How to create a polygon from other polygon boundaries?

Question

I want to write an algorithm that creates a polygon inside a hole made by a set of other polygons. It's something like a flood fill and it's like the "boundary" tool in Autocad.

The image shows an example: the algoritm should fill the enclosed white space in the center (after a click on it).

Any theory, pseudocode or examples would help a lot.

Answer 1

You can use the geoprocessing tools->symmetric difference from the QGIS 1.7 ftools plugin. Use as second layer a vector file with a polygon covering all the area you need.