Pixels and Resolution in Online Printing

From Online Printing

At its core, digital printing is simple. A binary data stream drives a print engine to render a digital image on an output device. End of story? Not quite. Like any production process, digital printing requires the right tools and the right information to make the right choices. Let’s begin at the beginning. A Digital Primer Photographers and artists are all, basically, image makers, so let’s start by looking inside a digital image. Anatomy of a Digital Image 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 Photo shop, 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 bit mapped image is a collection of pixels (picture elements) arranged on a rectangular grid (it’s a map of a bunch of bits); see 2.1. Each pixel can be described or quantized in terms of its colour 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-footed art are perfect for the vector format (see 2.2). And that’s why vector art often comes from drawing programs like Adobe Illustrator, Macro media Freehand, or Correlate 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 Post Script, which complicates the digital printing process (see more about Post Script in ). Alternatively, you can convert the vector graphic into a bitmap through the process of pasteurizing, 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 half toning and dithering. (Colour is another issue, but because it’s such a huge subject, it gets its own chapter .) Let’s take them one at a time. === Pixels and Bit Depth ===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-colour 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 argy). Keep going, and you see that three bits yields eight values, four bits 16, eight bits 256, and so on (see 2.3). In mathematical terms, this is called the power of two: equals four choices (2 × 2), 28 is 256 choices (2 × 2 × 2 × 2 × 2 × 2 × 2 × 2). Generally speaking, a one-colour digital image needs to be at least 8-bit (256 tones) to be photo realistic or continuous-tone in appearance. Study the eye image variations in 2.3, and you’ll see what I mean. Digital Equivalents 8 bits=1 byte 1024* bytes=1 kilobyte (KB) 1024 kilobytes=1 megabyte (MB) 1024 megabytes=1 gigabyte (GB) 1024 gigabytes=1 megabyte (TB) 10 *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 argy Since most people work in colour, you now have to apply the same thinking to each colour component of the image. So, in a 24-bit (8 bits per colour) 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 colours for each pixel (see 2.4). A CMOS colour image is described as 32-bit, or one 8-bit channel for each of the four printing colours: cyan, magenta, yellow, and black or K. There is no more colour information with CMOS; it’s just allocated differently t ham RGB. (For more about colour and colour models, see .) R G B Whether an image has one, two, four, eight, or even more bits of information per pixel per colour 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 for the pluses and minuses of going high-bit.) Resolution 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, bit mapped 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, bit mapped 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 pip Technically, it’s pixels per inch (pip) when you’re talking about image files, monitors, and cameras. But it’s dots per inch (dip) when it comes out of a printer because, if it’s an ink jet, the Printers software translates the pixels into tiny little s 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 pip looks much different and better than one of 50 pip at the same relative output size (see 2.5). However, there’s a downside to more pixels. The higher the pip 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 de vices as there is with commercial offset lithography. There, it’s well accepted that the pip-to-lip ratio (lip is the screen frequency), which is also called the half-tone 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 lip Multiply that by 1.5, and you get 225 pip Substitute 2.0, and you get 300 pip So your best image resolution in this example of commercial offset printing is usually between 225–300 pip at final print size. However, with most high-quality digital processes, there is no lip in the same sense as with offset. In the early days of ink jets, some people used the 1/3 Rule: Take the highest resolution of the printer and divide by 3. For example, an older Epson ink jet printer with a 720 maximum resolution would require a 240 pip file for optimal results (the Magic Resolution Number). But then Epson print head-based printers started coming out with 1440 , then 2880, and now 5760 resolutions. One-third of 5760 is 1920 pip, an absurdly high and unnecessary image resolution. Some photographers and artists still swear by the 240-pip formula for even the latest models of desktop printers, claiming, correctly, that, for desktop Stepsons, 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 cauterized at is 720 dip for desktops and 360 dip for wide formats.) However, Epson now recommends 300–360 pip 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 bit mapped images by going over 360 pip (Note: unlike bitmaps, vector art is resolution-independent, which means that you can blow it up or down without any loss of definition or clarity.) Hewlett-Packard (HP) has an internal render resolution of either 600 dip or 1200 dip, depending on the quality setting, and they recommend 150–200 pip (or even up to 300 pip) at final size for their ink jet printers. (HP likes to call it pixels per printed inch or PP PI) They claim that scientists doing satellite photo reproduction for the government on their printers typically find that 125 pip is adequate. In my own experience, 200 pip is a good image resolution target for most HP ink jet printers. Canon, also with a native print head resolution of 600 dip on many of its ink jets, says that an image must be greater than 180 pip to avoid pix elation that shows as staggering in contrast points. They go on to recommend 200 pip (see 2.1) as the target with 300 pip as the maximum needed for their ink jets (To see what print heads look like, go to the Ink jet section near the end of this chapter.) For continuous-tone printers that don’t use half toning or dithering (explained below), try to have your image resolution match the printer resolution. Most dye sublimation printers are around 300 dip, so make your final image also 300 pip Same for Light Jets and Lambdas, which are, respectively, 300 dip and 400 dip at their maximum settings; an image resolution of 300 pip should work well for them, too. Chances are that if you are anywhere between 240 to 360 pip 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 l 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 mega pixels By pixels per inch and image Pixel dimensions are one method of size: As long as you know both measuring image resolution. (See the intended output size and the more about sizing and scaling pip, you’re set. For example, an images in .) uncompressed, 24-bit, RGB, colour 300-pip image set to an out put size of 4 × 5 inches is just over a 5-megabyte (MB) file. By file size: Take the total bum brr of pixels (pixel dimensions), multiply that by 3 (total RGB colour 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 4 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. (For a much more complete look at determining the size, scale, and resolution of your digital files including the use of odd/even or interer resolution numbers, see .) 