11.6 Spirals

The Spiral tool (the key or ) creates another simple but versatile shape—a concentric spiral. While not quite as exciting as the star, it is sometimes very useful.

A spiral, like a star, is drawn from the center. Dragging with , as usual, snaps the drag point (the outer end of the spiral) to 15-degree increments (6.3 The Selector: Rotating and Skewing).

Unlike all other shape tools, the Spiral tool by default uses its own style for new spirals—no fill, black stroke—instead of the last used style (11.1.2 The Style of New Shapes).

Once drawn, a spiral displays two handles on both ends of the line. Simply dragging these handles circularly rolls the spiral in or out, from the inside or from the outside. In this way, you can, for example, create a spiral with only one turn.

The Turns parameter in the controls bar reflects how many full circles you will need to do to reach the outer end of the spiral. The maximum number of spiral turns is 1000. The inner end is controlled by the Inner radius value, which shows the percentage of the total turns at which the spiral starts (Figure 11-32). For example, an Inner radius of 0 means that the spiral starts right from the center; 0.5 means that it starts half-way between the center and the outer end.

Figure 11-32. Adjusting turns and inner radius

The Divergence of a spiral is the parameter controlling whether a spiral’s winding is equispaced throughout (divergence = 1), becomes denser towards its center (divergence > 1), or becomes denser towards its periphery (divergence < 1). You can change this parameter either numerically at the controls bar, or by -dragging the inner handle up or down:

Figure 11-33. Adjusting divergence

Other shortcuts for the outer handle:

• -drag to scale or rotate around the center (no rolling on unrolling, same behavior as when drawing the spiral).

• -drag to lock radius while rolling or unrolling (i.e., the entire spiral becomes denser or sparser without changing its overall size).

Inner handle shortcuts:

• -drag vertically to adjust divergence.

• -click to reset divergence to the default of 1.

• -click to reset the inner radius to 0 (i.e., move the inner handle all the way to the center).

When does it make sense to use spirals? A spiral fills in a given space with uniform concentric pattern—something that may not be easy to achieve manually or by any other tools. As such, it may well become a carrier or guide for other objects, such as markers (9.5.1 Mid Markers and Nodes), dash patterns (9.4 Dash Patterns), text on path (15.2.3 Text on a Path), or live path effects (13.1 Path Effects). Also, like the Ellipse tool, the Spiral tool can be convenient for creating curves with smoothly varying curvature; unlike a plain Bézier curve, an arc or a spiral can be made shorter or longer by dragging a handle along the curve without affecting its shape.