Rules for Construction

A valid starting point and vector follow two rules:

First, a circle of the given diameter and starting point should not intersect the scan. The image below shows a violation of this rule.

A - Scan
B - Start Point
C - Down Vector

Invalid starting point due to intersection with the scan

Second, the circle projected from the start point along the down vector must intersect the scan. The image below shows a violation of this rule.

A - Scan
B - Start Point
C - Down Vector

Invalid starting point due to the circle missing the scan