Add Weiler-Atherton clipping algorithm

Changes the default clipping algorithm for intersections.

Author: Nemanja Jovanovic

docs/src/algorithms/clipping.md

@@ -28,6 +28,30 @@ other = Box((0,1), (3,7))
 # clipped polygon
 clipped = clip(poly, other, SutherlandHodgmanClipping())
 
+viz(poly)
+viz!(other, color = :black, alpha = 0.2)
+viz!(boundary(clipped), color = :red, segmentsize = 3)
+Mke.current_figure()
+```
+
+## Weiler-Atherton
+
+```@docs
+WeilerAthertonClipping
+```
+
+```@example clipping
+# polygon to clip
+outer = Ring((8, 0), (4, 8), (2, 8), (-2, 0), (0, 0), (1, 2), (5, 2), (6, 0))
+inner = Ring((4, 4), (2, 4), (3, 6))
+poly = PolyArea([outer, inner])
+
+# clipping geometry can be a polygon with holes
+other = poly |> Rotate(π) |> Translate(8, 8)
+
+# clipped polygon
+clipped = clip(poly, other, WeilerAthertonClipping())
+
 viz(poly)
 viz!(other, color = :black, alpha = 0.2)
 viz!(boundary(clipped), color = :red, segmentsize = 3)

src/Meshes.jl

@@ -434,6 +434,7 @@ export
   # clipping
   ClippingMethod,
   SutherlandHodgmanClipping,
+  WeilerAthertonClipping,
   clip,
 
   # intersections

src/clipping.jl

@@ -16,8 +16,36 @@ Clip the subject `geometry` with `other` geometry using clipping `method`.
 """
 function clip end
 
+function clip(poly::Polygon, other::Geometry, method::ClippingMethod)
+  b = boundary(other)
+  if b isa Multi
+    for r in parent(b)
+      poly = clip(poly, r, method)
+      if isnothing(poly)
+        return nothing
+      end
+    end
+    return poly
+  else
+    return clip(poly, b, method)
+  end
+end
+
+function clip(gs::Domain, other::Geometry, method::ClippingMethod)
+  polys = PolyArea[]
+  for i in 1:nelements(gs)
+    poly = clip(element(gs, i), other, method)
+    if !isnothing(poly)
+      push!(polys, poly)
+    end
+  end
+  n = length(polys)
+  return n == 0 ? nothing : (n == 1 ? polys[1] : GeometrySet(polys))
+end
+
 # ----------------
 # IMPLEMENTATIONS
 # ----------------
 
 include("clipping/sutherlandhodgman.jl")
+include("clipping/weileratherton.jl")

src/clipping/sutherlandhodgman.jl

@@ -18,10 +18,17 @@ The Sutherland-Hodgman algorithm for clipping polygons.
 """
 struct SutherlandHodgmanClipping <: ClippingMethod end
 
-function clip(poly::Polygon, other::Geometry, method::SutherlandHodgmanClipping)
-  c = [clip(ring, boundary(other), method) for ring in rings(poly)]
-  r = [r for r in c if !isnothing(r)]
-  isempty(r) ? nothing : PolyArea(r)
+# function clip(poly::Polygon, other::Geometry, method::SutherlandHodgmanClipping)
+  # c = [clip(ring, boundary(other), method) for ring in rings(poly)]
+  # r = [r for r in c if !isnothing(r)]
+  # isempty(r) ? nothing : PolyArea(r)
+# end
+
+
+function clip(poly::Polygon, other::Ring, method::SutherlandHodgmanClipping)
+ c = [clip(ring, other, method) for ring in rings(poly)]
+ r = [r for r in c if !isnothing(r)]
+ isempty(r) ? nothing : PolyArea(r)
 end
 
 function clip(ring::Ring, other::Ring, ::SutherlandHodgmanClipping)