Boundary Representations

Boundary representation (Brep) support is realized in COMPAS using its plugin system. The expected interface for Brep related classes is defined in the compas.geometry.brep module whereas the actual implementation is context dependent and implemented using plugins.

Brep Basics

Brep is a data structure used to describe a shape by means of recording topological and geometrical information of the shape’s boundaries. Some topological properties are associated with an underlying geometry, while others are purely topological.

A Brep is comprised of the following:

Topology	Geometry	Description
Vertex	3D Point	The most basic element of a Brep, geometrically described as a point in 3D space.
Edge	3D Curve	An edge has a start vertex and an end vertex. The underlying 3D curve describes the geometry of the edge (Line, Circle etc.). Closed edges feature start_vertex == end_vertex.
Loop	None	A collection of trims which define the inner or outer boundary of a face.
Face	Surface	Defines the geometry of one of the shape’s faces using a surface. Associated with at least one loop which describes the trimmed outer boundary of the surface. Inner loops are referred to as holes in the face.
Trim	2D Curve	A 2D curve which trims a face. Trims are associated with a corresponding edge.

Getting Started with COMPAS Brep

To create an empty Brep

>>> from compas.geometry import Brep
>>> brep = Brep()

Notice that the type of the actual instance created by Brep() will differ depending on the currently available backend. For example, when in Rhino

>>> type(brep)
compas_rhino.geometry.RhinoBrep

Every backend is expected to implement some alternative constructors

>>> from compas.geometry import Box
>>> from compas.geometry import Brep
>>> ...
>>> box = Box.from_width_height_depth(5., 5., 5.)
>>> brep_box = Brep.from_box(box)

Brep can also be instantiated from an instance of a backend native Brep

>>> import Rhino
>>> from compas.geometry import Brep
>>> ...
>>> Brep.from_native(Rhino.Geometry.Brep())

Brep operations

Trimming a Brep in Grasshopper

from compas.geometry import Frame
from compas.geometry import Point
from compas.geometry import Brep

box = Box.from_width_height_depth(5, 5, 10)

brep = Brep.from_box(box)
cutting_plane = Frame(Point(0, 2.5, 0), [1, 0, 0], [0, 1, 1.5])

brep.trim(cutting_plane)

Splitting a Brep in Grasshopper

from compas.geometry import Brep, Box, Frame, Translation

brep = Brep.from_box(Box.from_width_height_depth(5,5,5))
cutter = Brep.from_box(Box.from_width_height_depth(1, 6, 6))

a, b, c = brep.split(cutter)

world_xy = Frame.worldXY()
translated_frame = Frame((0, 0, 1.), world_xy.xaxis, world_xy.yaxis)
t = Translation.from_frame_to_frame(world_xy, translated_frame)
a.transform(t)
b.transform(t)

result = [x.native_brep for x in [a, b, c]]