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 BonsaiBIM, 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 BonsaiBIM 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

Command Line Options

By default, ifcmerge prioritises the remote fork when renumbering entity IDs and resolving conflicts. You can reverse this behavior:

ifcmerge --prioritise-local base.ifc local_fork.ifc remote_fork.ifc result_merged.ifc

--prioritise-local: Preserves local entity IDs and renumbers remote IDs when there are conflicts. Also affects which values are selected during conflict resolution (though conflicts still cause the merge to fail).

Error Reporting

When conflicts are detected, ifcmerge outputs structured JSON error reports:

{
  "status": "failed",
  "message": "Merge failed due to conflicts",
  "conflicts": [
    {
      "type": "attribute_conflict",
      "entity_id": 748,
      "entity_class": "IFCWINDOW",
      "attribute_index": 3,
      "message": "Attribute conflict in both local and remote (prioritising remote)",
      "base_value": "'Window-1'",
      "local_value": "'Window-Local'",
      "remote_value": "'Window-Remote'",
      "selected_value": "'Window-Remote'"
    }
  ]
}

This JSON output makes it easier to integrate ifcmerge with automated tools and provides detailed information about what conflicts occurred and where.

Automatic Conflict Resolution

Some conflicts that previously required manual intervention are now resolved automatically:

• IfcLocalPlacement conflicts: When both forks modify object placements, ifcmerge automatically selects one based on the priority setting (remote by default, or local with --prioritise-local)

Other conflicts (attribute modifications, entity deletions, etc.) still cause the merge to fail and require manual resolution.

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

A second tool, ifcmerge-forward, uses --prioritise-local so that the local (checked-out) branch takes priority. BonsaiBIM configures both tools automatically, but you can add them manually:

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

The reason two tools are needed is that when step-IDs are renumbered to resolve overlapping additions, they should always be renumbered in the temporary branch, never in main or master. The choice of tool depends on which branch you have checked out:

• On a working branch, merging from main/master/origin: use ifcmerge. The remote is main, so main's step-IDs are preserved and your branch's IDs are renumbered.

• On main/master, merging in a temporary branch: use ifcmerge-forward. The local branch is main, so main's step-IDs are preserved and the temporary branch's IDs are renumbered.

In both cases the invariant holds: step-IDs in main/master are never rewritten.

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. ifcmerge exits 0 and reports no conflicts):

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

Installation with Windows

1. Download ifcmerge.exe from the latest release.

2. Add to PATH so that ifcmerge can be found by git and other tools:

   1. Create a directory such as C:\Program Files\ifcmerge\ and place ifcmerge.exe there.
   2. Open Environment Variables: right-click This PC → Properties → Advanced system settings → Environment Variables....
   3. Under System Variables (or User Variables), select Path and click Edit....
   4. Click New and add C:\Program Files\ifcmerge.
   5. Click OK to save.

3. Configure git to use ifcmerge as a merge tool (in a Command Prompt or Git Bash):

   git config --global mergetool.ifcmerge.cmd 'ifcmerge "$BASE" "$LOCAL" "$REMOTE" "$MERGED"'
   git config --global mergetool.ifcmerge.trustExitCode true
   git config --global mergetool.ifcmerge-forward.cmd 'ifcmerge --prioritise-local "$BASE" "$LOCAL" "$REMOTE" "$MERGED"'
   git config --global mergetool.ifcmerge-forward.trustExitCode true
   

   See Using ifcmerge with git for guidance on which tool to use depending on merge direction. GUI git clients such as Sourcetree and TortoiseGit only support a single custom merge tool, so prefer the command-line git configuration above to get both.

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