RealityKit如何使Entity及其PBR材质实现透明（幽灵效果）？

我有一个名为“Table”的Entity，代表带有贴纸的桌子，它包含多个子ModelEntity（leg1、leg2、leg3、leg4、plank和sticker），每个ModelEntity都应用了PBR纹理材质（桌腿为lightWood，桌板为darkWood，贴纸为图片材质）。我希望创建该Table Entity的“幽灵/透明”版本，在用户将其放置到场景前展示。请问是否有简单方法可在材质创建前后设置不透明度？是否无需编写Metal shader（我不会编写）即可实现此需求？

贴纸ModelEntity的材质代码 #

// Setup image material
var pictureMaterial = PhysicallyBasedMaterial()
var imageTexture: TextureResource? = nil

if let cgImage = image?.cgImage,
    let resource = try? TextureResource.generate(from: cgImage, options: TextureResource.CreateOptions.init(semantic: nil)) {
    imageTexture = resource
}
    
let baseColor = MaterialParameters.Texture(imageTexture ?? StickerEntity.defaultImage)
pictureMaterial.baseColor = PhysicallyBasedMaterial.BaseColor(texture:baseColor)

self.stickerEntity.components[ModelComponent.self] =
    ModelComponent(mesh: .generateBox(width: width!,
                                      height: self.pictureDepth,
                                      depth: height!,
                                      cornerRadius: 0,
                                      splitFaces: true),
                   materials: [canvasMaterial,
                                   pictureMaterial,
                                   canvasMaterial,
                                   canvasMaterial,
                                   canvasMaterial,
                                   canvasMaterial
                                  ])

用于创建桌腿和桌板PBR材质的函数 #

static func generateMaterialFromImages(uiColor: UIColor? = nil,
                                       baseColor: String? = nil,
                                       normal: String? = nil,
                                       roughness: String? = nil,
                                       ambientOcclusion: String? = nil,
                                       metallic: String? = nil,
                                       clearcoat: String? = nil,
                                       emissiveColor: String? = nil) -> PhysicallyBasedMaterial {
    
    var material = PhysicallyBasedMaterial()
    
    if let color = uiColor {
        material.baseColor = PhysicallyBasedMaterial.BaseColor(tint: color)
        
    } else if let resourceFileName = baseColor, let resource = try? TextureResource.load(named: resourceFileName) {
        let baseColor = MaterialParameters.Texture(resource)
        material.baseColor = PhysicallyBasedMaterial.BaseColor(texture: baseColor)
    }
    
    if let resourceFileName = normal, let resource = try? TextureResource.load(named: resourceFileName) {
        let normal = MaterialParameters.Texture(resource)
        material.normal = PhysicallyBasedMaterial.Normal(texture:normal)
    }
    if let resourceFileName = roughness, let resource = try? TextureResource.load(named: resourceFileName) {
        let roughness = MaterialParameters.Texture(resource)
        material.roughness = PhysicallyBasedMaterial.Roughness(texture:roughness)
    }
    if let resourceFileName = ambientOcclusion, let resource = try? TextureResource.load(named: resourceFileName) {
        let ambientOcclusion = MaterialParameters.Texture(resource)
        material.ambientOcclusion = PhysicallyBasedMaterial.AmbientOcclusion(texture:ambientOcclusion)
    }
    if let resourceFileName = metallic, let resource = try? TextureResource.load(named: resourceFileName) {
        let metallic = MaterialParameters.Texture(resource)
        material.metallic = PhysicallyBasedMaterial.Metallic(texture:metallic)
    }
    if let resourceFileName = clearcoat, let resource = try? TextureResource.load(named: resourceFileName) {
        let clearcoat = MaterialParameters.Texture(resource)
        material.clearcoat = PhysicallyBasedMaterial.Clearcoat(texture:clearcoat)
    }
    if let resourceFileName = emissiveColor, let resource = try? TextureResource.load(named: resourceFileName) {
        let emissiveColor = MaterialParameters.Texture(resource)
        material.emissiveColor = PhysicallyBasedMaterial.EmissiveColor(texture:emissiveColor)
    }
    
    return material
}  

我查找了可更改材质不透明度的文档或函数，但未找到相关内容。

完全不需要编写Metal shader就能实现，有两种简单的方式可以给你的Table Entity设置透明效果：

方式一：修改材质的基础颜色透明度 #

不管是用tint还是纹理创建的PBR材质，都可以通过调整基础颜色的alpha通道来控制透明度：

• 针对纯色基础材质：如果你的材质是用UIColor创建的，直接创建带alpha的颜色即可：

// 设置50%透明度的木质色
let transparentWoodColor = UIColor(red: 0.7, green: 0.5, blue: 0.3, alpha: 0.5)
let legMaterial = generateMaterialFromImages(uiColor: transparentWoodColor)

• 针对纹理基础材质：可以给纹理叠加带透明度的tint，不改变纹理本身颜色的前提下添加透明效果：

// 假设已加载好桌板纹理资源
if let darkWoodResource = try? TextureResource.load(named: "darkWood") {
    let baseColor = PhysicallyBasedMaterial.BaseColor(texture: darkWoodResource)
    // 叠加50%透明度的白色tint
    baseColor.tint = UIColor.white.withAlphaComponent(0.5)
    var plankMaterial = PhysicallyBasedMaterial()
    plankMaterial.baseColor = baseColor
    // 其他PBR属性保持不变
}

对于贴纸材质，同样可以用这个方法：

pictureMaterial.baseColor?.tint = UIColor.white.withAlphaComponent(0.5)

方式二：使用Entity的opacity属性 #

每个Entity都自带opacity属性，直接设置这个值可以让整个实体（包括所有子Entity）变成半透明，这是最快捷的幽灵效果实现方式：

// 设置整个Table实体为50%透明
tableEntity.opacity = 0.5

这个方法不需要修改任何材质，直接作用于整个实体层级，适合快速创建放置前的预览效果。如果需要部分子实体保持不透明，单独给这些子实体设置opacity = 1.0即可覆盖父实体的设置。

关键注意事项 #

确保你的ModelComponent的isOpaque属性设为false，否则透明度设置可能不会生效：

// 给贴纸实体设置非透明标记
if var modelComponent = self.stickerEntity.components[ModelComponent.self] {
    modelComponent.isOpaque = false
    self.stickerEntity.components[ModelComponent.self] = modelComponent
}

内容的提问来源于stack exchange，提问作者Seb