  //@{
  /**
   * Define the text to be placed in the caption. The text can be multiple
   * lines (separated by "\n").
   * 定义标注的文本，文本可以多行（通过"\n"进行分隔）
   */
  virtual void SetCaption(const char* caption);
  virtual char* GetCaption();
  //@}

2.2 设置标注附着点

  //@{
  /**
   * Set/Get the attachment point for the caption. By default, the attachment
   * point is defined in world coordinates, but this can be changed using
   * vtkCoordinate methods.
   * 设置标注的附着点，默认为世界坐标系。可以通过vtkCoordinate的方法进行更改
   */
  vtkWorldCoordinateMacro(AttachmentPoint);
  //@}

2.3 设置是否启用边框

  //@{
  /**
   * Enable/disable the placement of a border around the text.
   * 可用/不可用文本外的包围框
   */
  vtkSetMacro(Border, vtkTypeBool);
  vtkGetMacro(Border, vtkTypeBool);
  vtkBooleanMacro(Border, vtkTypeBool);
  //@}

2.4 设置是否启用标注到附着点的引导线

  //@{
  /**
   * Enable/disable drawing a "line" from the caption to the
   * attachment point.
   * 可用/不可用 标注到附着点的线
   */
  vtkSetMacro(Leader, vtkTypeBool);
  vtkGetMacro(Leader, vtkTypeBool);
  vtkBooleanMacro(Leader, vtkTypeBool);
  //@}

2.5 指定2D或3D箭头

  //@{
  /**
   * Indicate whether the leader is 2D (no hidden line) or 3D (z-buffered).
   * 指定箭头为2D或3D
   */
  vtkSetMacro(ThreeDimensionalLeader, vtkTypeBool);
  vtkGetMacro(ThreeDimensionalLeader, vtkTypeBool);
  vtkBooleanMacro(ThreeDimensionalLeader, vtkTypeBool);
  //@}

  //@{
  /**
   * Specify a glyph to be used as the leader "head". This could be something
   * like an arrow or sphere. If not specified, no glyph is drawn. Note that
   * the glyph is assumed to be aligned along the x-axis and is rotated about
   * the origin. SetLeaderGlyphData() directly uses the polydata without
   * setting a pipeline connection. SetLeaderGlyphConnection() sets up a
   * pipeline connection and causes an update to the input during render.
   * 设置箭头的头部输入数据
   */
  virtual void SetLeaderGlyphData(vtkPolyData*);
  virtual void SetLeaderGlyphConnection(vtkAlgorithmOutput*);
  virtual vtkPolyData* GetLeaderGlyph();
  //@}

  //@{
  /**
   * Specify the relative size of the leader head. This is expressed as a
   * fraction of the size (diagonal length) of the renderer. The leader
   * head is automatically scaled so that window resize, zooming or other
   * camera motion results in proportional changes in size to the leader
   * glyph.
   * 设置箭头符号的大小
   */
  vtkSetClampMacro(LeaderGlyphSize, double, 0.0, 0.1);
  vtkGetMacro(LeaderGlyphSize, double);
  //@}

  //@{
  /**
   * Specify the maximum size of the leader head (if any) in pixels. This
   * is used in conjunction with LeaderGlyphSize to cap the maximum size of
   * the LeaderGlyph.
   * 像素上设置箭头符号的最大像素值
   */
  vtkSetClampMacro(MaximumLeaderGlyphSize, int, 1, 1000);
  vtkGetMacro(MaximumLeaderGlyphSize, int);
  //@}

2.6 设置文本与包围盒的距离

  //@{
  /**
   * Set/Get the padding between the caption and the border. The value
   * is specified in pixels.
   * 设置/获取标注文本和包围框之间的距离
   */
  vtkSetClampMacro(Padding, int, 0, 50);
  vtkGetMacro(Padding, int);
  //@}

  //@{
  /**
   * Get the text actor used by the caption. This is useful if you want to control
   * justification and other characteristics of the text actor.
   * 获取文本Actor对象
   */
  vtkGetObjectMacro(TextActor, vtkTextActor);
  //@}

2.7 设置文本属性

  //@{
  /**
   * Set/Get the text property.
   * 设置文本的属性
   */
  virtual void SetCaptionTextProperty(vtkTextProperty* p);
  vtkGetObjectMacro(CaptionTextProperty, vtkTextProperty);
  //@}

