Add progress callback support to upstream MMG library

[kmarchais]

Summary

Currently, mmgpy's progress reporting only shows phase-level progress (0% at start, 100% at complete) because the underlying MMG library doesn't expose fine-grained progress callbacks. This issue tracks the work needed to propose and implement progress callbacks in the upstream MMG library.

Current Behavior

The progress bar jumps from 0% to 100% instantly when remeshing:

• Init phase: 0% → 100%
• Options phase: 0% → 100%
• Remesh phase: 0% → 100% (entire remeshing happens here, could take minutes for large meshes)

Desired Behavior

Discrete progress updates during the remeshing phase, showing actual algorithm progress:

• Iteration 1/10: 10%
• Iteration 2/10: 20%
• etc.

Technical Feasibility

Analysis of the MMG library source code (mmg3d1_delone.c) shows this is highly feasible:

1. Main adaptation loop (MMG5_adpdel(), line 642-744) runs up to maxit=10 iterations
2. Existing progress output at line 715-717 already tracks operations:

   fprintf(stdout,"     %8d filtered, %8d splitted, %8d collapsed,"
           " %8d swapped, %8d moved\n",ifilt,ns,nc,nf,nm);

3. MMG5_Info structure (libmmgtypes.h, lines 523-564) could be extended to hold callback function pointers

Proposed Changes to MMG Library

1. Add callback function pointer to MMG5_Info:

   typedef int (*MMG5_progressCallback)(int phase, int iteration, int max_iterations, void* user_data);
   MMG5_progressCallback progressCb;
   void* progressUserData;

2. Add setter functions in the public API:

   int MMG3D_Set_progressCallback(MMG5_pMesh mesh, MMG5_progressCallback cb, void* user_data);

3. Call the callback at key points:

   • Before each iteration in MMG5_adpdel()
   • At phase transitions (Analysis → Meshing → Packing)

4. Allow cancellation via callback return value (non-zero = cancel)

Implementation Steps

1. Fork/clone MMG repository
2. Implement callback infrastructure in libmmgtypes.h
3. Add callback invocations in mmg3d1_delone.c
4. Create setter functions in libmmg3d.c
5. Add corresponding functions for mmg2d and mmgs
6. Test with a simple C program
7. Submit PR to upstream MMG repository
8. Update mmgpy bindings once upstream accepts the changes

Related

• mmgpy PR feat(progress): add progress callbacks with cancellation support #149: Added progress callback framework (phase-level progress)
• mmgpy issue Add progress callbacks for long remeshing operations #126: Original feature request for progress callbacks