2d circular union nav-mesh

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Hi guys,

Does anyone have any references or pointers for how to constrain a small 
character (represented by a circle) within the interior of the union of a 
set of larger circles?

The background is, the larger circles define regions of land where the 
small circle can navigate around (like a nav mesh), and the union of the 
these circles defines the entire nav-mesh. The character is not allowed to 
leave the interior of the nav-mesh and must remain wholly inside it at all 
times.

Some diagrams:

The general case, the filled circle represents the character and the 
hollow circles the land

http://www.pfirth.co.uk/ex1.png

My first attempt was to ensure the character was wholly inside at least 
one land circle, but that fails in this case:

http://www.pfirth.co.uk/ex2.png

In that example, if the character moves any further up, it will not be 
wholly contained in either circle and will fail, yet it should be able to 
navigate between them.

Cheers, Paul.
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