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Dion Moult
221 lines
9.3 KiB
ReStructuredText
221 lines
9.3 KiB
ReStructuredText
Introduction to IFC
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===================
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An IFC model is a collection of elements (e.g. doors, windows, construction
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tasks, materials, etc) with relationships to other elements in a graph-like
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database. Together, these elements and their relationships describe the digital
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built environment.
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.. image:: images/ifc-graph.png
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Each element has a type known as an **IFC Class**. These classes define the
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attributes that the element may store. For example, the **IfcWall Class** is
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allowed to store a **Name** and **Description** attribute.
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This IFC database can be stored in many formats. The most common is the ``.ifc``
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format, which stores data in plain text. If you open a ``.ifc`` file in a text
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editor, you'll see something like this:
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::
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#1=IFCPROJECT('3Cbhu4euf1hfgM_SHZbeqM',$,'My Project',$,$,$,$,$,#4);
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#2=IFCSIUNIT(*,.LENGTHUNIT.,.MILLI.,.METRE.);
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#3=IFCSIUNIT(*,.AREAUNIT.,$,.SQUARE_METRE.);
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#4=IFCUNITASSIGNMENT((#2,#3));
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#5=IFCCARTESIANPOINT((0.,0.,0.));
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In this example there are 5 elements in the graph. The element with the ID of
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**#1** has an **IFC Class** of **IfcProject**. This element has 9
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comma-separated attributes. IFC defines how many attributes each **IFC Class**
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is allowed to have, attribute names, the order of attributes, data type,
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optional or mandatory status (i.e. cardinality), and more.
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::
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IFC Class Quoted string value Null value ID reference
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↓ ↓ ↓ ↓
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#1=IFCPROJECT('3Cbhu4euf1hfgM_SHZbeqM',$,'My Project',$,$,$,$,$,#4);
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↑ ↑
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Element ID Comma-separated list of attributes
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By selecting elements by their **IFC Class**, and reading their attributes, you
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can navigate from one element to another. The relationships between elements are
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called **IFC Concepts** and create meaning in our industry. For example, if a
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**IfcWall** element has an attribute that references an **IfcBuildingStorey**
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element in a particular way, it will mean that the wall is located in the
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ground floor of the building.
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.. image:: images/ifc-concepts.png
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The official IFC documentation describes hundreds of **IFC Classes**, ranging
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from walls, door, to tasks, cost items, parametric materials, and structural
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analysis constraints. There are also hundreds of **IFC Concepts**, which may
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describe how a wall is in a storey, a construction task might occur one after
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another, or how an surface bounds a space for energy analysis.
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It takes time to learn the many **IFC Classes** and **IFC Concepts** available.
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Once you do, you will be able to richly describe our built environment
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digitally. IfcOpenShell can help you navigate these IFC elements, read their
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attributes, and explore relationships. Your journey begins here.
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.. seealso::
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If you are already familiar with IFC and just want to learn how to use
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IfcOpenShell, you can jump to the `Core functionality crash course`_.
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Begin learning IFC
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------------------
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IFC has three versions published by ISO: **IFC2X3** from 2007, **IFC4** from
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2017, and **IFC4X3** in draft form. Each version improves on the previous
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version, and will have different **IFC Classes** with different attributes and
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different **IFC Concepts**.
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You can access the official documentation here:
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- `Official IFC2X3 documentation homepage <https://standards.buildingsmart.org/IFC/RELEASE/IFC2x3/TC1/HTML/>`__
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- `Official IFC4 documentation homepage <https://standards.buildingsmart.org/IFC/RELEASE/IFC4/ADD2_TC1/HTML/>`__
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- `Official IFC4X3 documentation homepage <http://ifc43-docs.standards.buildingsmart.org/>`__
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- `List of all IFC2X3 classes <https://standards.buildingsmart.org/IFC/RELEASE/IFC2x3/TC1/HTML/alphabeticalorder_entities.htm>`__
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- `List of all IFC4 classes <https://standards.buildingsmart.org/IFC/RELEASE/IFC4/ADD2_TC1/HTML/link/inheritance-general-usage-all%20entities.htm>`__
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- `List of all IFC4X3 classes <https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/annex-c.html>`__
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.. tip::
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It is recommended to use IFC4. However, the IFC4X3 documentation is a lot
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more friendly to newcomers.
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The official ISO documentation is written for a technical audience and may be
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overwhelming. This guide will take you slowly through the core concepts, and
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leave you with the knowledge you need to discover more.
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Before digging into theory, let's explore an existing IFC model. You can
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download this sample IFC for this guide.
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.. container:: blockbutton
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`Download sample IFC <https://www.ifcwiki.org/images/e/e3/AC20-FZK-Haus.ifc>`__
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If you open up the model with a text editor, you will see text similar to this:
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::
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ISO-10303-21;
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HEADER;FILE_DESCRIPTION(('ViewDefinition [, QuantityTakeOffAddOnView, SpaceBoundary2ndLevelAddOnView]'),'2;1');
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FILE_NAME('AC20-FZK-Haus.ifc','2016-12-21T17:54:06',('Architect'),(''),'','','');
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FILE_SCHEMA(('IFC4'));
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ENDSEC;
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DATA;
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#3= IFCORGANIZATION($,'Nicht definiert',$,$,$);
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#12= IFCOWNERHISTORY(#7,#11,$,.ADDED.,$,$,$,1482339244);
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#13= IFCSIUNIT(*,.LENGTHUNIT.,$,.METRE.);
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#14= IFCSIUNIT(*,.AREAUNIT.,$,.SQUARE_METRE.);
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...
