Draw a ring shape behind the knob and add following layer styles.

画一个圈背后的旋钮的外形和添加图层样式如下。

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Draw a black circle behind the ring shape.

后面画一个圈形黑圈。

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Draw a circle shape behind the ring.

后面画一个圆形的环。

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Draw a circle shape behind the ring.

后面画一个圆形的环。

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