问题，有时我们需要显示某个 Actor 在相机最前面，可以遮盖后面的物体;显示在顶层有点不准确；因为这个还相机位置也有关系；
这里讲三种情况：
1. 设置 Mapper 顶层，尝试了一下，可以用于某些场景；具体原理未看源码；
原理 ：设置 vtkPolyData的 point ，line ，polygon 值 //负值会靠近相机，正值会远离相机 初始值就是0.0
显然他只能用于 vtkPolyData 数据；
const double units0 = -66000;sphereMapper->SetInputData(sphere->GetOutput());sphereMapper->SetResolveCoincidentTopologyToPolygonOffset();sphereMapper->SetRelativeCoincidentTopologyLineOffsetParameters(0, units0);sphereMapper->SetRelativeCoincidentTopologyPolygonOffsetParameters(0, units0);sphereMapper->SetRelativeCoincidentTopologyPointOffsetParameter(units0);
样例：
#include <iostream>
#include <vtkSmartPointer.h>
#include <vtkSphereSource.h>
#include <vtkActor.h>
#include <vtkConeSource.h>
#include <vtkRenderer.h>
#include <vtkRenderWindow.h>
#include <vtkPolyDataMapper.h>
#include <vtkSphereSource.h>
#include <vtkRenderWindowInteractor.h>
#include <vtkProperty.h>
#include <vtkLineSource.h>#define vtkSPtr vtkSmartPointer
#define vtkSPtrNew(Var, Type) vtkSPtr<Type> Var = vtkSPtr<Type>::New();using namespace std;int main()
{vtkSPtrNew( sphere, vtkSphereSource );sphere->SetCenter( 0, 0, 0 );sphere->SetRadius( 1 );sphere->Update();vtkSPtrNew( sphereMapper, vtkPolyDataMapper );// set mapper to make sphere always on topconst double units0 = -66000;sphereMapper->SetInputData( sphere->GetOutput() );sphereMapper->SetResolveCoincidentTopologyToPolygonOffset();sphereMapper->SetRelativeCoincidentTopologyLineOffsetParameters(0, units0);sphereMapper->SetRelativeCoincidentTopologyPolygonOffsetParameters(0, units0);sphereMapper->SetRelativeCoincidentTopologyPointOffsetParameter(units0);vtkSPtrNew( sphereActor, vtkActor );sphereActor->SetMapper( sphereMapper );sphereActor->GetProperty()->SetColor( 1, 0, 0 );vtkSPtrNew( cone, vtkConeSource );cone->SetRadius( 2 );cone->SetHeight( 4 );cone->Update();vtkSPtrNew( coneMapper, vtkPolyDataMapper );coneMapper->SetInputData( cone->GetOutput() );vtkSPtrNew( coneActor, vtkActor );coneActor->SetMapper( coneMapper );vtkSPtrNew( renderer, vtkRenderer );renderer->AddActor( coneActor );renderer->AddActor( sphereActor );renderer->SetBackground( 0, 0, 0 );vtkSPtrNew( renderWindow, vtkRenderWindow );renderWindow->AddRenderer( renderer );vtkSPtrNew( renderWindowInteractor, vtkRenderWindowInteractor );renderWindowInteractor->SetRenderWindow( renderWindow );renderer->ResetCamera();renderWindow->Render();renderWindowInteractor->Start();return 0;
}2.设置 VTK的图层 0层最低在后面：
RenderWindow 可以设置多个图层，最前面的显示在上面；
volumeRenderer_->SetLayer(0);
tagRenderer_->SetLayer(1);
GetRenderWindow()->SetNumberOfLayers(2);
GetRenderWindow()->AddRenderer(tagRenderer_);
GetRenderWindow()->AddRenderer(volumeRenderer_);参考资料：
https://blog.csdn.net/lpsl1882/article/details/52107117尝试后发现并不能实现这的场景；应该可以适应某些场景；
3.设置相机 前后裁剪平面，只有 前后裁剪平面内可见；可见相机的作用；
前后裁剪平面。裁剪平面与投影方向相交，一般与投影方向也是垂直的。裁剪平面主要用于评估Actor与相机距离的远近，只有在前后裁剪平面之间的Actor才是可见的。裁剪平面的位置可以用vtkCamera::SetClippingRange()方法设置。
这里可以看一下 vtkImageView2 源码 UpdateDisplayExtent，怎么设置 某个图层可见：
个面这个我修改了部分代码
vtkCamera *cam = imageViewer->GetRenderer()->GetActiveCamera();if (cam){double bounds[6];actor->GetBounds(bounds);double spos = bounds[2 * 2];double cpos = cam->GetPosition()[2];double range = fabs(spos - cpos);double *spacing = mImageData->GetSpacing();double avg_spacing =(spacing[0] + spacing[1] + spacing[2]) / 3.0;cam->SetClippingRange(range - avg_spacing * 3.0, range + avg_spacing * 3.0);}我最终要实现的是特定 Actor 显示在某个图层的上面，可以需要调整 相机的 SetClippingRange;
