Decimate based on curvature metric

lib/mittsu/mesh_analysis/modifiers/decimator.rb

@@ -5,6 +5,7 @@ def initialize(geometry)
   end
 
   def decimate(target_face_count)
+    @geometry.flatten!
     edge_collapses = edge_collapse_costs.sort_by { |x| x[:cost] }
     loop do
       break if @geometry.faces.count <= target_face_count
@@ -15,12 +16,12 @@ def decimate(target_face_count)
   end
 
   def edge_collapse_costs
-    @geometry.edges.map { |e|
+    @geometry.edges.filter_map { |e|
       e ? {
         edge_index: e.index,
-        cost: @geometry.edge_length(e.index)
+        cost: @geometry.collapse_cost(e.index)
       } : nil
-    }.compact
+    }
   end
 
   def collapse(edge)

lib/mittsu/mesh_analysis/winged_edge_geometry.rb

@@ -51,11 +51,27 @@ def from_geometry(geometry, merge: true, normalize: true)
       flatten!
     end
 
+    def collapse_cost(index)
+      length = edge_length(index) || 100
+      curvature = edge_curvature(index) || 100
+      length * curvature
+    end
+
     def edge_length(index)
       e0 = edge(index)
+      return nil if e0.nil?
       @vertices[e0.start].distance_to(@vertices[e0.finish])
     end
 
+    def edge_curvature(index)
+      e0 = edge(index)
+      unless e0.nil?
+        vl = @faces[e0.left]&.normal
+        vr = @faces[e0.right]&.normal
+        (1 - vl.normalize.dot(vr.normalize)) / 2 if vl && vr
+      end
+    end
+
     def flatten!
       @faces = @face_indices.map do |face|
         next if face.nil?

spec/lib/mittsu/mesh_analysis/winged_edge_geometry_spec.rb

@@ -25,6 +25,10 @@
       # These are the joined opposite corner vertices in the PlaneGeometry
       let(:edge) { geometry.between(1, 2) }
 
+      it "calculates curvature" do
+        expect(geometry.edge_curvature(edge.index)).to eq 0
+      end
+
       it "calculates edge lengths" do
         expect(geometry.edge_length(edge.index)).to be_within(0.1).of(1.414)
       end