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Adobe Illustrator CS5 : Working with Paths (part 2) - Understanding control handles and control handle lines

10/22/2014 9:35:49 PM
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3. Understanding control handles and control handle lines

If an anchor point has a control handle coming out of it, the next segment is curved. No control handle, no curve. Couldn't be simpler.

As stated before, control handles are connected to anchor points with control handle lines. Figure 3 shows what happens when an anchor point with no control handle and an anchor point with a control handle are connected to another anchor point. Figure 4 shows the anchor points, control handles, and control handle lines on a path.

Figure 3. An anchor point without a control handle and an anchor point with a control handle are connected to new anchor points, resulting in a straight line segment and a curved line segment.

Figure 4. Anchor points, control handles, and control handle lines along a path

The basic concept to remember about control handles is that they act as magnets by pulling the curve toward them. This presents an interesting problem because each curved line segment usually has two control handles. Just as you might suspect, the control handle exerts the greatest amount of force on the half of the curved segment nearest to it. If there's only one control handle, the segment curves more on the side of the segment with the control handle than on the side with no control handle.

The greater the distance between a control handle and its corresponding anchor point, the farther the curve (on that end of the curve segment) pulls away from an imaginary straight segment between the two points. If the control handles on either end of the segment are on different sides of the curved segment, the curved segment takes on a reversed S shape, as shown in Figure 5. If the control handles on the ends of the curved segment are on the same side, the curve takes on a U shape.

Regardless of whether the anchor point is a smooth point, a curved corner point, or a combination corner point, control handle lines coming out of an anchor point are always tangent to the curved segment where it touches the anchor point. Tangent refers to the touching of the control handle line to the curved segment as it crosses the anchor point, as shown in Figure 6.

To adjust the curves without moving the control handles, click the curve and then drag it. Keep in mind that you're changing both control handles at once, which can make adjusting the curve hard to control.


Figure 5. Control handles pull the line segment away from the straight line that would normally exist between them. The bottom path is a reversed S shape because the control handles are pulling in opposite directions.

Figure 6. Control handle lines run tangent to the path where the path meets the anchor point.

A Little History on Bézier Curves

If you don't know much about geometry (or maybe don't remember much — it was back in high school, after all), you may find the very concept of creating curves by using math frightening. But most of the curve creation in Illustrator takes place behind the scenes when you use a drawing tool, such as the Pen tool.

PostScript curves are based on Bézier curves (pronounced bez-ee-ay), which were created by Pierre Bézier as a way of controlling mechanical cutting devices, commonly known as Numerical Control. Bézier worked for Renault (the car manufacturer) in France, and his mission was to streamline the process by which machines were controlled.

A mathematician and engineer, Bézier developed a method for creating curves using four points for every curved segment. He placed two points at either end of the segment — in Illustrator, these correspond to the anchor points — and made two points float around the curve segment to control the curve's shape; these are control handles in Illustrator. Using these four points, you can create any curve; using multiple sets of these curves, you can create any possible shape. John Warnock and Chuck Geschke of Adobe decided that Bézier curves were the best method for creating curves in a page description language (PostScript), and suddenly those curves became a fundamental part of high-end graphic design.

Bézier curves are anything but intuitive, and in fact, they represent the most significant stumbling block for beginners learning Illustrator. After you master the concept and use of these curves, everything about Illustrator suddenly becomes easier and friendlier. Don't try to ignore them because they won't go away. You'll find it easier in the long run to try to understand how they work.


 
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