VTK基本概念之光照

光照

　　物体的光照属性主要包括环境光系数、漫反射光系数、镜反射光系数和镜面指数四个，分别用方法SetAmbient()，SetDiffuse()，SetSpecular()和SetSpecularPower()进行设置。
　　
- 环境光系数表示各种光线照射到物体材质上，经过很多次发射后最终在环境中的光线强度。
- 漫反射光系数表示光线照射到物体材质上，经过漫反射后形成的光线强度。
- 镜反射系数表示光线照射到物体材质上，经过镜面反射后形成的光线强度。

　　镜面指数的取值范围是0~128。该值越小，表示材质越粗糙，点光源发射的光线照射到上面，也可以产生较大的亮点；该值越大，表示材质越类似于镜面，光源照射到上面后，产生较小的亮点。

示例说明

CMakeLists.txt文件代码如下：

CMAKE_MINIMUM_REQUIRED(VERSION 2.8)
PROJECT(Ambient)
FIND_PACKAGE(VTK REQUIRED)
INCLUDE(${VTK_USE_FILE})
ADD_EXECUTABLE(Ambient  Ambient.cpp)
TARGET_LINK_LIBRARIES(Ambient ${VTK_LIBRARIES})

Ambient.cpp文件代码如下：

#include "vtkSmartPointer.h"
#include "vtkSphereSource.h"
#include "vtkPolyDataMapper.h"
#include "vtkActor.h"
#include "vtkRenderer.h"
#include "vtkRenderWindow.h"
#include "vtkRenderWindowInteractor.h"
#include "vtkProperty.h"
#include "vtkCamera.h"
#include "vtkLight.h"

int main()
{
    //创建多边形球体
    vtkSmartPointer<vtkSphereSource> sphere = vtkSmartPointer<vtkSphereSource>::New();
    sphere->SetThetaResolution(100);
    sphere->SetPhiResolution(50);

    vtkSmartPointer<vtkPolyDataMapper> sphereMapper = vtkSmartPointer<vtkPolyDataMapper>::New();
    sphereMapper->SetInputConnection(sphere->GetOutputPort());

    //actor是一个分组机制，除了将处理映射器的信息外，还具有属性及转换矩阵、光谱系数等参数
    //下面八个球体因为环境光的设置不同，其可见度将呈现变化
    vtkSmartPointer<vtkActor> sphereActor1 = vtkSmartPointer<vtkActor>::New();
    sphereActor1->SetMapper(sphereMapper);
    sphereActor1->GetProperty()->SetColor(1, 0, 0);
    sphereActor1->GetProperty()->SetAmbient(0.125);
    sphereActor1->GetProperty()->SetDiffuse(0.0);
    sphereActor1->GetProperty()->SetSpecular(0.0);

    vtkSmartPointer<vtkActor> sphereActor2 = vtkSmartPointer<vtkActor>::New();
    sphereActor2->SetMapper(sphereMapper);
    sphereActor2->GetProperty()->SetColor(1, 0, 0);
    sphereActor2->GetProperty()->SetAmbient(0.25);
    sphereActor2->GetProperty()->SetDiffuse(0.0);
    sphereActor2->GetProperty()->SetSpecular(0.0);
    sphereActor2->AddPosition(1.25, 0, 0);

    vtkSmartPointer<vtkActor> sphereActor3 = vtkSmartPointer<vtkActor>::New();
    sphereActor3->SetMapper(sphereMapper);
    sphereActor3->GetProperty()->SetColor(1, 0, 0);
    sphereActor3->GetProperty()->SetAmbient(0.375);
    sphereActor3->GetProperty()->SetDiffuse(0.0);
    sphereActor3->GetProperty()->SetSpecular(0.0);
    sphereActor3->AddPosition(2.5, 0, 0);

    vtkSmartPointer<vtkActor> sphereActor4 = vtkSmartPointer<vtkActor>::New();
    sphereActor4->SetMapper(sphereMapper);
    sphereActor4->GetProperty()->SetColor(1, 0, 0);
    sphereActor4->GetProperty()->SetAmbient(0.5);
    sphereActor4->GetProperty()->SetDiffuse(0.0);
    sphereActor4->GetProperty()->SetSpecular(0.0);
    sphereActor4->AddPosition(3.75, 0, 0);

