clipper - polygon clipping

`local clipper = require'clipper'`

A Lua+C+ffi binding of Clipper, Angus Johnson's free polygon clipping library.

screenshot

Features

polygon clipping: intersection, union, difference, xor
polygon simplificaion with even_odd, non_zero, positive and negative fill types
polygon offsetting with square, round and miter join types

`clipper.polygon([n]) -> poly`	create a polygon object of size `n` (default 0)
`poly:size() -> n`	number of vertices
`poly:add(x, y)`	add a vertex to the polygon
`poly:get(i).x -> n` `poly:get(i).y -> n`	get vertex coordinates
`poly:get(i).x = n` `poly:get(i).y = n`	set vertex coordinates
`poly:simplify(['even_odd'\|'non_zero'\|'positive'\|'negative']) -> polys`	simplify a polygon
`poly:clean([distance]) -> polys`	clean a polygon
`poly:reverse()`	reverse the order (and hence orientation) of vertices
`poly:orientation() -> true \| false`	polygon orientation (true = clockwise)
`poly:area() -> n`	polygon area
Polygon lists
`clipper.polygons([n \| poly1, poly2, ...]) -> polys`	create a polygon list
`polys:size() -> n`	list size
`polys:add(poly)`	add a polygon to the end of the list
`polys:get(i) -> poly`	get a polygon from the list
`polys:set(i, poly)`	set a polygon in the list
`polys:simplify(['even_odd'\|'non_zero'\|'positive'\|'negative']) -> polys`	simplify polygons (default is `'even_odd'`)
`polys:clean([distance]) -> polys`	clean polygons (default distance is `~= sqrt(2)`)
`polys:reverse()`	reverse the order (and hence orientation) of vertices
`polys:offset(delta, ['square'\|'round'\|'miter'], [limit]) -> polys`	offset polygons (default is 'square', 0)
Clipping
`clipper.new() -> cl`	create a clipper object
`cl:add_subject(poly \| polys)`	add polygons to be clipped
`cl:add_clip(poly \| polys)`	add polygons to be clipped against
`cl:get_bounds() -> x1, y1, x2, y2`	bounding box of all the polygons in the clipper

cl:execute(operation, [subj_fill_type], [clip_fill_type], [reverse]) -> polys

Clip subject polygons against clip polygons, optionally setting the fill type for each polygon list and optionally reversing the order of the vertices.

operation = 'intersection'|'union'|'difference'|'xor'
*_fill_type = 'even_odd'|'non_zero'|'positive'|'negative'
reverse = true | false

Notes

input and output vertices are int64_t cdata, not Lua numbers; use simple scaling on the input and output points to preserve sub-pixel accuracy.
all objects are garbage collected.
adding a polygon to a polygon list copies the polygon and all its elements to the list so there's no need to keep a reference to the polygon afterwards.
poly:get(1) returns a pointer to the beginning of the vertex array so pointer arithmetic and memcpy are allowed on it.