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2D polygon robustness #1493

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2D polygon robustness #1493

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bmhan12
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@bmhan12 bmhan12 commented Jan 21, 2025

This PR:

  • Adds robustness example to unit tests from axom::primal::clip() for polygons is unreliable. #1481
  • Fixes robustness issue with 2D polygon clipping by:
    • Adding missing tolerance parameter to intersect overload between plane and segment
    • Generalizing, simplifying logic to handle duplicate vertices in Sutherland–Hodgman algorithm by performing a pass at the end to remove duplicates.

Closes #1481

@bmhan12 bmhan12 added the Primal Issues related to Axom's 'primal component label Jan 21, 2025
rhornung67
rhornung67 approved these changes Jan 21, 2025
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Thank you @bmhan12

BradWhitlock
BradWhitlock approved these changes Jan 21, 2025
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Thanks for fixing this so soon @bmhan12 !

kennyweiss
kennyweiss approved these changes Jan 22, 2025
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Thanks @bmhan12!

Please note the bugfix in the RELEASE-NOTES before merging

BradWhitlock
BradWhitlock reviewed Jan 22, 2025
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src/axom/primal/operators/detail/clip_impl.hpp
for(int i = 0; i < outputList.numVertices(); i++)
{
int prevIndex = ((i - 1) == -1) ? (outputList.numVertices() - 1) : (i - 1);
if(outputList[i] != outputList[prevIndex])
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I have a late follow-up question about this point test. Might tolerances need to apply here too? I ask because I tried the fixed code with 2 quads where the resulting output ought to be a triangle but it outputs a quad where the first 2 points are essentially the same.

void test4()
{
  using Polygon = axom::primal::Polygon<double, 2>;
  using Point = axom::primal::Point<double, 2>;
  constexpr double eps = 1.e-6;

  Polygon p1;
  p1.addVertex(Point{0., 0.});
  p1.addVertex(Point{1., 0.});
  p1.addVertex(Point{1., 1.});
  p1.addVertex(Point{0., 1.});

  Polygon p2;
  p2.addVertex(Point{0., 0.});
  p2.addVertex(Point{0.3, 0.3});
  p2.addVertex(Point{0., 1.});
  p2.addVertex(Point{-0.3, 0.7});

  const auto overlapPoly = axom::primal::clip(p1, p2, eps);
  const auto area = overlapPoly.area();
  std::cout << "subject=" << p1
            << ", clip=" << p2
            << ", overlap=" << overlapPoly
            << ", area=" << area
            << std::endl;

// Leads to a polygon with an extra vertex... area is good.
// subject={4-gon:(0,0),(1,0),(1,1),(0,1)}, clip={4-gon:(0,0),(0.3,0.3),(0,1),(-0.3,0.7)}, overlap={4-gon:(-6.66134e-17,1),(0,1),(0,0),(0.3,0.3)}, area=0.15

}

image

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@BradWhitlock BradWhitlock BradWhitlock approved these changes

@Arlie-Capps Arlie-Capps Arlie-Capps approved these changes

@rhornung67 rhornung67 rhornung67 approved these changes

@adayton1 adayton1 adayton1 approved these changes

@kennyweiss kennyweiss kennyweiss approved these changes

@jcs15c jcs15c Awaiting requested review from jcs15c

@gunney1 gunney1 Awaiting requested review from gunney1

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axom::primal::clip() for polygons is unreliable.
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@bmhan12 @BradWhitlock @kennyweiss @adayton1 @rhornung67 @Arlie-Capps