adding volume calculation

Author: aza

lib/stl.rb

@@ -132,4 +132,13 @@ def translate(offset)
     def write(filename, format=:binary)
 	self.class.write(filename, faces, format)
     end
-end
+
+    #   @return [Float]  the volume in cube stl units (usually milimeters)
+    def volume
+      vol = 0
+      faces.each do |face|
+        vol = vol + face.signed_volume
+      end
+      vol.abs
+    end
+end

lib/stl/face.rb

@@ -15,5 +15,18 @@ def initialize(normal, *vertices)
 	    @normal = normal
 	    @points = vertices
 	end
+
+	#   @return [Float]  The signed volume
+	def signed_volume
+ 	    # math voodoo acording to:
+ 	    # http://stackoverflow.com/questions/1406029/how-to-calculate-the-volume-of-a-3d-mesh-object-the-surface-of-which-is-made-up
+	    v321 = @points[2][0] * @points[1][1] * @points[0][2]
+	    v231 = @points[1][0] * @points[2][1] * @points[0][2]
+	    v312 = @points[2][0] * @points[0][1] * @points[1][2]
+	    v132 = @points[0][0] * @points[2][1] * @points[1][2]
+	    v213 = @points[1][0] * @points[0][1] * @points[2][2]
+	    v123 = @points[0][0] * @points[1][1] * @points[2][2]
+ 	    (1.0/6.0)*(-v321 + v231 + v312 - v132 - v213 + v123)
+	end
     end
 end

test/fixtures/box.stl

@@ -0,0 +1,86 @@
+solid OpenSCAD_Model
+  facet normal -1 0 0
+    outer loop
+      vertex -15 -10 10
+      vertex -15 10 10
+      vertex -15 -10 -10
+    endloop
+  endfacet
+  facet normal -1 0 0
+    outer loop
+      vertex -15 -10 -10
+      vertex -15 10 10
+      vertex -15 10 -10
+    endloop
+  endfacet
+  facet normal 0 0 1
+    outer loop
+      vertex -15 -10 10
+      vertex 15 -10 10
+      vertex 15 10 10
+    endloop
+  endfacet
+  facet normal 0 0 1
+    outer loop
+      vertex -15 10 10
+      vertex -15 -10 10
+      vertex 15 10 10
+    endloop
+  endfacet
+  facet normal 0 -1 0
+    outer loop
+      vertex -15 -10 -10
+      vertex 15 -10 -10
+      vertex 15 -10 10
+    endloop
+  endfacet
+  facet normal 0 -1 0
+    outer loop
+      vertex -15 -10 10
+      vertex -15 -10 -10
+      vertex 15 -10 10
+    endloop
+  endfacet
+  facet normal 0 0 -1
+    outer loop
+      vertex -15 10 -10
+      vertex 15 10 -10
+      vertex -15 -10 -10
+    endloop
+  endfacet
+  facet normal 0 0 -1
+    outer loop
+      vertex -15 -10 -10
+      vertex 15 10 -10
+      vertex 15 -10 -10
+    endloop
+  endfacet
+  facet normal 0 1 0
+    outer loop
+      vertex -15 10 10
+      vertex 15 10 10
+      vertex -15 10 -10
+    endloop
+  endfacet
+  facet normal 0 1 0
+    outer loop
+      vertex -15 10 -10
+      vertex 15 10 10
+      vertex 15 10 -10
+    endloop
+  endfacet
+  facet normal 1 0 0
+    outer loop
+      vertex 15 -10 -10
+      vertex 15 10 -10
+      vertex 15 10 10
+    endloop
+  endfacet
+  facet normal 1 0 0
+    outer loop
+      vertex 15 -10 10
+      vertex 15 -10 -10
+      vertex 15 10 10
+    endloop
+  endfacet
+endsolid OpenSCAD_Model

test/stl.rb

@@ -66,6 +66,19 @@
 	end
     end
 
+    describe 'when reading a box' do
+	subject { STL.read('test/fixtures/box.stl') }
+
+	it 'face must have a signed volume' do
+	    subject.faces.first.signed_volume.must_equal 1000.0
+	end
+
+	it 'must have a volume' do
+	    subject.volume.must_equal 12000.0
+ 	    subject.scale(2).volume.must_equal 96000.0
+	end
+    end
+
     describe 'when reading from a binary file' do
 	subject { STL.read('test/fixtures/binary_triangle.stl') }