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I am working on a track editor and have found myself in a situation where I need to define two touching circles. Ideally I would like to know the centre point, and radius of these circles.

The information I have is a point on the circumference of each of the circles, and the tangent to the circle at that point.

On my own I have figured out that if I know the tangents at those points I know the lines on which the centre points must lie.

C1 is the centre of the first circle I am looking for
P1 is a point on the circumference of the circle at which I know the tangent
A is the normal to the tangent that I know at P1

C2 is the centre of the second circle I am looking for
P2 is a point on the circumference of the circle at which I know the tangent
B is the normal to the tangent that I know at P2

C1 = P1 - t1 * A
C2 = P2 - t2 * B

I also know that the distance between the two centres will be equal to the sum of the distance of the centres from the points on the circumference. Because I am looking for two circles that touch, but do not overlap.

|C1 - P1| + |C2 - P2| = |C1 - C2|

It seems that finding C1 and C2 is dependent on finding the scalars t1 and t2 such that the last equation is true.

Hopefully this is really simple, and I just don't know the technique to use to get the answer. Please help.

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1 Answer 1

You have one equation in two unknowns $t_1$ and $t_2$, so there should generally be a one-parameter family of solutions. In fact, choose $t_1$ (and thus $C_1$ and $r_1 = |C_1 - P_1|$). Then you want $C_2$ to be a point on the line $P_2 - t_2 B$ whose distance to $C_1$ is $r_1$ more than its distance from $P_2$. The locus of a point whose distance to $C_1$ is $r_1$ more than its distance from $P_2$ is a branch of a hyperbola (if nonempty).

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I have realised t1 = t2 How do I solve for this? –  user9062 Apr 4 '11 at 3:44

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