Add support for slicing (#339)

* Allow constructing infinite planes

* Preliminary section implementation

* Better error message

* Section related tests

cadquery/cq.py

@@ -3420,6 +3420,27 @@ def text(
             newS = newS.clean()
         return newS
 
+    def section(self, height=0.0):
+        """
+        Slices current solid at the given height.
+        
+        :param float height: height to slice at (default: 0)
+        :return: a CQ object with the resulting face(s).
+        """
+
+        solidRef = self.findSolid(searchStack=True, searchParents=True)
+
+        if solidRef is None:
+            raise ValueError("Cannot find solid to slice")
+
+        plane = Face.makePlane(
+            basePnt=self.plane.origin + self.plane.zDir * height, dir=self.plane.zDir
+        )
+
+        r = solidRef.intersect(plane)
+
+        return self.newObject([r])
+
     def _repr_html_(self):
         """
         Special method for rendering current object in a jupyter notebook

cadquery/occ_impl/shapes.py

@@ -1208,17 +1208,20 @@ def makeNSidedSurface(
         return cls.cast(face).fix()
 
     @classmethod
-    def makePlane(cls, length, width, basePnt=(0, 0, 0), dir=(0, 0, 1)):
+    def makePlane(cls, length=None, width=None, basePnt=(0, 0, 0), dir=(0, 0, 1)):
         basePnt = Vector(basePnt)
         dir = Vector(dir)
 
         pln_geom = gp_Pln(basePnt.toPnt(), dir.toDir())
 
-        return cls(
-            BRepBuilderAPI_MakeFace(
+        if length and width:
+            pln_shape = BRepBuilderAPI_MakeFace(
                 pln_geom, -width * 0.5, width * 0.5, -length * 0.5, length * 0.5
             ).Face()
-        )
+        else:
+            pln_shape = BRepBuilderAPI_MakeFace(pln_geom).Face()
+
+        return cls(pln_shape)
 
     @classmethod
     def makeRuledSurface(cls, edgeOrWire1, edgeOrWire2, dist=None):

tests/test_cadquery.py

@@ -3395,3 +3395,18 @@ def testUnionCompound(self):
         obj = Workplane("XY").newObject(list_of_shapes).cut(shape_to_cut)
 
         assert obj.val().isValid()
+
+    def testSection(self):
+
+        box = Workplane("XY", origin=(1, 2, 3)).box(1, 1, 1)
+
+        s1 = box.section()
+        s2 = box.section(0.5)
+
+        self.assertAlmostEqual(s1.faces().val().Area(), 1)
+        self.assertAlmostEqual(s2.faces().val().Area(), 1)
+
+        line = Workplane("XY").hLine(1)
+
+        with self.assertRaises(ValueError):
+            line.section()