ShapeUpgrade_UnifySameDomain with UnifyFaces=True causes severe performance degradation #1477

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1. tpaviot commented on Jan 29, 2026

   

   tpaviot

   on Jan 29, 2026

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   Can you please include the test step file and a code ready to be executed/tested
2. ChaitanyaYeole02 commented on Jan 29, 2026

   

   ChaitanyaYeole02

   on Jan 29, 2026

   Last edited by ChaitanyaYeole02

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   Test step file:
   Step File with Multiple Threads

   Executable Code:

   from pathlib import Path
   
   from OCC.Core.BRepBuilderAPI import (
       BRepBuilderAPI_Sewing, BRepBuilderAPI_MakeSolid
   )
   from OCC.Core.IFSelect import IFSelect_RetDone
   from OCC.Core.STEPControl import STEPControl_Reader
   from OCC.Core.ShapeFix import ShapeFix_Shape
   from OCC.Core.ShapeUpgrade import ShapeUpgrade_UnifySameDomain
   from OCC.Core.TopAbs import TopAbs_SHELL, TopAbs_SOLID
   from OCC.Core.TopExp import TopExp_Explorer
   from OCC.Core.TopoDS import topods
   
   
   class CADReader:    
       def __init__(self, file_path: str):
           self.file_path = Path(file_path)
           self.shape = None
   
       def read_step_file(self):
           reader = STEPControl_Reader()
           transfer_result = reader.ReadFile(str(self.file_path))
           
           if transfer_result != IFSelect_RetDone:
               raise ValueError(
                   f"Failed to read STEP file. Error code: {transfer_result}"
               )
           
           transfer_result = reader.TransferRoots()
           if transfer_result != IFSelect_RetDone:
               raise ValueError(
                   f"Failed to transfer STEP file. Error code: {transfer_result}"
               )
           
           self.shape = reader.OneShape()
           if self.shape.IsNull():
               raise ValueError("No valid shape found in STEP file")
       
       def fix_shape(self):
           fixer = ShapeFix_Shape(self.shape)        
           fixer.Perform()
           self.shape = fixer.Shape()
   
       def sew_shape(self):
           sewing = BRepBuilderAPI_Sewing(1e-4)
           sewing.Add(self.shape)
           sewing.Perform()
           self.shape = sewing.SewedShape()
       
       def ensure_solid(self):
           solid_explorer = TopExp_Explorer(self.shape, TopAbs_SOLID)
           
           if not solid_explorer.More():
               shell_explorer = TopExp_Explorer(self.shape, TopAbs_SHELL)
               
               if shell_explorer.More():
                   ms = BRepBuilderAPI_MakeSolid()
                   
                   while shell_explorer.More():
                       shell = topods.Shell(shell_explorer.Current())
                       ms.Add(shell)
                       shell_explorer.Next()
                   
                   if ms.IsDone():
                       self.shape = ms.Solid()
       
       def unify_domains(self):        
           unify = ShapeUpgrade_UnifySameDomain(self.shape, True, True, False)
           unify.Build()
           self.shape = unify.Shape()
       
       def read(self) -> 'CADReader':
           print("Reading STEP file")
           self.read_step_file()
   
           print("Fixing shape")
           self.fix_shape()
           
           print("Sewing shape")
           self.sew_shape()
           
           print("Ensuring solid")
           self.ensure_solid()
           
           print("Unifying domains")
           self.unify_domains()
           
           print("Done")
   
           return self
       
       def get_shape(self):
           """
           Return the processed OCC shape.
   
           Raises if `read()` has not been called yet.
           """
           if self.shape is None:
               raise ValueError("Shape not loaded. Call read() first.")
           
           return self.shape
   
   
   def main():
       step_file = "sample_files/threadalltypes.step"
       
       # Create reader and process the file
       reader = CADReader(step_file)
       shape = reader.read().get_shape()
       print(shape)
   
   
   if __name__ == "__main__":
       main()

   @tpaviot
3. ChaitanyaYeole02 commented on Feb 16, 2026

   

   ChaitanyaYeole02

   on Feb 16, 2026

   Author

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   Hi @tpaviot ,
   Just wanted to follow up on issue, when you get a chance. Just checking if you've had time to take a look at it.
4. tpaviot commented on Sep 22, 2026

   

   tpaviot

   on Sep 22, 2026

   Owner

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   ShapeUpgrade_UnifySameDomain has no cache or lazy initialization: the time is spent comparing the faces of your shape, so a warm-up on another file shouldn't make any difference. If you measured one, please share the timings of each step (read, fix, sew, unify) for the first and second call.

   Also note that OCCT 8.0 fixes a case where UnifySameDomain could loop with 100% CPU on periodic surfaces (OCCT #557). It's not in any 7.x release. The next pythonocc-core release is based on OCCT 8.0.1. It would be worth testing your file with it once released (in progress).
5. tpaviot

   closed this as completedon Sep 22, 2026