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Auto Structural Mapping

Use Automated Structural Mapping to segment a triangulated mesh (TIN) into coherent planar patches using dip/azimuth orientation and an optional coplanarity metric. The tool writes per‑triangle attributes and can create per‑patch geometry objects for downstream analysis.

Prerequisites​

  • A selected TriangulatedMesh in the Project Tree.
  • Triangle attributes available on the mesh:
    • Dip (degrees) and Azimuth (degrees) for orientation‑based growth.
    • CoPlanarity (scalar) for seeding and optional non‑orientation growth/filtering.

If any of these are missing, compute or import them first. When dip/azimuth are missing, the algorithm falls back to face normals (forced to the +Z hemisphere), but results may be less geologically meaningful.

Launch​

  • Project Tree: select a triangulated mesh, then run Attributes → Patch Decomposition (opens “Automated Structural Mapping”).
  • Buttons: Apply runs without closing; OK runs and closes.

Dialog Fields​

Attributes​

  • Coplanarity: Choose the layer used to order seeds and, optionally, to filter triangles during growth.
  • Use Coplanarity Filter: When on, respects the coplanarity mapper’s min/max window. Triangles with any vertex filtered out are excluded; seeds are limited to values within the window.
  • Dip: Choose the dip attribute layer for orientation growth.
  • Azimuth: Choose the azimuth attribute layer for orientation growth.

Settings​

  • Minimum Patch Size: Minimum number of triangles required for a patch to be accepted.
  • Tolerance Angle: Orientation mode: angular tolerance in degrees for neighbor acceptance. Non‑orientation mode: treated as a scalar tolerance on coplanarity values.
  • Relative Variation: Orientation reference. When on, compares each neighbor to the current triangle (“relative”). When off, compares to the seed triangle. See also “Use Mean Reference”.
  • Fix Single Holes: After growth, fill single‑triangle holes fully surrounded by the patch to clean up small gaps.
  • Create Objects: Create a per‑patch Orientation object named Auto_<ID> (<count>) with collected patch vertices.
  • Dispersion (0..1): Acceptance control for patch coherence when RMin is not used. Patch is accepted if the average axial angle to the patch mean is ≤ Dispersion * Tolerance Angle. Default 0.75.
  • Use Mean Reference: Growth reference is a running axial mean of the patch, not the seed/current triangle. More robust on smoothly varying surfaces.
  • RMin (0..1): Mean resultant length threshold for acceptance in orientation mode. If > 0, supersedes Dispersion. Typical values: 0.7–0.9 (tight), 0.5–0.7 (looser).

Filter​

  • Dip: Enable and set [min, max] in degrees. The patch’s axial mean dip must fall within this window to be accepted.
  • Azimuth: Enable and set [min, max] in degrees. Wrap‑around is supported: e.g., 350–10 accepts angles ≥ 350 or ≤ 10.
  • Get From Selected: Populates Dip/Azimuth windows from selected orientation objects or measurements (Strike/Dip). The +/- margin expands the window on both sides.
  • +/-: Margin in degrees used by “Get From Selected”.

What It Writes​

  • Triangle attributes added:
    • Patches (int): Patch ID (0 = unassigned).
    • Patch Size (int): Number of triangles in the patch.
    • Patch Difference (float):
      • Orientation mode: angular difference to the patch mean in radians (0..π/2, axial).
      • Non‑orientation mode: absolute difference in coplanarity values.
    • Patch Similarity (float): |dot(pole, patch_mean)| in [0..1] (orientation mode only).
  • Optional objects: A per‑patch Orientation object when Create Objects is enabled.

How It Works (Summary)​

Implementation: CAutoStructureMappingDlg drives CTINPatchDecomposer.

  1. Attribute setup
  • SetAttributes(Dip, Azimuth, CoPlanarity, UseCoplanarityFilter). If Coplanarity is present and filtering is enabled, seed candidates are restricted to the mapper’s range and triangles with filtered vertices are excluded during growth.
  1. Seed ordering
  • Triangles are ordered by average coplanarity (highest first) when available; otherwise by index. Filtered triangles are skipped when filter is on.
  1. Patch growth
  • Orientation mode (default): Accepts neighbor triangles whose axial angle to the chosen reference is ≤ Tolerance Angle.
    • Reference can be: current triangle (Relative), seed triangle, or running mean (Use Mean Reference).
  • Non‑orientation mode: Uses coplanarity differences instead of angles.
  1. Acceptance
  • Must meet Minimum Patch Size and Dip/Azimuth filter windows (if enabled).
  • Coherence controls (orientation mode):
    • If RMin > 0: compute mean resultant length R = |Σ aligned poles| / N and require R ≥ RMin.
    • Else: require average axial angle to the patch mean ≤ Dispersion * Tolerance Angle.
  1. Post‑processing
  • Optional single‑triangle hole fill; attribute writing; hide consumed triangles during sweep to avoid reseeding; unhide at end.

Tips For Best Results​

  • Start conservative: Tolerance Angle 10–20°. If patches are too fragmented, increase gradually or enable Use Mean Reference.
  • Use RMin for consistency: Prefer RMin (e.g., 0.8) when you want patches with uniform orientation, independent of the chosen tolerance.
  • Target known sets: Enable Dip/Azimuth filters to focus on expected planes (e.g., bedding or dominant fracture sets).
  • Wrap‑around azimuth: For ranges that cross 0°, set min > max (e.g., 350–10). The filter treats this as a wrap.
  • Smoothly varying surfaces: Enable Use Mean Reference to let the patch “follow” gentle curvature without drifting.
  • Clean boundaries: Leave Relative Variation off when boundaries are crisp; turn it on to allow local adaptation, but watch for gradual drift.
  • Faster iteration: Try Apply without Create Objects first. Create Objects can be heavier on large meshes.

Troubleshooting​

  • “Missing Attributes: Need Dip, Azimuth and Coplanarity”: Ensure the mesh has all three attribute layers selected in the dialog. If coplanarity is not used, you still need to choose a layer; you can disable Use Coplanarity Filter if you don’t want it to constrain growth.
  • No patches created or far too few:
    • Increase Tolerance Angle, reduce Minimum Patch Size, disable Dip/Azimuth filters, or relax Dispersion/increase RMin.
    • Verify Dip/Azimuth are in degrees and valid per triangle.
  • Over‑merged patches (too large/indistinct):
    • Reduce Tolerance Angle, increase RMin, or lower Dispersion.
    • Narrow Dip/Azimuth filter windows to isolate sets.
  • Azimuth window behaves oddly: Check wrap‑around. Use ranges like 350–10 for values straddling 0°.
  • Attributes look constant or zero:
    • Confirm you selected the intended layers in Attributes. Recompute Dip/Azimuth/Coplanarity if needed.
  • Performance concerns:
    • Subsequent Apply runs in the same dialog avoid recomputing connectivity. Close/reopen only when necessary.

Reference: Parameter Mapping​

  • Minimum Patch Size → MinTriangles
  • Tolerance Angle → MaxAng (degrees in orientation mode)
  • Relative Variation → IsRelative
  • Fix Single Holes → bFixHoles
  • Create Objects → bSaveData
  • Dip/Azimuth filters → SetDipFilter, SetAzimuthFilter
  • Use Coplanarity Filter → passed to SetAttributes(..., UseCoplanarityFilter)
  • Dispersion → SetDispersionFraction
  • Use Mean Reference → UseMeanReference
  • RMin → SetResultantMin

See also: internal notes in VRGS2020/CTINPatchDecomposer.md for lower‑level details of the algorithm and outputs.