我希望创建一个类似于太阳使用 SceneKit 照亮地球的方式的光源。这是一个业余天文学课程项目。由于太阳比地球大，所以全向 SCNLight 不会这样做，因为它发出的光是从一个点发出的。从太阳发出的光基本上是从一个非常大的球体发出的，而不是一个点。
此图像是使用全向光源创建的，但并未真实显示地球的阴影区域。具体来说，北极没有亮，但应该亮（在这种情况下，我们是在夏天。
第二张图像更逼真，你可以看到北极被照亮，就像在夏天一样。
问题是要获得第二张图像，我必须非常繁琐地定位定向光以使图像正确（这是我手动完成的。对于第一张图像，我只是将全向光定位在与太阳相同的位置领域）。由于整个物体是动画的，使用定向光并且必须随着地球沿着其轨道前进而不断地重新定位它，这将需要一些相当复杂的数学运算。
所以我想创建一个使用以编程方式创建的模型 I/O 灯光对象初始化的 SCNLight。根据 Apple 的“文档”，在 Swift 中，可以使用来自指定模型 I/O 灯光对象的初始化程序创建 SCNLight ，据我了解，它允许创建“光源”，从一个圆盘状区域"
“文档”为“创建灯光”说明了以下内容：
init(mdlLight: MDLLight)
它被定义为便利初始化器：
我期望能够做到以下几点：
let lightModelObject = MDLLight()
lightModelObject.lightType = .discArea
let discShapedSceneLight = SCNLight(lightModelObject) //cannot convert value ... error.
但最后一条语句让我知道：“无法将 'MDLLight' 类型的值转换为预期的参数类型 'NSCoder'”错误。我也试过：
let discShapedSceneLight = SCNLight.init(lightModelObject)
但那里也没有运气。
我完全被困住了！感觉在 Swift 中使用初始化器有一些基本的东西我不明白。
任何意见将不胜感激。
编辑：根据相同的文档，我还在objective-C中尝试了以下内容：
#import <ModelIO/MDLLight.h>
...
SCNLight *discLight = [SCNLight light];
MDLPhysicallcPlausibleLight *ppl = [MDLPhysicallyPlausibleLight lightWithSCNLight:discLight];
但得到这个错误：“选择器'lightWithSCNLight：'没有已知的类方法”
编辑：感谢@EmilioPelaez 解决了这个问题。
我已经将完整的代码与所需的照明放在下面，也许它会对其他人有所帮助。
import UIKit
import QuartzCore
import SceneKit
import SceneKit.ModelIO
class GameViewController: UIViewController {
var scnView:SCNView!
var scnScene:SCNScene!
var cameraNode:SCNNode!
override func viewDidLoad() {
super.viewDidLoad()
setupView()
setupScene()
setupCamera()
drawSolarSystem()
}
func setupView() {
scnView = self.view as! SCNView
scnView.showsStatistics = true
scnView.allowsCameraControl = true
scnView.autoenablesDefaultLighting = false
}
func setupScene() {
scnScene = SCNScene()
scnView.scene = scnScene
scnScene.background.contents = UIColor.systemBlue
}
func setupCamera() {
cameraNode = SCNNode()
cameraNode.camera = SCNCamera()
cameraNode.position = SCNVector3(x: 0, y: 0, z: 10)
scnScene.rootNode.addChildNode(cameraNode)
}
func drawSolarSystem() {
var geometryObject:SCNGeometry
//SUN object and light source
let sunX:Float = -3.5 ///position a little off to the left of center
//create the material for the Sun's surface
let sunMaterial = SCNMaterial()
sunMaterial.emission.contents = UIColor.yellow ///the color the material emits.
// sunMaterial.transparency = 0.3
// sunMaterial.diffuse.contents = UIColor.systemYellow ///the color the material reflects when lit.
//create the Sphere and assign it the material
geometryObject = SCNSphere(radius: 1.0)
geometryObject.firstMaterial=sunMaterial
//create the node and assign it the geometry (object) previously created.
let sunNode = SCNNode(geometry: geometryObject)
sunNode.position = SCNVector3(x:sunX, y:0, z:0)
scnScene.rootNode.addChildNode(sunNode)
//create the light source and position it same place as the sun
//create an MDLAreaLight, since the "normal" SCNLight types - such as omni - are not suitable.
//The .omni type emanates from a point, and so doesn't correctly represent the sun lighting the earth
let lightModelObject = MDLAreaLight()
lightModelObject.lightType = .discArea
// lightModelObject.areaRadius = 5.01 ///This doesn't appear to affect the light.
//create the node and assign it the MDLAreaLight
let sunLightNode = SCNNode()
sunLightNode.light = SCNLight(mdlLight:lightModelObject)
sunLightNode.light?.color = UIColor .white
sunLightNode.position = SCNVector3(x:sunX, y:0, z:0)
scnScene.rootNode.addChildNode(sunLightNode)
//EARTH EQUATORIAL PLANE but centered on the Sun
let floorObject = SCNFloor()
floorObject.reflectivity = 0
floorObject.width = 2
floorObject.length = 3
let earthEquatorialPlaneNode = SCNNode(geometry: floorObject)
earthEquatorialPlaneNode.position = SCNVector3(x:sunX, y:0, z:0)
scnScene.rootNode.addChildNode(earthEquatorialPlaneNode)
//EARTH main node - node with 2 subnodes, one sphere and one axis
///a node can only have a single geometry object attached. In order to attach multiple geometries, create a (parent) node without any geometry, and then attach subnodes with one geometry each.
//The parent node
let earthNode = SCNNode()
earthNode.position = SCNVector3(x: 0, y:-1.2, z:0)
scnScene.rootNode.addChildNode(earthNode)
//the child node for the earth axis of rotation object
geometryObject = SCNCylinder(radius: 0.01, height: 1.2)
let earthAxisNode = SCNNode(geometry: geometryObject)
earthNode.addChildNode(earthAxisNode)
//the child node for the earth sphere object
geometryObject = SCNSphere(radius: 0.5)
let earthSphereNode = SCNNode(geometry: geometryObject)
earthNode.addChildNode(earthSphereNode)
//put some meridians and an equator onto the sphere.
let earthSphereMaterial = SCNMaterial()
geometryObject.firstMaterial = earthSphereMaterial
earthSphereMaterial.diffuse.contents = "coordinateGrid.png"
earthSphereMaterial.lightingModel = .lambert
}
override var shouldAutorotate: Bool {
return true
}
override var prefersStatusBarHidden: Bool {
return true
}
}