`ShapeUpgrade_UnifySameDomain` with `UnifyFaces=True` causes severe performance degradation

[ChaitanyaYeole02]

Problem Summary

When using ShapeUpgrade_UnifySameDomain with UnifyFaces=True, I experience:
First call in a Python session takes 75-90 seconds (lazy initialization)

Environment

• OpenCascade Version: 7.8.1 (via pythonocc-core)
• Python Version: 3.12
• OS: macOS (darwin 25.2.0)

Minimal Reproduction

Original Code

from OCC.Core.STEPControl import STEPControl_Reader
from OCC.Core.ShapeUpgrade import ShapeUpgrade_UnifySameDomain

# Read STEP file
reader = STEPControl_Reader()
reader.ReadFile("file_with_multiple_threads.step")  # 465 faces
reader.TransferRoots()
shape = reader.OneShape()

# Unify faces (SLOW!)
unify = ShapeUpgrade_UnifySameDomain(
    shape,
    True,   # UnifyEdges
    True,   # UnifyFaces - THIS IS THE BOTTLENECK
    False   # ConcatBSplines
)
unify.Build()
unified_shape = unify.Shape()

Fast Alternative

# Just read the file, no unification
reader = STEPControl_Reader()
reader.ReadFile("file_with_multiple_threads.step")
reader.TransferRoots()
shape = reader.OneShape()

# Result: 465 faces (no reduction)
# Problem: Circular planes split into hemispheres, cylinders split

Current Workaround

I observed that a warm-up strategy to mitigate the cold start:

# Warm up with small file
def warmup_cad_reader(warmup_file: str = "small_file.stp"):
    """Process small file to initialize OpenCascade caches"""
    reader = STEPControl_Reader()
    reader.ReadFile(warmup_file)
    reader.TransferRoots()
    shape = reader.OneShape()
    
    unify = ShapeUpgrade_UnifySameDomain(shape, True, True, False)
    unify.Build()
    return True

Current Optimizations Applied

unify = ShapeUpgrade_UnifySameDomain(shape, True, True, False)
unify.SetLinearTolerance(5e-4)      # 0.5mm tolerance
unify.SetAngularTolerance(0.1)      # ~5.7 degrees
unify.SetSafeInputMode(False)       # Skip extra validation
unify.AllowInternalEdges(True)      # Handle complex topology
unify.Build()

Questions

1. Cold Start Penalty:

   • Why does the first call to ShapeUpgrade_UnifySameDomain.Build() take 75-90 seconds?
   • Is there a way to pre-initialize these internal caches without processing a file?

2. Performance Optimization:

   • Is there a way to make face unification faster while maintaining accuracy?
   • Can the O(n²) face comparison be optimized (spatial indexing, parallel processing)?

3. Alternative Approaches:

   • Is there a different algorithm or class that can merge split faces more efficiently?

Would love to discuss and know more about this @tpaviot

[tpaviot]

Can you please include the test step file and a code ready to be executed/tested

[ChaitanyaYeole02]

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

[ChaitanyaYeole02]

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.