VTK实现电影级渲染效果（CVR）

        目前vtk9.2.2版本中已经集成了ray marching（光线步进）算法实现的体渲染功能，官方博客中已经介绍为电影级体渲染了，如图：

        此效果是在3d显示器上的显示效果，就此效果来看说是电影级渲染效果也没什么问题，但是和光线追踪来实现的CVR还是差一些，下图为光线追踪的方法来实现的CVR。

        首先可以先了解下这几个名词概念

Ray Marching 光线步进 基本只用于volumetric(体积绘制)

RayTracing 光线追踪 在与物体相交后会选一个随机方向继续跟踪，并根据BRDF计算颜色(全局照明 (GI),反射)

Ray Casting 光线投射 该算法的计算不会停留在物体的表面，而会沿着射线穿过物体内部进行采样，且不会产生二次射线。我们在VTK中使用的vtkGPUVolumeRayCastMapper就是光线投射原理。

        VTK实现Ray Marching也是基于光线投射的mapper，只是加了两个接口控制。并且最好自己加上灯光，且控制灯光的旋转等。

    vtkNew<vtkGPUVolumeRayCastMapper> mapper;
	mapper->SetAutoAdjustSampleDistances(0);
	mapper->SetSampleDistance(0.5);
	//mapper->UseJitteringOn();
	mapper->SetInputData(reader->GetOutput());
	mapper->SetGlobalIlluminationReach(0.75);
	mapper->SetVolumetricScatteringBlending(1.0);

SetGlobalIlluminationReach 控制着射向光线的次级光线的范围/范围。值0.0表示只渲染局部阴影(但它更快)，值1.0表示渲染所有阴影(但它更慢)。

SetVolumetricScatteringBlending 控制散射模型混合的效果。

 效果实现：

 代码：

#include <vtkNew.h>
#include <vtkNIFTIImageReader.h>
#include <vtkFixedPointVolumeRayCastMapper.h>
#include <vtkImageGaussianSmooth.h>
#include <vtkVolumeProperty.h>
#include <vtkPiecewiseFunction.h>
#include <vtkColorTransferFunction.h>
#include <vtkVolume.h>
#include <vtkRenderer.h>
#include <vtkCamera.h>
#include <vtkRenderWindow.h>
#include <vtkRenderWindowInteractor.h>
#include <vtkInteractorStyleTrackballCamera.h>
#include <vtkGPUVolumeRayCastMapper.h>
#include <vtkPiecewiseFunction.h>
#include <vtkColorTransferFunction.h>
#include <vtkWindowToImageFilter.h>
#include <vtkLight.h>
using namespace std;

int main()
{
	vtkNew<vtkNIFTIImageReader> reader;
	reader->SetFileName(R"(./00001741.nii.gz)");
	reader->Update();

	vtkNew<vtkGPUVolumeRayCastMapper> mapper;
	mapper->SetAutoAdjustSampleDistances(0);
	mapper->SetSampleDistance(0.5);
	//mapper->UseJitteringOn();
	mapper->SetInputData(reader->GetOutput());
	mapper->SetGlobalIlluminationReach(0.75);
	mapper->SetVolumetricScatteringBlending(1.0);

	vtkNew<vtkVolume> volume;
	volume->SetMapper(mapper);


	vtkNew<vtkVolumeProperty> volumeProperty;
	volumeProperty->SetInterpolationTypeToLinear();
	volumeProperty->ShadeOn();  //打开或者关闭阴影测试
	volumeProperty->SetAmbient(0.55);
	volumeProperty->SetDiffuse(0.85);  //漫反射
	volumeProperty->SetSpecular(0.54); //镜面反射
	volumeProperty->SetSpecularPower(69); //镜面反射

	//设置不透明度
	vtkNew<vtkPiecewiseFunction> compositeOpacity;
	compositeOpacity->AddPoint(-1000, 0.00);
	compositeOpacity->AddPoint(100, 0.00);
	compositeOpacity->AddPoint(300, 0.879);
	compositeOpacity->AddPoint(478, 0.879);
	compositeOpacity->AddPoint(1500, 0.901);
	volumeProperty->SetScalarOpacity(compositeOpacity); //设置不透明度传输函数

	vtkNew<vtkColorTransferFunction> color;
	color->AddRGBPoint(-1000, 0.00, 0.00, 0.00);
	color->AddRGBPoint(100, 220/255., 21/255., 3/255.);
	color->AddRGBPoint(300, 250/255., 1.00, 189/255.);
	color->AddRGBPoint(478, 224/255., 1, 249/255.);
	color->AddRGBPoint(1500, 1, 1, 1);
	volumeProperty->SetColor(color);

	volume->SetProperty(volumeProperty);

	vtkNew<vtkRenderer>	ren;
	ren->AddActor(volume);
	ren->SetBackground(0, 0, 0);
	ren->ResetCamera();
	vtkCamera* camera = ren->GetActiveCamera();
	camera->Elevation(-70);

	ren->ClearLights();
	ren->RemoveAllLights();

	double* lightPosition = camera->GetPosition();
	double lightPositions[3] = { lightPosition[0], lightPosition[1], lightPosition[2]-100 };
	vtkNew<vtkLight> light;
	light->SetLightTypeToSceneLight();
	light->SetPosition(lightPositions);
	light->SetPositional(true);
	light->SetAmbientColor(0.3, 0.2, 0.1);
	light->SetConeAngle(60);
	light->SetFocalPoint(camera->GetFocalPoint());
	light->SetIntensity(1.2);
	ren->AddLight(light);

	vtkNew<vtkRenderWindow> renWin;
	renWin->AddRenderer(ren);

	vtkNew<vtkRenderWindowInteractor> iren;
	vtkNew<vtkInteractorStyleTrackballCamera> style;
	iren->SetRenderWindow(renWin);
	iren->SetInteractorStyle(style);
	renWin->SetSize(1024, 768);
	renWin->Render();

	iren->Start();

	return 0;
}