Method ComputeStickyAxesBoundingSphere

Rendering/Annotation/vtkCubeAxesActor.cxx:2562–2720

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void vtkCubeAxesActor::ComputeStickyAxesBoundingSphere(vtkViewport* viewport,
  const double originalBounds[6], double sphereCenter[3], double& sphereRadius)
{
  double aspect[2];
  viewport->GetAspect(aspect);
  vtkPlanes* frustumPlanes = vtkPlanes::New();
  double frustumPlanesArray[24];
  this->GetCamera()->GetFrustumPlanes(aspect[0], frustumPlanesArray);
  frustumPlanes->SetFrustumPlanes(frustumPlanesArray);

  vtkFrustumSource* frustumSource = vtkFrustumSource::New();
  frustumSource->SetPlanes(frustumPlanes);
  frustumPlanes->Delete();
  frustumSource->Update();

  vtkPoints* points = frustumSource->GetOutput()->GetPoints();

  // From http://gamedev.stackexchange.com/questions/60104/largest-sphere-inside-a-frustum
  // Point indices are set up to match the second figure.
  double p0[3], p1[3], p2[3], p3[3], p4[3];
  double q0[3], q1[3], q2[3], q3[3], q4[3];
  points->GetPoint(0, p1); // left bottom near
  points->GetPoint(1, p2); // right bottom near
  points->GetPoint(2, p4); // right top near
  points->GetPoint(3, p3); // left top near

  points->GetPoint(4, q1); // left bottom far
  points->GetPoint(5, q2); // right bottom far
  points->GetPoint(6, q4); // right top far
  points->GetPoint(7, q3); // left top far

  for (int i = 0; i < 3; ++i)
  {
    p0[i] = 0.25 * (p1[i] + p2[i] + p3[i] + p4[i]); // near center
    q0[i] = 0.25 * (q1[i] + q2[i] + q3[i] + q4[i]); // far center
  }
  frustumSource->Delete();

  double view[3];
  vtkMath::Subtract(p0, q0, view);
  double d = vtkMath::Norm(view);

  double v0[3], v1[3];
  vtkMath::Subtract(p1, q1, v0);
  vtkMath::Subtract(q2, q1, v1);
  double l = 0.5 * vtkMath::Norm(v1);
  double alpha = atan(vtkMath::Dot(v0, v1) / (d * vtkMath::Norm(v1)));
  double halfWidth = l * tan((vtkMath::Pi() - 2.0 * alpha) / 4.0);

  vtkMath::Subtract(q3, q1, v1);
  l = 0.5 * vtkMath::Norm(v1);
  alpha = atan(vtkMath::Dot(v0, v1) / (d * vtkMath::Norm(v1)));
  double halfHeight = l * tan((vtkMath::Pi() - 2.0 * alpha) / 4.0);

  sphereRadius = std::min(halfWidth, halfHeight);

  vtkMath::Normalize(view);
  sphereCenter[0] = q0[0] + sphereRadius * view[0];