You are teasing me, aren't you? It is absurd to think that you could actually print a file on paper with 2400 dpi since this is similar to the resolution slide film writers use when making digital slides for gigantic projection. I know you to be a savvy guy and not likely to be taken in by the phlogiston of numbers. Just in case, let me wise you up [and our colleagues who always ask this question in my seminars] as to exactly what is going on.
In this essay I will address two issues: mechanical printer resolution and why the darned inkjet printer has eight colors.
Resolution: When you look at 2400 or 4800 pixels or dots per inch, you are seeing the truth: the machine is capable of making that many dots per inch. But it doesn’t do them side-by-each for supreme sharpness and detail as you might wish, it overlaps them to create smoothness. If your inkjet printer lets you watch the process, you’ll see smooth, saturated color slowly emerge from numerous overlapping printhead passes. This is achieved in two ways: jogging the printhead and squirting a tiny ink droplet. Since the absolute limit of the printhead is 360 dots per inch, each pass of the printhead is jogged a fraction of a dot width prior to its return. This jogging is how they get their big number claim: one jog = 720, 2 jogs = 1440 3 jogs =2880 and 4 jogs = 5600ish. The overlapping dots plus their tiny 2 picolitre size make smooth color gradation. I use the Epson values because I use the Epson Sylus Pro 3800 in my work and it claims higher resolution than HP does.
Now: as to file size: that 360 dpi is actually all four process colors. Each color therefore is only sprayed out at 90 dpi. The rule of thumb in the digital printing business is to oversample by a factor of two, therefore we arrive at an optimal file size of 180dpi. This oversampling gives the rasterizing engine [the printer driver] that processes the digital file into actual printing instructions [when to squirt what color where during a given print head pass] choices in making a smooth surface or a sharp edge. The driver decides how to make the best print.
You can use a lower resolution than 180 dpi and still get reasonable results. Modern prnter drivers have “sharpening” routines contained in them. When a file lacking data arrives, it is processed to emphasize edges. This sharpening disguises the paucity of data in the file. It also disguises the staircasing of pixels that plagued the early days of printing-too-big. A 4800 dpi PDF is absurdly oversampled [but will have extremely sharp-edged text and vectors to the delight of architectural precision obsessives like myself], but specially-designed offset presses and film writers for printing plates can employ them. A good example of high resolution offset printing is the photographic magazine Linhof FotoTeknik that reveals amazing large format depth of color in detail of engraved gunstocks and woodgrain, and the Time-Life Library of Photography that has a special fine screen for printing monochrome images on clay coated paper.
More ink colors: The print head overlapping strategy was developed to increase smoothness in printing, NOT increase sharpness. In the old days, inkjet printers were really good at the darks but not so good fading to white. As the color density dropped below 20%, the dots got spread so far apart that you could see the individual spots of color. At around 10%, you could see an edge where the ink ended so that highlights looked artificially cut out. We all know that even though we work in the RGB color space in Archicad, that four process colors make up every printed image. Cyan, Magenta, Yellow and Knockout [black] are the CMYK color mode. Dots of these colors combine to form every color in the universe [or so they would like us to think], limited by the gamut of the colors. A few years ago an offset printing system called Hexachrome promised more vibrant greens with the addition of extra colors but it is now as popular as Home Betamax was and HD-DVD will soon be. If you have ever had a porcelain enamel sign made from a photo, you know that they have their own color separation scheme because frit behaves different than printing ink does.
Anyway, when diluted magenta and cyan ink got added to the inkjet realm, those bridging-to-white situations got smoother [altho is remains a good practice to place a little ink on all surfaces of a printed photo - white just looks like glare - but there is no such thing as white!!!!]. When they added the subtle photographic greys to the ink choices, we could then make rich blacks and monochrome/duotone photos that avoided the harshness of printer’s black.
Dwight Atkinson
