Geometry processing#

Geometry is specified in many ways in IFC. Some geometry is defined explicitly with coordinates, vertices, and faces. Some geometry is defined implicitly with equations, boolean operations, and parametric shapes.

Individual processing#

The simplest way to process any geometry in a standardised fashion is to use the IfcOpenShell create_shape() function. This will provide a list of vertices, edges, and faces, or alternatively an OpenCASCADE BRep.

Warning

This section describes individual processing only. This is useful for learning how geometry processing works, but is not recommended for practical applications. See the Geometry iterator section below after reading this to see how to process geometry with multiple threads.

Here is a simple example of processing a single wall into a list of vertices and faces. In this example, a shape variable is returned, which holds geometry related information in shape.geometry:

import ifcopenshell
import ifcopenshell.geom
import ifcopenshell.util.shape

ifc_file = ifcopenshell.open('model.ifc')
element = ifc_file.by_type('IfcWall')[0]

settings = ifcopenshell.geom.settings()
shape = ifcopenshell.geom.create_shape(settings, element)

# The GUID of the element we processed
print(shape.guid)

# The ID of the element we processed
print(shape.id)

# The element we are processing
print(ifc_file.by_guid(shape.guid))

# A unique geometry ID, useful to check whether or not two geometries are
# identical for caching and reuse. The naming scheme is:
# IfcShapeRepresentation.id{-layerset-LayerSet.id}{-material-Material.id}{-openings-[Opening n.id ...]}{-world-coords}
print(shape.geometry.id)

# A 4x4 matrix representing the location and rotation of the element, in the form:
# [ [ x_x, y_x, z_x, x   ]
#   [ x_y, y_y, z_y, y   ]
#   [ x_z, y_z, z_z, z   ]
#   [ 0.0, 0.0, 0.0, 1.0 ] ]
# The position is given by the last column: (x, y, z)
# The rotation is described by the first three columns, by explicitly specifying the local X, Y, Z axes.
# The first column is a normalised vector of the local X axis: (x_x, x_y, x_z)
# The second column is a normalised vector of the local Y axis: (y_x, y_y, y_z)
# The third column is a normalised vector of the local Z axis: (z_x, z_y, z_z)
# The axes follow a right-handed coordinate system.
# Objects are never scaled, so the scale factor of the matrix is always 1.
matrix = shape.transformation.matrix.data

# For convenience, you might want the matrix as a nested numpy array, so you can do matrix math.
matrix = ifcopenshell.util.shape.get_shape_matrix(shape)

# You can also extract the XYZ location of the matrix.
location = matrix[:,3][0:3]

# X Y Z of vertices in flattened list e.g. [v1x, v1y, v1z, v2x, v2y, v2z, ...]
# These vertices are local relative to the shape's transformation matrix.
verts = shape.geometry.verts

# Indices of vertices per edge e.g. [e1v1, e1v2, e2v1, e2v2, ...]
# If the geometry is mesh-like, edges contain the original edges.
# These may be quads or ngons and not necessarily triangles.
edges = shape.geometry.edges

# Indices of vertices per triangle face e.g. [f1v1, f1v2, f1v3, f2v1, f2v2, f2v3, ...]
# Note that faces are always triangles.
faces = shape.geometry.faces

# Since the lists are flattened, you may prefer to group them like so depending on your geometry kernel
# A nested numpy array e.g. [[v1x, v1y, v1z], [v2x, v2y, v2z], ...]
grouped_verts = ifcopenshell.util.shape.get_vertices(shape.geometry)
# A nested numpy array e.g. [[e1v1, e1v2], [e2v1, e2v2], ...]
grouped_edges = ifcopenshell.util.shape.get_edges(shape.geometry)
# A nested numpy array e.g. [[f1v1, f1v2, f1v3], [f2v1, f2v2, f2v3], ...]
grouped_faces = ifcopenshell.util.shape.get_faces(shape.geometry)

# A list of styles that are relevant to this shape
styles = shape.geometry.materials

for style in styles:
    # Each style is named after the entity class if a default
    # material is applied. Otherwise, it is named "surface-style-{SurfaceStyle.name}"
    # All non-alphanumeric characters are replaced with a "-".
    print(style.original_name())

    # A more human readable name
    print(style.name)

    # Each style may have diffuse colour RGB codes
    if style.has_diffuse:
        print(style.diffuse)

    # Each style may have transparency data
    if style.has_transparency:
        print(style.transparency)

# Indices of material applied per triangle face e.g. [f1m, f2m, ...]
material_ids = shape.geometry.material_ids

# IDs representation item per triangle face e.g. [f1i, f2i, ...]
item_ids = shape.geometry.item_ids

Alternatively, you may choose to retrieve an OpenCASCADE BRep:

import ifcopenshell
import ifcopenshell.geom

ifc_file = ifcopenshell.open('model.ifc')
element = ifc_file.by_type('IfcWall')[0]

settings = ifcopenshell.geom.settings()
settings.set(settings.USE_PYTHON_OPENCASCADE, True)

try:
    shape = geom.create_shape(settings, element)
    geometry = shape.geometry # see #1124
    # These are methods of the TopoDS_Shape class from pythonOCC
    shape_gpXYZ = geometry.Location().Transformation().TranslationPart()
    # These are methods of the gpXYZ class from pythonOCC
    print(shape_gpXYZ.X(), shape_gpXYZ.Y(), shape_gpXYZ.Z())
except:
    print("Shape creation failed")

When an entire element is passed into create_shape(), the 3D representation is processed by default with all openings applied. However, it is also possible to only process a single shape representation with no openings, representation item, or profile definition.