2.8 设置箭头是否在Edge上附着

  //@{
  /**
   * Enable/disable whether to attach the arrow only to the edge,
   * NOT the vertices of the caption border.
   * 箭头在包围框上的附着点；若为On时，表示附着点考虑包围框四个边的中点；否则不考虑
   */
  vtkSetMacro(AttachEdgeOnly, vtkTypeBool);
  vtkGetMacro(AttachEdgeOnly, vtkTypeBool);
  vtkBooleanMacro(AttachEdgeOnly, vtkTypeBool);
  //@}

3. 原理解析

3.1 vtkBorderRepresentation原理解析

定义一个矩形，左下角点为（0.，0.，0.），右上角点为（1.，1.，0.）。

定义由这四个点组成的矩形边界的PolyData数据

  // Create the geometry in canonical coordinates
  this->BWPoints = vtkPoints::New();
  this->BWPoints->SetDataTypeToDouble();
  this->BWPoints->SetNumberOfPoints(4);
  this->BWPoints->SetPoint(0, 0.0, 0.0, 0.0); // may be updated by the subclass
  this->BWPoints->SetPoint(1, 1.0, 0.0, 0.0);
  this->BWPoints->SetPoint(2, 1.0, 1.0, 0.0);
  this->BWPoints->SetPoint(3, 0.0, 1.0, 0.0);

  vtkCellArray* outline = vtkCellArray::New();
  outline->InsertNextCell(5);
  outline->InsertCellPoint(0);
  outline->InsertCellPoint(1);
  outline->InsertCellPoint(2);
  outline->InsertCellPoint(3);
  outline->InsertCellPoint(0);

  this->BWPolyData = vtkPolyData::New();
  this->BWPolyData->SetPoints(this->BWPoints);
  this->BWPolyData->SetLines(outline);
  outline->Delete();

给PolyData数据添加Transform变换，并将其映射为actor2D 数据。

  this->BWTransform = vtkTransform::New();
  this->BWTransformFilter = vtkTransformPolyDataFilter::New();
  this->BWTransformFilter->SetTransform(this->BWTransform);
  this->BWTransformFilter->SetInputData(this->BWPolyData);

  this->BWMapper = vtkPolyDataMapper2D::New();
  this->BWMapper->SetInputConnection(this->BWTransformFilter->GetOutputPort());
  this->BWActor = vtkActor2D::New();
  this->BWActor->SetMapper(this->BWMapper);

  this->BorderProperty = vtkProperty2D::New();
  this->BWActor->SetProperty(this->BorderProperty);

通过两个vtkCoordinate对象约束矩形的左下角点和右上角点。

  this->PositionCoordinate = vtkCoordinate::New();
  this->PositionCoordinate->SetCoordinateSystemToNormalizedViewport();
  this->PositionCoordinate->SetValue(0.05, 0.05);
  this->Position2Coordinate = vtkCoordinate::New();
  this->Position2Coordinate->SetCoordinateSystemToNormalizedViewport();
  this->Position2Coordinate->SetValue(0.1, 0.1); // may be updated by the subclass
  this->Position2Coordinate->SetReferenceCoordinate(this->PositionCoordinate);

在每次Render的时候，获取当前的Display坐标对原始PolyData数据进行Transform，即可看到一个一直处于视平面的矩形框。

    // Set things up
    int* pos1 = this->PositionCoordinate->GetComputedViewportValue(this->Renderer);
    int* pos2 = this->Position2Coordinate->GetComputedViewportValue(this->Renderer);

    // If the widget's aspect ratio is to be preserved (ProportionalResizeOn),
    // then (pos1,pos2) are a bounding rectangle.
    if (this->ProportionalResize)
    {
    }

    // Now transform the canonical widget into display coordinates
    double size[2];
    this->GetSize(size);
    double tx = pos1[0];
    double ty = pos1[1];
    double sx = (pos2[0] - pos1[0]) / size[0];
    double sy = (pos2[1] - pos1[1]) / size[1];

    this->BWTransform->Identity();
    this->BWTransform->Translate(tx, ty, 0.0);
    this->BWTransform->Scale(sx, sy, 1);

    this->BuildTime.Modified();

保持位置不变的原因：每次旋转或移动渲染窗口时，都重新获取两个vtkCoordinate对象的ViewPort坐标系下的坐标，并对其进行重新绘制。

3.2 vtkCaptionActor2D

结论：

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