900s is the lone desktop except at the time of this writing. All the green or any of the other colours of the rainbow must come from a visual blending of primary colours 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 Photocell Half toning, actually a type of error-diffusion-type dithering. Canon uses what it calls Precision Colour Distribution Technology for its dot layering technique to ensure uniform colour Moving away from ink jets, the Xerox Phaser 7750 colour laser printer uses a combination of digital half toning and a special dithering pattern to render the image (see 2.16). Why is all this talk about dithers and half-tones important? Because the type of screen rendering will partially determine the look of an image when printed using that particular screening or half toning technology. This is a big part of what makes up a prints 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 ink jet, 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) colours 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.
