The algorithm proposed in $[1]$ is used to continuously update the radius of the bubble cursor, such that there is always exactly one target within its activation area. To describe the algorithm in an environment with objects T$1,$ T$2,...,$Tn$,$ we adopt the following definitions as $[1]$:

Intersecting Distance i $($IntDi$)$: The length of the shortest line connecting the centre of the bubble cursor and any point on the border of Ti$.$IntDi can be calculated as the distance between the cursor and the object centre minus the radius of the object $($Fig$.2).$