Getting to Know Your Pixels: Computers and Printers
Think about it. Without pixels, digital imaging would be impossible. We looked at camera pixels last week. This week, let's get to know the pixels on your computer and printer.
Pixels are everywhere. Indeed, we have so many that we have to count them by the millions. Yet in the mountains of megapixels, it's easy to lose sight that they're not all created the same, what with each one being so tiny. But if there were an imaginary microscope that could look within a pixel, the differences would become apparent. Since none of us likely have access to such a magical contraption, allow me to try shedding light on things this week through words.
Since I brought it up, quantifying the light shining on things is why those pixels exist in the first place. The function of a pixel is to translate real-world colors and shades into numeric values that computer hardware and software can work with. The only problem is that there are so many ways of accomplishing that goal. Sometimes, the variations stem from optimizations for different needs. The best compact formats aren't likely to be capable of the highest precision and accuracy, and vice versa. Engineers try to purpose build specifications to meet different demands. And yes, some profusion has resulted from good, old competition and market factors. People say it takes all kinds, and if pixels varieties are any indication, they're positively inevitable.
Most of the pixels you will encounter on your computer will be of the RGB variety, although there are other kinds. An RGB pixel uses three values to represent color, one channel value each for red, green, and blue. As with colored lights, combine the maximum values for all three, and you get blinded by bright white light. Turn them all entirely off and get left in the dark. Or explore the rainbow possible by mixing them in countless other combinations. Many image editing applications work in RGB because your computer monitor does. But commercial printers care more about how things will appear on the page where ink density matters and not light brightness. They prefer CMYK over RGB, a system that uses four channels for color to match the standard inks used in printing: cyan, magenta, yellow and black. There are other channel formats as well, but I'll leave them for perhaps another day.
Yet even limiting our discussions to RGB pixels, the plot thickens further. There's the matter of how many bits of information we should use to represent each of those color values, a measure known as "bit depth." It takes at least eight bits to render images well, but more bits will allow greater precision and range. There's that inescapable trade-off between space and quality again. Jumping to sixteen bits gives us even more room to work without problems.
But we're not done yet. No matter how many bits we choose, there are only so many number combinations possible. It makes sense to reserve most to shades that computer monitors can display. They're only machines, after all. Industry scientists agreed to a system known as sRGB, or "Standard" RGB, years ago. But high-end monitors these days have begun to exceed the range of sRGB. Something known as Adobe RGB covers a wider gamut of colors than sRGB, at the expense of making each one spread further apart. Consider the problem of arranging chairs in a room. You'll need more chairs to cover a larger room if you want to maintain the same separation for good coverage. For this reason, you should be using 16-bit color before making use of Adobe RGB. When we map RGB gamut into the spectrum possible in commercial printing, the use of Adobe RGB makes even more sense. Some printable colors lie outside of sRGB but more closely to Adobe. Indeed, that's why Adobe invented the darned thing. Thanks, Adobe. And most modern cameras can produce colors over an even wider gamut, pushing users to ever-wider systems such as ProPhoto RGB.
Photoshop gives you the choice of both bit depth and color space. If you're a Lightroom user, such matters are less relevant since they only happen when you finally export to something other than RAW. Consult your manual if your preference runs to something else for image editing, but make sure it can support at least 16-bit RGB or RAW workflow.
For web display, a format more compact than either would be more appropriate. Web standards grew out of the days when we all dialed up a thing called a modem that sent data as warbles and chirps over a phone line. Back then, we were lucky to share even text data online without exceeding the patience of the best of us. Pictures were a luxury and compressed 8-Bit jpegs were all that were practical for photos. To create a jpeg out of a standard RGB file, programmatically block it up into regions, squeeze out most of the variability within each, then stomp on the result with heavy-soled shoes for good measure. PNG images use a different compression format that is more forgiving and works just at least as well now that we've moved from dial-up to broadband.
When it comes time to print, it's no longer the technical limitations of computer monitors that matter but those of computer printers. I'm not going to get into CMYK commercial printing here, but home desktop inkjet printers clearly have their limits, too. It may surprise some, but the printers from Canon and Epson that you and I use are still RGB devices. Despite the array of colored inks they insist we keep feeding them, it's their internal drivers that make the final conversions to the amount of printing ink required. Since they produce color in such a contrary way than monitors do, the details of their limits differ significantly. Printers can't hit every color in Adobe RGB but cover vast expanses outside sRGB. Color management in your printer driver does its best to keep things looking right.
And so, we end where we began. Computer pixels vary so widely for many reasons beyond that of software makers having fun at our expense. Now, I'm not saying those geeks at Adobe don't enjoy a good laugh. But most of the seeming confusion of formats represents deliberate choices to optimize things for so many competing purposes. Perhaps one day, everyone will have enough bandwidth and storage space to allow a single specification to handle it all, but don't count on it. Our expectations and the capabilities of our equipment keep going up, too. Progress always involves compromise.