Boolean algorithm steps

1. Prepare graph

- convert the polygons into graphs
- round vertexes to GRID and simplify if possible
- what is MARGE
- merge all graphs into one big graph

1. Calculate Intersections

- Merge nodes .
- Merge node to link.
- Process crossings.
- Rotate the drawing 90 degrees.
- Merge node to link.
- Process crossings.
- Rotate back (not really needed)
- Calculate Link to Link crossings.
- Process crossings.
- Merge nodes

2. prepare for boolean

- left and/or right side is inside or outside
- set group A & B for each link
- remove redundant links .

3. perform operation

- remove not needed
- extract the resulting graphs for a given operation.
- convert the result graphs back to polygons.