Image-Editing Theory

Like any image editor, Photoshop enables you to alter photographs and other scanned artwork. You can retouch an image, apply special effects, swap details between photos, introduce text and logos, adjust color balance, and even add color to a grayscale scan.

Photoshop also provides the tools you need to create images from scratch. These tools are fully compatible with pressure-sensitive tablets, so you can create naturalistic images that look for all the world like watercolors and oils.

Image editors fall into the larger software category of painting programs. In a painting program, you draw a line, and the application converts it to tiny square dots called pixels. The painting itself is called a bitmapped image, but bitmap and image are equally acceptable terms.

Photoshop uses the term bitmap exclusively to mean a black-and-white image, the logic being that each pixel conforms to one bit of data, 0 or 1 (off or on).

In order to avoid awkward syllabic mergers such as pix-map and because forcing a distinction between a painting with exactly two colors and one with anywhere from four to 16 million colors is entirely arbitrary I use the term bitmap more broadly to mean any image composed of a fixed number of pixels, regardless of the number of colors involved.

What about other graphics applications, such as Adobe Illustrator?

Illustrator, Macromedia FreeHand, CorelDraw, and others fall into a different category of software called drawing programs. Drawings comprise objects, which are independent, mathematically defined lines and shapes. For this reason, drawing programs are sometimes said to be object-oriented.

Some folks prefer the term vector-based, but I shy away from it because vector implies the physical components direction and magnitude, which generally are associated with straight lines.

Besides, my preference suggests an air of romance, as in, “One day, I’m going to shake off the dust of this three-horse town and pursue a life of romantic adventure in the Object Orient!”

Photoshop introduces object-oriented layers, which permit you to add highresolution text and shapes to your photographic images, all inside a single piece of artwork.

In that regard, the program has become a kind of painting and drawing hybrid. These features don’t altogether take the place of a drawing program, they merely help to make Photoshop that much more flexible and capable.

Painting programs and drawing programs each have their strengths and weaknesses. The strength of a painting program is that it offers an extremely straightforward approach to creating images. For example, although many of Photoshop’s features are complex exceedingly complex on occasion its core painting tools are as easy to use as a pencil.

You alternately draw and erase until you reach a desired effect, just as you’ve been doing since grade school. Of course, for all I know, you’ve been using computers since grade school. If you’re pushing 20, you probably managed to log in many happy hours on paint programs in your formative years.

Then again, if you’re under 20, you’re still in your formative years. Shucks, we’re all in our formative years. Wrinkles, expanding tummies, falling arches, longer nose hairs . . . if that’s not a new form, I don’t know what is. In addition to being simple to use, each of Photoshop’s core painting tools is fully customizable.

It’s as if you have access to an infinite variety of crayons, colored pencils, pastels, airbrushes, watercolors, and so on, all of which are entirely erasable. Doodling on the phone book was never so much fun.

The downside of a painting program is that it limits your resolution options. Because bitmaps contain a fixed number of pixels, the resolution of an image the number of pixels per inch is dependent upon the size at which the image is printed, as demonstrated in Figure below.

Print the image small, and the pixels become tiny, which increases resolution; print the image large, and the pixels grow, which decreases resolution. An image that fills up a low-resolution screen (640 × 480 pixels) prints with smooth color transitions when reduced to, say, the size of a business card.

But if you print that same image so it fills an 81⁄2-by-1inch page, you’ll probably be able to distinguish individual pixels, which means you can see jagged edges and blocky transitions. The only way to remedy this problem is to increase the number of pixels in the image, which increases the size of the file on disk.

Bear in mind that this is a very simplified explanation of how images work. For a more complete description that includes techniques for maximizing image performance.

Painting programs provide tools reminiscent of traditional art tools. A drawing program, on the other hand, features tools that have no real-world counterparts.

The process of drawing might more aptly be termed constructing, because you actually build lines and shapes point by point and stack them on top of each other to create a finished image. Each object is independently editable—one of the few structural advantages of an object-oriented approach—but you’re still faced with the task of building your artwork one chunk at a time.

Nevertheless, because a drawing program defines lines, shapes, and text as mathematical equations, these objects automatically conform to the full resolution of the output device, whether it’s a laser printer, imagesetter, or film recorder. The drawing program sends the math to the printer and the printer renders the math to paper or film.

In other words, the printer converts the drawing program’s equations to printer pixels. Your printer offers far more pixels than your screen—a 300-dots-per-inch (dpi) laser printer, for example, offers 300 pixels per inch (dots equal pixels), whereas most screens offer 72 pixels per inch.

So the printed drawing appears smooth and sharply focused regardless of the size at which you print it, as shown in Figure below.

Another advantage of drawings is that they take up relatively little room on disk. The file size of a drawing depends on the quantity and complexity of the objects the drawing contains. Thus, the file size has almost nothing to do with the size of the printed image, which is just the opposite of the way bitmapped images work.

A thumbnail drawing of a garden that contains hundreds of leaves and petals consumes several times more disk space than a poster-sized drawing that contains three rectangles.