The cage tool is based on the Green coordinates (
http://www.math.tau.ac.il/~lipmanya/GC/gc.htm). These coordinates are not
defined outside the cage, so the cage tool wont be able to affect point
outside the cage. There is a mathematical possibility to expand the
cage trough one edge of the cage, but i think that the way it would work
isn't relevant for the gimp use case.
However, the cage tool already implement almost half of a future iwarp tool.
Most of the deformation backend can be reused as is, and the deformation
logic of the cage tool can be adapted to code an iwarp tool.
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