IFC Merge

Collaborative BIM

ifcmerge is a three-way-merge tool for IFC files, intended to support a modern fork, branch, pull-request and merge workflow using revision control systems such as git or mercurial. This enables multiple people to work on separate copies of the same IFC data, periodically merging their work.

This tool requires that a Native IFC application, such as BlenderBIM, is used for all authoring and editing.

A Native IFC application behaves in the following ways when editing a pre-existing IFC (STEP/SPF) file:

1. IFC entities must be written in the same format as received, with the same numeric IDs as before.

2. Attribute changes to entities must be written in-place.

3. Numeric IDs of deleted entities must not be reused for new entities.

If you use a traditional BIM application that saves in a proprietary format, and that exports IFC files, you probably do not have a Native IFC application :(

This whitepaper shows why you might want to use Native IFC for your work: https://github.com/brunopostle/ifcmerge/blob/main/docs/whitepaper.rst

This video presentation shows interactive use of ifcmerge in the BlenderBIM application: https://peertube.linuxrocks.online/w/jotEqADodmuYz8J1Sku7B2

Quickstart

Given a base IFC file and two different forked versions of it, combine the changes from the two forks into a merged result like so:

ifcmerge base.ifc local_fork.ifc remote_fork.ifc result_merged.ifc

Using ifcmerge with git

Configure git to add ifcmerge to the list of available merge tools (set the path to suit your installation location):

git config --global mergetool.ifcmerge.cmd '/path/to/ifcmerge $BASE $LOCAL $REMOTE $MERGED'
git config --global mergetool.ifcmerge.trustExitCode true

Assuming you already have a git repository containing test_model.ifc. Create a new branch, edit and commit some changes to the IFC file in this branch:

git branch my_branch
git switch my_branch
  [some editing of the IFC file]
git commit test_model.ifc

(The procedure is similar with a remote pull request: create a temporary local branch and use git pull to update it with the remote changes rather than making those changes yourself)

Switch back to the original main branch where the IFC file remains unmodified, edit and commit some different changes:

git switch main
  [some editing of the IFC file]
git commit test_model.ifc

At this point the two branches have diverged, instruct git to merge them:

git merge my_branch

This will not complete, resulting in an unresolved conflict, because the default git merge will always find conflicts between two versions of the same IFC file. Complete the merge by resolving the conflict using ifcmerge:

git mergetool --tool=ifcmerge

Commit the merge if it is successful (i.e. with no error messages):

git commit -i test_model.ifc

Otherwise, if ifcmerge refuses because it can't safely merge the branches, such as when an entity has been modified in one branch and deleted in the other, you can always abandon the merge:

git merge --abort

If your repository only contains IFC files, you can set git mergetool to default to using ifcmerge:

git config merge.tool ifcmerge

About

Copyright 2022, Bruno Postle bruno@postle.net License: GPLv3