Paths and Printing

Which is faster — a square or a square?

For basic shapes, path-based artwork prints much faster than pixel-based ?artwork. For instance, a 5-x-5-inch path-based square prints in a fraction of the time that it takes to print the same pixel-based square. Why? The printer needs only the four corner locations for the path-based square. For the pixel-based square, the printer needs to see each and every pixel that’s on the way to the printer. (Imagine trying to count sand grains with tweezers, really fast.)

Printing paths: The evolution of B?zier curves

Printers path in much the same way that Illustrator does — using points ?connected by lines. Think of a Connect-the-Dot game you did as a kid. Instead of processing each pixel, the Illustrator print code says, “Connect this point to that point, that point to this one over here, and so on.” All it takes is another bit of code to say, “Now fill that shape with solid red” or some other color.

This approach works well for objects with flat sides, but curves are another matter entirely. You’d need to connect an infinite number of dots to make a perfect curve. Here’s where Illustrator gets really clever. Instead of using straight lines between points, PostScript uses curves between points: B?zier curves, named after their creator, world-famous mathematician Pierre B?zier (pronounced BEZ-zee-ay).

The idea behind B?zier curves is that you need no more than four points to define any curved line: One point to say where the path begins, one point to say where the path ends, and two control points in between. Where you put each control point (relative to the end points) determines how much the line curves — and in what direction — on its way to meet the end point.

Fortunately, Illustrator spares users the headache of having to work out the math; you have a little magnet-like handle on-screen (the B?zier control point) that changes the direction of the curve. Okay, actually using it may be far from intuitive, but it does give Illustrator the capability to generate complex shapes with curves.

What’s my vector, Victor?

You often hear the words vector graphics used to describe the kind of art ?created in Illustrator. In mathematics, a vector is a quantity, such as a force or velocity, having direction and magnitude and a line representing such a quantity drawn from its point of origin to its final position. In artists’ terms, a vector is a line of a specific size drawn in a specific place. Hmm, vector graphics are simply graphics made with lines! The basic building blocks of every Illustrator graphic are just a bunch of lines. Those lines may have other information attached to them (such as color and width), but no matter how fancy they get, underneath they’re still just lines.

So why not call such graphics line art? Unfortunately, that term is already taken. Line art is a specific type of traditional graphic. I could call such illustrations Illustrator illustrations, but that term is a little alliterative (is there an echo in here?) and not quite accurate. You can also create vector graphics in CorelDRAW and Macromedia FreeHand (though you’d be doing yourself a great disservice if you were to do so). So this book follows convention, using vector graphics to describe the kind of graphics you create in Illustrator. Pretty soon you’ll be doing the same!