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#62= IFCGEOMETRICREPRESENTATIONCONTEXT($,'Model',3,1.00000000000E-5,#59,#60);
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#66= IFCPROJECT('0lY6P5Ur90TAQnnnI6wtnb',#12,'Projekt-FZK-Haus','Projekt FZK-House create by KHH Forschuungszentrum Karlsruhe',$,$,$,(#62,#374),#49);
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#77= IFCPROPERTYSINGLEVALUE('GS_TimeStamp',$,IFCTIMESTAMP(9685146),$);
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#85= IFCPROPERTYSET('1mnk_H9cG6eU2r9ped0WRu',#12,'GSPset_TimeStamp',$,(#77));
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...
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#15033= IFCSHAPEREPRESENTATION(#15026,'Axis','Curve2D',(#15031));
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#15037= IFCPRODUCTDEFINITIONSHAPE($,$,(#15016,#15024,#15033));
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#15042= IFCWALLSTANDARDCASE('2XPyKWY018sA1ygZKgQPtU',#12,'Wand-Int-ERDG-4',$,$,#14983,#15037,'BC6F0F70-6195-495E-A2-FC-239713029DB1',$);
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#15046= IFCMATERIAL('Leichtbeton 102890359',$,$);
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...
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#15231= IFCRELDEFINESBYPROPERTIES('3Q0nMR5elnJFWzAhgkZqe1',#12,$,$,(#15042),#15229);
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#15234= IFCWALLTYPE('2AEMyYvIjlsz7LRzqYHy64',#12,'Leichtbeton 102890359 240',$,$,$,(#15244,#15248,#15250,#17288,#17290,#17292,#18637,#18639,#18641,#19015,#19017,#19019,#20770,#20772,#20774),'8A396F22-E52B-6FDB-D1D5-6FDD2247C184',$,.NOTDEFINED.);
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#15237= IFCDIRECTION((1.,0.,0.));
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#15239= IFCDIRECTION((0.,0.,1.));
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... etc
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The first thing you should notice is the line that defines that this is an
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**IFC4** version. This determines what **IFC Classes** and **IFC Concepts** are
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available.
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::
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FILE_SCHEMA(('IFC4'));
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You'll notice certain **IFC Class** keywords jump out at you: things like
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**IFCSIUNIT** which defines the length unit of metres, or **IFCPROJECT** which
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defines the project, or **IFCPROPERTYSINGLEVALUE** which defines a property of
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something, or **IFCWALLSTANDARDCASE** which defines a wall, or **IFCMATERIAL**
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which defines a material, and so on.
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Let's see how to fetch this data with code. Let's start with loading the model.
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Import the IfcOpenShell module, then use the ``open`` function to load the
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model into a variable called ``model``. The first piece of information we want
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to check is what IFC schema version we are using. We assume the model you are
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learning with is IFC4. We'll then fetch all entities that use the **IfcSlab**
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class.
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.. code-block:: python
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import ifcopenshell
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model = ifcopenshell.open('/path/to/your/model.ifc')
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print(model.schema) # May return IFC2X3, IFC4, or IFC4X3.
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print(model.by_type("IfcSlab")) # Will return a list of IFCSLAB entities, like below:
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# [
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# #34509=IfcSlab('1pPHnf7cXCpPsNEnQf8_6B',#12,'Bodenplatte',$,$,#34464,#34505,'E4D9CD4B-CA43-4735-94-BD-1FD4376BD455',.BASESLAB.),
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# #59290=IfcSlab('2RGlQk4xH47RHK93zcTzUL',#12,'Slab-033',$,$,#59253,#59286,'DA0A17AC-B773-47AC-99-C5-D390C73AD5CC',.FLOOR.),
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# #59553=IfcSlab('07Enbsqm9C7AQC9iyBwfSD',#12,'Dach-1',$,$,#59508,#59549,'E142B455-80E4-4B96-83-EC-E1589CA998DB',.ROOF.),
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# #59753=IfcSlab('2IxUUNUVPB6Ob$eicCfP2N',#12,'Dach-2',$,$,#59716,#59749,'BD6D9414-37DF-40A8-88-40-301A32A9A5B5',.ROOF.)
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# ]
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.. tip::
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Try changing ``model.by_type("IfcSlab")`` to fetch different types of
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entities based on their **IFC Class**.
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An overview of all IFC classes
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------------------------------
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There are hundreds of **IFC Classes**. You don't need to know them all, but
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we'll help describe the general breakdown so you know where to find the
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appropriate class for what you're after.
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**IFC Classes** are defined using an **Object Oriented** tree hierarchy. Child
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**IFC Classes** inherit the attributes defined by the parent **IFC Class**.
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This means that **IFC Classes** with common attributes are grouped together in
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the tree.
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For example, because all **IfcObject** classes can have a **GlobalId**
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attribute, that means that because **IfcWall** is a subtype of **IfcObject**,
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it can also have a **GlobalId** attribute.
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.. image:: images/ifc-tree.png
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Important IFC concepts
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----------------------
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There are hundreds of **IFC Concepts** that allow you to describe relationships
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between **IFC Classes**. In this guide, we'll focus on the five most common
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**IFC Concepts** to get you started.
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Concept 1: the project context
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------------------------------
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Concept 2: spatial decomposition
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--------------------------------
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Concept 3: object typing
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------------------------
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Concept 4: attributes and property sets
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---------------------------------------
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Concept 5: material assignment
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------------------------------
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Self-learning IFC: how to learn more
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------------------------------------
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