In these scenarios, a geometry is returned directly, equivalent to shape.geometry in the example above.

ifc_file = ifcopenshell.open('model.ifc')
element = ifc_file.by_type('IfcWall')[0]

# Process a shape representation
body = ifcopenshell.util.representation.get_representation(element, "Model", "Body")

# Note: geometry is returned directly, equivalent to shape.geometry when passing in an element
geometry = geom.create_shape(settings, body)

# Process a representation item
geometry = geom.create_shape(settings, ifc_file.by_type("IfcExtrudedAreaSolid")[0])

# Process a profile
geometry = geom.create_shape(settings, ifc_file.by_type("IfcProfileDef")[0])

When an element contains multiple shape representations with the same identifier or when you want more explicit control over which representation is processed (e.g Body or Tessellation), you can use the third parameter of create_shape() to nominate a specific shape representation to be processed in the context of a product. The element in your ifc file might look like this.

#1=IFCSHAPEREPRESENTATION(#4,'Body','BRep',(#1617476));
#2=IFCSHAPEREPRESENTATION(#4,'Body','BRep',(#1617583));
#3=IFCSHAPEREPRESENTATION(#4,'Body','BRep',(#1617630));
#5=IFCPRODUCTDEFINITIONSHAPE($,$,(#1,#2,#3));
#6=IFCWINDOW('0Rrp2csNr07QrVCrEBJezu',#9,'test','test',$,#7,#5,'test',$,$,$,$,$);

In order to get the geometry data (e.g. vertices) for this IfcWindow, we can use the Python code below:

representations = window.Representation.Representations
for representation in representations:
    # ... code that filters which representation you want ...
    shape = ifcopenshell.geom.create_shape(settings, window, representation)

See also

You may find the ifcopenshell.util.representation module useful to filter out specific representations.

Geometry iterator#

IfcOpenShell provides a geometry iterator function to efficiently process geometry in an IFC model. The iterator is always used in IfcConvert, and may also be invoked in C++ or in Python. It offers the same features as the create_shape() function for Individual processing.

The geometry iterator makes it easy to collect possible geometry in a model, supports multicore processing, and implements caching and reuse to improve the efficiency of geometry processing. For any bulk geometry processing, it is always recommended to use the iterator.

By default, the geometry iterator processes all 3D geometry in a model from all elements, and returns a list of X Y Z vertex ordinates in a flattened list, as well as a flattened list of triangulated faces denoted by vertex indices.

Here is a simple example in Python:

import multiprocessing
import ifcopenshell
import ifcopenshell.geom

ifc_file = ifcopenshell.open('model.ifc')

settings = ifcopenshell.geom.settings()
iterator = ifcopenshell.geom.iterator(settings, ifc_file, multiprocessing.cpu_count())
if iterator.initialize():
    while True:
        shape = iterator.get()
        matrix = shape.transformation.matrix.data
        faces = shape.geometry.faces
        edges = shape.geometry.edges
        verts = shape.geometry.verts
        materials = shape.geometry.materials
        material_ids = shape.geometry.material_ids
        # ... write code to process geometry here ...
        if not iterator.next():
            break

There are a variety of configuration settings to get different output. For example, you may filter elements from processing, extract 2D data, or return non-triangulated OpenCASCADE BReps. For more information on the various settings, see Geometry Settings.

One of the more common settings used is the include setting, which specifies only to process certain geometry. For example, this iterator will only process wall elements.

walls = ifc.by_type('IfcWall')
iterator = ifcopenshell.geom.iterator(settings, ifc, multiprocessing.cpu_count(), include=walls)

Note

The iterator can only be used to process whole elements, not individual shape representations, representation items, and profiles.

Manual parsing#

IfcOpenShell lets you traverse any IFC entity graph. This means it is possible for you to manually browse through the Representation attribute of IFC elements, and parse the corresponding IFC shape representations yourself instead of using generic geometric processing such as Individual processing and the Geometry iterator.

This approach requires an in-depth understanding of IFC geometry representations, as well as its many caveats with units and transformations, but can be very simple and extremely fast to extract specific types of geometry. For example, if you know you are dealing with IfcCircle geometry, you can specifically pinpoint the Radius parameter.

unit_scale = ifcopenshell.util.unit.calculate_unit_scale(ifc_file)

for circle in ifc_file.by_type("IfcCircle"):
    # In project length units
    print(circle.Radius)

    # In SI meters
    print(circle.Radius * unit_scale)

Given the advanced nature of manual processing, it is generally not recommended except in specific tasks.

Geometry serialisation#

Geometry may be serialised into many different formats using IfcConvert. Alternatively, you may also access the serialiser with Python to customise the conversion, such as by writing a script the modifies the IFC on the fly before converting it, or writing complex include and exclude filters.

Here is a typical example to serialising to glTF / glb.

import ifcopenshell
import ifcopenshell.geom
import multiprocessing

settings = ifcopenshell.geom.settings()
settings.set(settings.STRICT_TOLERANCE, True)
settings.set(settings.INCLUDE_CURVES, True)
# Setting element GUIDs is optional, but useful to uniquely identify objects in non-semantic formats.
settings.set(settings.USE_ELEMENT_GUIDS, True)
# Note that applying default materials is required in glTF serialisation.
settings.set(settings.APPLY_DEFAULT_MATERIALS, True)

serialiser = ifcopenshell.geom.serializers.gltf("output.glb", settings)
# Alternatively, this is an example for OBJ
# serialiser = ifcopenshell.geom.serializers.obj('output.obj', 'output.mtl', settings)
serialiser.setFile(self.file)
serialiser.setUnitNameAndMagnitude("METER", 1.0)
serialiser.writeHeader()

iterator = ifcopenshell.geom.iterator(settings, self.file, multiprocessing.cpu_count())
if iterator.initialize():
    while True:
        serialiser.write(iterator.get())
        if not iterator.next():
            break
serialiser.finalize()