Recipe: Detecting Circles

In a direct manipulation interface like iOS, you’d imagine that most people could get by just pointing to items onscreen. And yet, circle detection remains one of the most requested gestures. Developers like having people circle items onscreen with their fingers. In the spirit of providing solutions that readers have requested, Recipe 8-12 offers a relatively simple circle detector, which is shown in Figure 8-6.

Figure 8-6. The dot and the outer ellipse show the key features of the detected circle.

In this implementation, detection uses a multistep test. A time test checks that the stroke was not lingering. A circle gesture should be quickly drawn. There’s an inflection test checking that the touch did not change directions too often. A proper circle includes four direction changes. This test allows for five. There’s a convergence test. The circle must start and end close enough together that the points are somehow related. A fair amount of leeway is needed because when you don’t provide direct visual feedback, users tend to undershoot or overshoot where they began. The pixel distance used here is generous, approximately a third of the view size.