VTK笔记——轴对齐包围盒（AABB）

什么是包围盒

包围盒是指能够包容物体的立方体或者二维长方形，是包围体的一种，常常用于模型的碰撞检测。包围体主要包括球体、轴对齐包围盒(AABB)、有向包围盒(OBB)和凸包(Convex Hull)。
在较早的时候，包围盒按照坐标系的坐标轴进行排列，这被称为轴对齐的包围盒(AABB/Axis-aligned bounding box)。为了将AABB与通用的情况区分开来，将任意的包围盒称为有向包围盒(OBB/Oriented bounding box)。AABB检测模型的相交要比OBB更简单，但是他的缺点是当模型旋转的时候无法随之旋转，而必须重新计算。

轴对齐包围盒

在VTK中，轴对齐包围盒被定义为包含该模型，且边平行于坐标轴的最小六面体。AABB比较简单，存储空间小，仅需六个标量。
获取bounds的方法
1. vtkPolyData
vtkPolyData通过继承关系拥有GetBounds()方法，实际在vtkDataSet中定义，

double *GetBounds() VTK_SIZEHINT(6);
void GetBounds(double bounds[6]);

获取包围盒的参数，即三个轴向上的最大值和最小值。

	vtkSmartPointer<vtkPolyData> polydata =
		vtkSmartPointer<vtkPolyData>::New();
	double bounds[6];
	polydata->GetBounds(bounds);

需要注意的是，通过reader或者source来的模型，需要先Update()；
而有时候仅仅是获取这些参数并不直观，还需要显示包围盒，vtkOutlineFilter提供了这一方便的操作，输入模型的数据，就很容易的可视化显示。

	vtkSmartPointer<vtkOutlineFilter> outline =
		vtkSmartPointer<vtkOutlineFilter>::New();
	outline->SetInputData(polydata);
	outline->Update();

如果需要看实心表面，则需要打开，默认是关闭的。

	outline->GenerateFacesOn();

2. vtkBoundingBox
另一种方法是用vtkBoundingBox的GetBounds方法

  void GetBounds(double bounds[6]) const;
  void GetBounds(double &xMin, double &xMax,
                 double &yMin, double &yMax,
                 double &zMin, double &zMax) const;

vtkBoundingBox是专门用来处理3D bounds的，快速小巧，用了很多内联函数。它没有继承vtkObject，因此可以不用new，直接在栈上分配。
它的输入对的是点，所以对于点云来说很方便。

  double p0[3] = {3, 4, 5};
  vtkBoundingBox boundingBox;
  boundingBox.AddPoint(p0);
  double bounds[6];
  boundingBox.GetBounds(bounds);

效果

示例代码

#include <vtkSmartPointer.h>
#include <vtkPolyData.h>
#include <vtkSphereSource.h>
#include <vtkXMLPolyDataReader.h>
#include <vtkActor.h>
#include <vtkPolyDataMapper.h>
#include <vtkRenderer.h>
#include <vtkRenderWindow.h>
#include <vtkRenderWindowInteractor.h>
#include <vtkOutlineFilter.h>
#include <vtkProperty.h>

int main(int argc, char *argv[])
{
	vtkSmartPointer<vtkPolyData> polydata =
		vtkSmartPointer<vtkPolyData>::New();
	if (argc > 1)
	{
		vtkSmartPointer<vtkXMLPolyDataReader> reader =
			vtkSmartPointer<vtkXMLPolyDataReader>::New();
		reader->SetFileName(argv[1]);
		reader->Update();
		polydata = reader->GetOutput();
	}
	else
	{
		vtkSmartPointer<vtkSphereSource> modelSource =
			vtkSmartPointer<vtkSphereSource>::New();
		modelSource->Update();
		polydata = modelSource->GetOutput();
	}

	vtkSmartPointer<vtkOutlineFilter> outline =
		vtkSmartPointer<vtkOutlineFilter>::New();
	outline->SetInputData(polydata);
	outline->Update();

	double bounds[6];
	polydata->GetBounds(bounds);
	std::cout << "Polydata bounds: " << std::endl;
	std::cout << "xmin: " << bounds[0] << " "
			  << "xmax: " << bounds[1] << std::endl
			  << "ymin: " << bounds[2] << " "
			  << "ymax: " << bounds[3] << std::endl
			  << "zmin: " << bounds[4] << " "
			  << "zmax: " << bounds[5] << std::endl;

	outline->GetOutput()->GetBounds(bounds);
	std::cout << "Outline bounds: " << std::endl;
	std::cout << "xmin: " << bounds[0] << " "
			  << "xmax: " << bounds[1] << std::endl
			  << "ymin: " << bounds[2] << " "
			  << "ymax: " << bounds[3] << std::endl
			  << "zmin: " << bounds[4] << " "
			  << "zmax: " << bounds[5] << std::endl;

	vtkSmartPointer<vtkPolyDataMapper> modelMapper =
		vtkSmartPointer<vtkPolyDataMapper>::New();
	modelMapper->SetInputData(polydata);

	vtkSmartPointer<vtkActor> modelActor =
		vtkSmartPointer<vtkActor>::New();
	modelActor->SetMapper(modelMapper);

	vtkSmartPointer<vtkPolyDataMapper> outlineMapper =
		vtkSmartPointer<vtkPolyDataMapper>::New();
	outlineMapper->SetInputConnection(outline->GetOutputPort());

	vtkSmartPointer<vtkActor> outlineActor =
		vtkSmartPointer<vtkActor>::New();
	outlineActor->SetMapper(outlineMapper);
	outlineActor->GetProperty()->SetColor(1, 0, 0);
	outlineActor->GetProperty()->SetLineWidth(1.5);

	vtkSmartPointer<vtkRenderer> renderer =
		vtkSmartPointer<vtkRenderer>::New();
	vtkSmartPointer<vtkRenderWindow> renderWindow =
		vtkSmartPointer<vtkRenderWindow>::New();
	renderWindow->SetSize(300, 300);
	renderWindow->AddRenderer(renderer);
	vtkSmartPointer<vtkRenderWindowInteractor> renderWindowInteractor =
		vtkSmartPointer<vtkRenderWindowInteractor>::New();
	renderWindowInteractor->SetRenderWindow(renderWindow);

	renderer->AddActor(modelActor);
	renderer->AddActor(outlineActor);

	renderWindow->Render();
	renderWindowInteractor->Start();

	return EXIT_SUCCESS;
}

Ref

维基百科.包围体
DataBounds
BoundingBox