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 LIP v DIP Spatial resolution is a measure of how finely the image information is grouped to be reproduced or rendered by the output device. With the dig ta image setters used in commercial printing, this is where the line screen (or screen frequency) comes into play. Using the typical 150 lines per inch (lip) as the assumption, the printing dots are arranged in rows that are placed 1/150 apart. The spatial rose elution is then 150 lip Now output the same image at 85 lip, and you’ve lowered the spatial resolution (and reduced the detail of the image). See 2.6 for an exaggerated example. How does lines-per-inch (lip) relate to dots-per-inch (dip)? A 150 lines-per-inch image will probably be output on a commercial image setter at 2,400 dots per inch. The addressable resolution of this device is, then, 2400 dip; the spatial resolution is 150 lip The 2400 dots are used to print the 150 lines. Clear as mud, right? A each dot or spot having its own address on the paper, and all this is measured in dots per inch (dip). (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 sec sheet and in every advertisement only give you part of the picture. And each print-device manufacturer talks about it differently. Take ink jet printers. The Epson Stylus Pro 4000 printer’s maximum resolution is listed as 2880 × 1440 dip (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 Design jet 130 is 2400 × 1200 dip For the Canon 900s, it’s 4800 × 2400 dip 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 print head=s travel (see 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. 2.7 Ink jet printers have the higher-resolution numbers in the horizontal or print head-travel direction. Printer image Hewlett Packard Company Why are the horizontal numbers usually higher? Because it’s a lot easier to position the print head precisely than it is to position the paper precisely. As software developer Robert Krakatoa explains it, The print head 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 print head 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.’ (See 2.8.) The idea is the same for the other ink jet brands, although each has its own way to arrive at the maximum resolution numbers. HP do things like colour 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 print head) to reach high dip 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 dip 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 high r 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 dip 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 dip resolution numbers on a sec 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 colours, the size of the ink droplets, the precise positioning of the dots, how the ink jet nozzles are arranged and fire, the order of the colours, the direction of printing, and the screening or dithering pattern of the image pixels they all come into play. My advice: Don’t up t too much stock in the dip 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. Dots, Drops, & Spots If all this talk of dots, drops, and spots is making your head hurt, it’s time to sort all this out. I asked ink jet expert Dr. Ray Work, an internationally recognized authority on the subject, to help me clarify the differences from an ink jet printing point of view. Dots: A dot is the on the paper or other ink jet receptive material resulting from the printing of one or more drops of ink. It is the smallest component of an ink jet-printed image. Drops: A drop (or droplet) is that small amount of ink that’s ejected from the orifice in the ink jet print head that lands on the paper and forms a or dot. Spots: With printing, a spot is the same as a dot. When ink jet printers translate pixels into printed dots, I T=s not a 1:1 conversion. Each pixel typically requires lots of dots depending on its colour and value. In addition, ink jet 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 colours per dot. And there’s more. Ink jet printers can eject drops from their print heads 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 ink jet printers on the et today. (Learn more about ink jet printers in the Comparing Digital Printing Technologies section.) Dots produced by Epson 3.5 politer drops, some overlapping to give secondary colours Epson America, Inc. 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 half toning, dithering, or screening involved; the full pixel information in terms of colour and tone/value is output directly to paper. Connote printers are playing a different game on the digital ball field. Since these types of printers can only list relatively lowly 200 pip, 300 pip, or at the most, 400 pip as their addressable resolutions, the manufacturers have come up with a ting term apparent resole Toronto put them on equal footing with all the ink jets that are claiming much higher numbers. Using the Oct Light Jet 430 photo laser printer as an example, here’s how it works. The Light Jet accepts 24-bit, RGB colour data. We know that each colour is 8-bit, which represents 256 possible values per pixel. The equivalent commercial half-tone printing device would need a 16 × 16 cell to equal that same 256 levels (16 × 16=256). (If you don’t know what a half-tone cell is, don’t worry; you’ll learn about it soon. Just stick with me for now.) So if you take 300 pip (one of the Light Jet=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 ink jets 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 ink jet printers covering the land. Unfortunately these virtual dots are of no use in forming sharp-edged vector element , so dye subs and photo printers are at a disadvantage in printing fine text. Some ink jets themselves have used apparent resolution to compete in the emplace The now-discontinued-but-still-in-use, drum-based, wide-format ink jet printers IRIS and Coloration=s Gigacycle Printmaker Fa have addressable resolutions of 300 dip (the IRIS was replaced by the ILIA, which is still being sold). However, they both claim 1800–2000 dip apparent resolution, based on either variable-drop technology, the ability to layer colour dots, or additional ink colours, or all three.