first rough partial draft of alignment handling in python

src/ifcopenshell-python/ifcopenshell/alignment.py

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+from dataclasses import dataclass
+
+import numpy
+
+import ifcopenshell                             
+import ifcopenshell.geom                            
+import ifcopenshell.express
+import ifcopenshell.transition_curve
+
+# geometric primitives
+
+# @notes
+# - not sure if the separation of geometric primitives make sense
+#   does it make handling the variety of distance expressions and
+#   interpolation harder?
+
+@dataclass
+class line:
+    start_point: numpy.ndarray
+    direction_vector: numpy.ndarray
+
+    def __call__(self, u):     
+        return self.start_point + \
+            self.direction_vector * u
+
+
+@dataclass
+class circle:
+    radius: numpy.ndarray
+
+    def __call__(self, u):        
+        return numpy.array([
+            self.radius * numpy.cos(u),
+            self.radius * numpy.sin(u)
+        ])
+
+
+def place(matrix, func):
+    """
+    Higher order function for application of a 3x3 matrix
+    to a 2D point. Assumes a functor such as line or circle.
+    """
+    def inner(*args):
+        v = func(*args)
+        # homogenize
+        v = numpy.insert(v, v.shape[-1], 1, axis=-1)
+        return (matrix @ v)[0:2]
+    return inner
+
+
+# mapping functions from IFC entities
+
+def map_inst(inst):
+    """
+    Looks up one of the implementation functions below in the global namespace
+    """
+    return globals()[f"impl_{inst.is_a()}"](inst)
+
+
+def impl_IfcLine(inst):
+    return line(
+        numpy.array(inst.Pnt.Coordinates),
+        numpy.array(inst.Dir.Orientation.DirectionRatios) * inst.Dir.Magnitude
+    )
+
+
+def impl_IfcCircle(inst):
+    return place(map_inst(inst.Position), circle(
+        inst.Radius
+    ))
+
+
+def impl_IfcClothoid(inst):
+    # @todo
+    # place = map_inst(inst.Position)
+    # ifcopenshell.transition_curve.TransitionCurve(
+    #     StartPoint          = place.T[2]
+    #     StartDirection      = numpy.arctan2(place.T[0][1], place.T[0][0]),
+    #     SegmentLength       = 
+    #     IsStartRadiusCCW    = 
+    #     IsEndRadiusCCW      = 
+    #     TransitionCurveType = 
+    #     StartRadius         = 
+    #     EndRadius           = 
+    # )
+    return lambda *args: numpy.array((0.,0.))
+
+
+def impl_IfcAxis2Placement2D(inst):
+    arr = numpy.eye(3)
+
+    if inst is None:
+        return arr
+
+    arr.T[2, 0:2] = inst.Location.Coordinates
+
+    if inst.RefDirection is None:
+        return arr
+
+    arr.T[0, 0:2] = inst.RefDirection.DirectionRatios
+    arr.T[0, 0:2] /= numpy.linalg.norm(arr.T[0, 0:2])
+    arr.T[1, 0:2] = -arr.T[0,1], arr.T[0,0]
+
+    return arr
+
+
+# conversion functions for semantic design parameters (not used atm)
+
+def convert(inst):
+    """
+    Looks up one of the conversion functions below in the global namespace
+    """
+    yield from globals()[f"convert_{inst.is_a()}_{inst.PredefinedType}"](inst)
+
+
+def convert_IfcAlignmentHorizontalSegment_LINE(data):
+    xy = numpy.array(data.StartPoint.Coordinates)
+    yield xy
+    di = numpy.array([
+        numpy.cos(data.StartDirection),
+        numpy.sin(data.StartDirection)
+    ])
+    yield xy + di * data.SegmentLength
+
+# Two approaches, either DesignParameters or Representation  
+
+def interpret_linear_element_semantics(settings, crv):
+    # traverse decomposition
+    for rel in crv.IsNestedBy:
+        for obj in rel.RelatedObjects:
+            yield from interpret_linear_element_semantics(settings, obj)
+
+    # lookup design parameters and dispatch to conversion function
+    if crv.is_a("IfcAlignmentSegment"):
+        dp = crv.DesignParameters
+        yield from convert(dp)
+
+
+def interpret_linear_element_geometry(settings, crv):
+    for segment in crv.Representation.Representations[0].Items[0].Segments:
+
+        print(segment)
+        print(segment.ParentCurve)
+        print()        
+
+        func = place(
+            map_inst(segment.Placement), 
+            map_inst(segment.ParentCurve)
+        )
+
+        for u in numpy.linspace(
+            segment.SegmentStart[0],
+            segment.SegmentStart[0] + segment.SegmentLength[0],
+            num=32
+        ):
+            yield func(u)
+
+
+interpret_linear_element = interpret_linear_element_geometry
+
+def create_shape(settings, elem):
+    if elem.is_a("IfcLinearPositioningElement") or elem.is_a("IfcLinearElement"):
+        return numpy.row_stack(list(interpret_linear_element(settings, elem)))
+    else:
+        return ifcopenshell.geom.create_shape(settings, elem)
+
+
+def print_structure(alignment, indent=0):
+    """
+    Debugging function to print alignment decomposition
+    """
+    print(" " * indent, str(alignment)[0:100])          
+    for rel in alignment.IsNestedBy:                    
+        for child in rel.RelatedObjects:        
+            print_structure(child, indent+2)
+
+
+if __name__ == "__main__":
+    import sys
+    from matplotlib import pyplot as plt
+
+    s = ifcopenshell.express.parse("IFC4x3_RC3.exp")
+    ifcopenshell.register_schema(s)
+    f = ifcopenshell.open(sys.argv[1])
+    print_structure(f.by_type("IfcAlignment")[0])
+
+    al_hor = f.by_type("IfcAlignmentHorizontal")[0]
+    xy = create_shape({}, al_hor)
+
+    plt.plot(xy.T[0], xy.T[1])
+    plt.savefig("horizontal_alignment.png")