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 top
    const 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;