Each pixel is independent. You might think that you see a car or a circle or a tree or Uncle Bob in an image, but the image is actually only a bunch of little colored squares. Although you can read about various ways to work with groups of pixels throughout this book, each pixel exists unto itself.

       Each pixel is square (except on TV). Really! Each pixel in a digital image is square except when you’re creating images for some television formats, which use nonsquare pixels. It’s important that you understand the squareness of pixels because you sometimes have to deal with those pointy little corners.

       Each pixel can be exactly one color. That color can change as you edit or alter the image, but each pixel consists entirely of a single color — there’s no such thing as a two-tone pixel. Figure 2-2, at 3,200 percent zoom, shows each pixel distinctly.FIGURE 2-2: Each pixel is monotone, containing a single color throughout the pixel.

       Smaller is better (generally speaking). The smaller each pixel, the better the detail in an image. (However, when you are preparing images for the web, you need smaller images that invariably have less detail.) If you capture an image of a house with an older cellphone camera and capture the same shot with a new DSLR (digital single-lens reflex camera — you know, one of the cameras with interchangeable lenses) that captures 3 or 7 or 15 times as many pixels — it’s pretty obvious which image has better detail. Take a look at Figure 2-3, which illustrates how lots more smaller pixels present a better image than do fewer-and-larger pixels.Smaller pixels also help hide those nasty corners of pixels that are sometimes visible along curves and diagonal lines. When the corners of pixels are noticeable and degrade the image, you call it a bad case of the jaggies.FIGURE 2-3: More pixels (top) means better detail. Note the zoom factors in the lower left of each window. Keep in mind that the size at which an image can be printed — and still look good — depends on the number of pixels available. Sure, these days every cellphone seems to capture at least 10 megapixels, which is fine for 8-x-10 prints and perhaps even as large as 16-x-20 inches. But how about when your 10-megapixel pocket camera doesn’t have a long enough zoom to capture little Tommy’s exploits on the far side of the soccer field? That’s when you might need to crop and resample the image to increase the number of pixels. I cover resampling later in this chapter.

       Pixels are aligned in a raster. The term raster appears regularly when you discuss images created from pixels. Raster, in this case, refers to the nice orderly rows and columns in which pixels appear. Each image has a certain number of rows of pixels, and each row is a certain number of pixels wide — the columns. Within the raster, the pixels perfectly align side to side and top to bottom.

       Every picture created with pixels is rectangular. Some images might appear to be round, or star-shaped, or missing a hole from the middle, but they aren’t unless you print them and grab your scissors. The image file itself is rectangular, even if it appears round. Pixels actually exist in those seemingly empty areas; the pixels are, however, transparent. When printing, the transparent areas show the color of the paper you’re using.

       Image resolution: Image resolution is the size of your image’s individual pixels when you print. I go into greater detail about this concept in the upcoming section, “Picking an image resolution.”

       Camera resolution: Digital cameras capture each image in a specific number of pixels. Check your camera’s user guide or open one of the images in Photoshop and choose Image ⇒ Image Size. Take a look at the number of pixels that your camera records for the width and for the height. Multiply the numbers together, divide by one million, and round off the result. (If you’re in the camera maker’s marketing department, make sure that you round up.) That’s the megapixel (MP) rating for the camera. Use it as a general guideline when shopping. But remember that a camera with lower resolution using an excellent lens generally produces a better print than a camera with more megapixels using a less expensive lens.

       Monitor resolution: Monitor resolution determines how many pixels are visible onscreen. Whether you use a Mac or a PC, you set the monitor resolution at the system level (as shown in Figure 2-4). When you use a higher monitor resolution, you get a larger workspace, but each pixel is smaller, which might make some jobs tougher. Experiment to find a monitor resolution that works just right for you.

       Printer resolution: Unlike the three preceding terms, printer resolution doesn’t involve pixels. Rather, a printer resolution tells you how many tiny droplets of ink are sprayed on the paper. Remember that it takes several droplets to reproduce a single image pixel — you certainly don’t need an image resolution anywhere close to the printer’s resolution! (See the following section for more on image resolution.)