util.unit: scale RepresentationContext Precision on unit conversion #6127

IfcGeometricRepresentationContext.Precision is typed as a plain IfcReal
but is interpreted in the project length unit, so the IfcLengthMeasure
traversal in convert_file_length_units never touched it. A model
converted from mm to m kept a Precision of e.g. 0.01 (fine in mm, huge
in m), which breaks downstream geometry interpretation such as
IfcConvert boolean cleanup.

Subcontexts derive Precision from their parent, so only root contexts
are scaled.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/ifcopenshell-python/ifcopenshell/util/unit.py

@@ -912,6 +912,13 @@ def convert_file_length_units(ifc_file: ifcopenshell.file, target_units: str = "
             new_value = convert_value(val)
             setattr(element, attr.name(), new_value)
 
+    # IfcGeometricRepresentationContext.Precision is typed as a plain IfcReal
+    # but is interpreted in the project length unit, so it must be scaled too.
+    # Subcontexts derive Precision from their parent and cannot be set.
+    for context in file_patched.by_type("IfcGeometricRepresentationContext", include_subtypes=False):
+        if context.Precision is not None:
+            context.Precision = convert_unit(context.Precision, old_length, new_length)
+
     has_map_unit = False
     if (
         ifc_file.schema == "IFC2X3"

src/ifcopenshell-python/test/util/test_unit.py

@@ -19,6 +19,7 @@
 from math import pi
 
 import numpy as np
+import pytest
 
 import ifcopenshell.api.context
 import ifcopenshell.api.georeference
@@ -258,6 +259,23 @@ class TestConvertFileLengthUnits(test.bootstrap.IFC2X3):
         assert max(i.id() for i in output) == len(output.wrapped_data.entity_names()) + 1
         assert subject.get_full_unit_name(subject.get_project_unit(output, "LENGTHUNIT")) == "METRE"
 
+    def test_precision_conversion(self):
+        # Regression test for #6127: IfcGeometricRepresentationContext.Precision
+        # is typed IfcReal but interpreted in the project length unit, so it must
+        # be scaled along with the length measures.
+        ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcProject")
+        unit = ifcopenshell.api.unit.add_si_unit(self.file, unit_type="LENGTHUNIT", prefix="MILLI")
+        ifcopenshell.api.unit.assign_unit(self.file, units=[unit])
+        context = ifcopenshell.api.context.add_context(self.file, context_type="Model")
+        context.Precision = 0.01
+        # Subcontexts derive Precision from the parent and must be left alone.
+        ifcopenshell.api.context.add_context(
+            self.file, context_type="Model", context_identifier="Body", target_view="MODEL_VIEW", parent=context
+        )
+        output = subject.convert_file_length_units(self.file, target_units="METER")
+        new_context = output.by_type("IfcGeometricRepresentationContext", include_subtypes=False)[0]
+        assert new_context.Precision == pytest.approx(0.00001)
+
     def test_attribute_conversion(self):
         ifcopenshell.api.root.create_entity(self.file, ifc_class="IfcProject")
         unit = ifcopenshell.api.unit.add_si_unit(self.file, unit_type="LENGTHUNIT", prefix="MILLI")