    vtkSmartPointer<vtkActor> sphereActor5 = vtkSmartPointer<vtkActor>::New();
    sphereActor5->SetMapper(sphereMapper);
    sphereActor5->GetProperty()->SetColor(1, 0, 0);
    sphereActor5->GetProperty()->SetAmbient(0.625);
    sphereActor5->GetProperty()->SetDiffuse(0.0);
    sphereActor5->GetProperty()->SetSpecular(0.0);
    sphereActor5->AddPosition(0, 1.25, 0);

    vtkSmartPointer<vtkActor> sphereActor6 = vtkSmartPointer<vtkActor>::New();
    sphereActor6->SetMapper(sphereMapper);
    sphereActor6->GetProperty()->SetColor(1, 0, 0);
    sphereActor6->GetProperty()->SetAmbient(0.75);
    sphereActor6->GetProperty()->SetDiffuse(0.0);
    sphereActor6->GetProperty()->SetSpecular(0.0);
    sphereActor6->AddPosition(1.25, 1.25, 0);

    vtkSmartPointer<vtkActor> sphereActor7 = vtkSmartPointer<vtkActor>::New();
    sphereActor7->SetMapper(sphereMapper);
    sphereActor7->GetProperty()->SetColor(1, 0, 0);
    sphereActor7->GetProperty()->SetAmbient(0.825);
    sphereActor7->GetProperty()->SetDiffuse(0.0);
    sphereActor7->GetProperty()->SetSpecular(0.0);
    sphereActor7->AddPosition(2.5, 1.25, 0);

    vtkSmartPointer<vtkActor> sphereActor8 = vtkSmartPointer<vtkActor>::New();
    sphereActor8->SetMapper(sphereMapper);
    sphereActor8->GetProperty()->SetColor(1, 0, 0);
    sphereActor8->GetProperty()->SetAmbient(1.0);
    sphereActor8->GetProperty()->SetDiffuse(0.0);
    sphereActor8->GetProperty()->SetSpecular(0.0);
    sphereActor8->AddPosition(3.75, 1.25, 0);

    vtkSmartPointer<vtkRenderer> ren = vtkSmartPointer<vtkRenderer>::New();
    ren->AddActor(sphereActor1);
    ren->AddActor(sphereActor2);
    ren->AddActor(sphereActor3);
    ren->AddActor(sphereActor4);
    ren->AddActor(sphereActor5);
    ren->AddActor(sphereActor6);
    ren->AddActor(sphereActor7);
    ren->AddActor(sphereActor8);
    ren->SetBackground(0.1, 0.2, 0.4);
    //设置灯光及属性
    vtkSmartPointer<vtkLight> light = vtkSmartPointer<vtkLight>::New();
    light->SetFocalPoint(1.875, 0.6125, 0);
    light->SetPosition(0.875, 1.6125, 1);
    ren->AddLight(light);
    //如果想消除背景的阴暗，可以采用平行投射模式
    ren->GetActiveCamera()->SetFocalPoint(0, 0, 0);
    ren->GetActiveCamera()->SetPosition(0, 0, 1);
    ren->GetActiveCamera()->SetViewUp(0, 1, 0);
    ren->GetActiveCamera()->ParallelProjectionOn();
    ren->ResetCamera();
    ren->GetActiveCamera()->SetParallelScale(1.5);

    vtkSmartPointer<vtkRenderWindow> renWin = vtkSmartPointer<vtkRenderWindow>::New();
    renWin->AddRenderer(ren);
    renWin->SetSize(400, 200);
    vtkSmartPointer<vtkRenderWindowInteractor> iren = vtkSmartPointer<vtkRenderWindowInteractor>::New();
    iren->SetRenderWindow(renWin);

    //启动事件进入循环并开始渲染
    iren->Initialize();
    iren->Start();
    return EXIT_SUCCESS;
}

运行结果：

　　在截图上，每个球体上方添加它的环境光系数，如下图所示。我们可以看出：随着环境光系数属性值的增加，球体越来越凉，但是看上去不像球体，而像个 2D的圆盘。这是因为其他光反射系数都为零了，因而没有很好地反映出现实中物体的实际光照属性。