Adobe Illustrator Tutorial CS CS2 | Pen Tool Line Quality
Controlling Line Weights Using Adobe Illustrator's Pen tool & Stroke Pallet
Line Weight "Stroke" Attributes
In Adobe Illustrator CS drawing line-art is done via the Pen tool and it's four attributes shown in Fig. 1. You can also use Illustrator's "Line" tool to drag a line from point to point, but the Pen tool can accomplish the same tasks by clicking point to point making it unnecessary to constantly change between tools.
Pen tool line weights are controlled by the "Stroke" pallet shown in Fig. 2. To make adjustments to the line weight (or line thickness) you would use the pulldown menu labeled "Weight" (Fig. 2). The standard line weights vary in thickness from 0.25 point to 100 point. Most line art used in technical illustration would range from 0.25 points to 2 points. You can manually input smaller line weights for hairlines, centerlines and construction lines.
Illustrator Pen Tool - End Cap Attributes
In our sample illustration of a Crankshaft Journal and Connecting Rod (Fig. 4) your will observe that when we zoom in at various levels, flaws in the line quality begin to appear at each intersecting point between two lines. When we zoom in at 1600% Fig. 5 there are noticeable gaps where lines intersect. One solution might be to simply join each corner together (via Command>J or the Pen Tool) but that would negate the effect of using variable line weights to add readability to the illustration. Once two lines are joined together they take on the same attributes (weight, color, end & miter limit).
Although zooming in on your subject matter to 1600% or even 2400% may seem a bit excessive, flaws such as those shown in Fig 5 can and will translate into unsightly gaps when exported to Photoshop or a printed piece.
Technical Note: If you are going to export the line art from Illustrator to use as a painting template in Photoshop, these gaps can cause leaks when using the Selection (Magic Wand) Tool.
A simple solution is to set the "End Cap" of your lines (strokes) to the "Round Cap" setting in the "Stroke Pallet". Our example in Fig. 6 has the line-art strokes selected and the End Cap is set to the "Butt Cap" or blunt setting. This results in a squared-off ending to the line or stroke. In Fig. 7 we have set the end caps to the "Round Cap" setting and this step has cleaned up the line quality considerably.
Pen Tool - Corner Joint Attributes
If you elect to join two lines together at their intersecting point (using Command>J) you will still need to control the corner effect. This is done through the Stroke Pallet's "Miter Limit" and "Miter Joint" settings.
In Fig. 8 the Miter Limit is set to "4" and the corner is set to "Miter Joint". This creates a corner that is too sharp when two lines intersect at such extreme angles. This type of setting is useful on 90º corners.
In Fig. 9 the Miter Limit now is set to "3" (or lower) but the corner is still set to the "Miter Joint" setting. This creates a corner that is too blunt.
In Fig. 10 the corner is set to "Round Joint". You will notice that the "Miter Limit" setting is deactivated when using the "Round Joint" setting. Using the Round Joint setting on a corner of this type creates a corner that is nicely rounded and visually appealing.
Varying or Tapered Line Weights - Copy and Paste Method
I have never found an automated way to create tapered line thickness with any predictability or consistency, and the location of the taper is critical to the overall appearance of the illustration. One way to manually achieve the "appearance" of a "calligraphy style" thick-to-thin line is to select a stoked path, then use the Command>Option>Arrow (move) keys to copy and paste the original line (Fig. 11) multiple times. By using the up-down or left-right keys you can keep one point of the arc small while thickening the other side of the arc (Fig. 12).
You will need to set the "Keyboard Increment" setting (located in Illustrator's "General" Preferences window) to 0.0010 so that each incremental movement is very small. This technique makes a perfect arc (only as perfect as the original) with a "calligraphy style" thick-to-thin appearance.
Conclusion
As with all of the multitude of setting possibilities that exist in Adobe Illustrator, there are usually several ways in which to accomplish the same task or effect. I find that the settings discussed in this Illustrator Tutorial are the one's that I use most often to create the line-art for a technical illustration.
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