VTK笔记——寻找最近点(vtkCellLocator)

当你看到找最近点时，可能会立刻想到在一堆点中，通过距离找出最近的点，没错，这是一种寻找最近点的需求。今天我们要说的是另一种寻找最近点的需求，就是在Mesh上找最近点，这个点不一定是Mesh的顶点。
多边形网格是由很多个小的单元组成，类似于这样：

我们拿其中一个单元格cell来简化模型，
空间中有一个点p，p到cell的最近点要怎么求得？
如果我们把cell想象成一个平面，那就变成了，p到平面ABCD的投影，也是从p点做垂线，求垂线与平面的交点。
没错，这就是vtkCellLocator的基本思想。

vtkCellLocator

vtkCellLocator本身是一个octree。典型的用法就是求最近的单元或者点。

使用方法

// Create the tree
  auto cellLocator = vtkSmartPointer<vtkCellLocator>::New();
  cellLocator->SetDataSet(sphereSource->GetOutput());
  cellLocator->BuildLocator();
  
  double testPoint[3] = {0.6, 0.5, 0.0};
  
  //Find the closest points to TestPoint
  auto assistCell = vtkSmartPointer<vtkGenericCell>::New();
  double closestPoint[3];//the coordinates of the closest point will be returned here
  double closestPointDist2; //the squared distance to the closest point will be returned here
  vtkIdType cellId; //the cell id of the cell containing the closest point will be returned here
  int subId;
  cellLocator->FindClosestPoint(testPoint, closestPoint, assistCell, cellId, subId, closestPointDist2);

注意：

1. 这里FindClosestPoint方法中用到的vtkGenericCell，是为了提高效率，在循环调用时，免除频繁分配和释放资源。
2. 这个方法并不是线程安全的，线程安全，可以考虑，vtkStaticCellLocator

示例

在一个球体上找离测试点最近的点。绿色表示测试点，红色表示求得到最近点。

CellLocator.cxx

#include <vtkSmartPointer.h>
#include <vtkSphereSource.h>
#include <vtkCellLocator.h>
#include <vtkGenericCell.h>
#include <vtkPolyDataMapper.h>
#include <vtkActor.h>
#include <vtkRenderer.h>
#include <vtkRenderWindow.h>
#include <vtkRenderWindowInteractor.h>
#include <vtkProperty.h>

int main(int, char *[])
{
  auto sphereSource = vtkSmartPointer<vtkSphereSource>::New();
  sphereSource->Update();
  
  // Create the tree
  auto cellLocator = vtkSmartPointer<vtkCellLocator>::New();
  cellLocator->SetDataSet(sphereSource->GetOutput());
  cellLocator->BuildLocator();
  
  double testPoint[3] = {0.6, 0.5, 0.0};
  
  //Find the closest points to TestPoint
  auto assistCell = vtkSmartPointer<vtkGenericCell>::New();
  double closestPoint[3];//the coordinates of the closest point will be returned here
  double closestPointDist2; //the squared distance to the closest point will be returned here
  vtkIdType cellId; //the cell id of the cell containing the closest point will be returned here
  int subId;
  cellLocator->FindClosestPoint(testPoint, closestPoint, assistCell, cellId, subId, closestPointDist2);
  
  std::cout << "Coordinates of closest point: " << closestPoint[0] << " " << closestPoint[1] << " " << closestPoint[2] << std::endl;
  std::cout << "Squared distance to closest point: " << closestPointDist2 << std::endl;
  std::cout << "CellId: " << cellId << std::endl;

  auto testSource = vtkSmartPointer<vtkSphereSource>::New();
  testSource->SetRadius(0.01);
  testSource->SetCenter(testPoint);
  testSource->Update();

  auto pointSource = vtkSmartPointer<vtkSphereSource>::New();
  pointSource->SetRadius(0.01);
  pointSource->SetCenter(closestPoint);
  pointSource->Update();

  vtkSmartPointer<vtkPolyDataMapper> sphereMapper = vtkSmartPointer<vtkPolyDataMapper>::New();
  sphereMapper->SetInputConnection(sphereSource->GetOutputPort());
  vtkSmartPointer<vtkPolyDataMapper> testMapper = vtkSmartPointer<vtkPolyDataMapper>::New();
  testMapper->SetInputConnection(testSource->GetOutputPort());
  vtkSmartPointer<vtkPolyDataMapper> pointMapper = vtkSmartPointer<vtkPolyDataMapper>::New();
  pointMapper->SetInputConnection(pointSource->GetOutputPort());

  auto sphereActor = vtkSmartPointer<vtkActor>::New();
  sphereActor->SetMapper(sphereMapper);
  sphereActor->GetProperty()->EdgeVisibilityOn();
  auto pointActor = vtkSmartPointer<vtkActor>::New();
  pointActor->SetMapper(pointMapper);
  pointActor->GetProperty()->SetColor(1, 0, 0);
  auto testActor = vtkSmartPointer<vtkActor>::New();
  testActor->SetMapper(testMapper);
  testActor->GetProperty()->SetColor(0, 1, 0);

  auto render = vtkSmartPointer<vtkRenderer>::New();
  render->AddActor(sphereActor);
  render->AddActor(pointActor);
  render->AddActor(testActor);

  auto renWin = vtkSmartPointer<vtkRenderWindow>::New();
  renWin->SetSize(640, 480);
  renWin->AddRenderer(render);

  auto interactor = vtkSmartPointer<vtkRenderWindowInteractor>::New();
  interactor->SetRenderWindow(renWin);

  renWin->Render();
  interactor->Initialize();
  interactor->Start();
  
  return EXIT_SUCCESS;
}

Ref

vtkCellLocator Class Reference
VTKExamples/Cxx/PolyData/CellLocator