如何在visionOS的RealityKit中实现Vignette暗角效果？

可行方案与最佳实践 #

方案一：叠加全屏半透明Plane（最简单高效） #

无需接触Metal，直接在RealityKit场景最上层添加全屏Plane，通过半透明纹理或动态着色实现暗角，完全覆盖AR视图内容。

实现步骤：

• 创建覆盖相机视野的ModelEntity，用PlaneResource生成平面，调整尺寸适配相机视锥体
• 准备Vignette纹理：四角纯黑、中心透明的径向渐变PNG图，导入项目
• 基于纹理创建UnlitMaterial，开启透明混合模式，控制透明度调整暗角强度
• 将Plane的渲染优先级设为最高，确保在所有AR内容之上

代码示例：

import RealityKit

func createVignetteEntity() -> ModelEntity {
    // 生成适配视野的平面（可根据相机FOV调整宽高）
    let planeMesh = MeshResource.generatePlane(width: 2.2, height: 1.6)
    guard let vignetteTexture = try? TextureResource.load(named: "vignette_gradient") else {
        fatalError("Vignette texture not found")
    }

    var material = UnlitMaterial()
    material.baseColor = .init(texture: vignetteTexture)
    material.transparency = .init(alpha: 0.6) // 调整暗角深浅
    material.blending = .transparent

    let vignetteEntity = ModelEntity(mesh: planeMesh, materials: [material])
    // 放置在相机前方0.5米处，确保覆盖视野
    vignetteEntity.position = [0, 0, -0.5]
    // 设置最高渲染优先级
    vignetteEntity.components.set(RenderPriorityComponent(priority: 1000))

    return vignetteEntity
}

// 在ARView中添加暗角实体
let arView = ARView(frame: .zero)
let vignette = createVignetteEntity()
arView.scene.addAnchor(vignette)

方案二：Metal + RealityKit 自定义后期处理（动态可调） #

如果需要实时调整暗角强度、大小等参数，可通过Metal拦截ARView的渲染帧，应用自定义着色器实现动态后期效果。

步骤1：编写Metal着色器（.metal文件）

#include <metal_stdlib>
using namespace metal;

struct VertexIn {
    float4 position [[position]];
    float2 uv;
};

struct VertexOut {
    float4 position [[position]];
    float2 uv;
};

vertex VertexOut vertexShader(VertexIn in [[stage_in]]) {
    VertexOut out;
    out.position = in.position;
    out.uv = in.uv;
    return out;
}

fragment float4 fragmentShader(VertexOut in [[stage_in]],
                               texture2d<float, access::sample> inputTexture [[texture(0)]],
                               constant float& strength [[buffer(0)]],
                               constant float& size [[buffer(1)]]) {
    float4 sourceColor = inputTexture.sample(sampler(coord::normalized), in.uv);
    
    // 计算暗角因子
    float2 center = float2(0.5, 0.5);
    float distanceToCenter = distance(in.uv, center);
    float vignetteFactor = smoothstep(size, size - strength, distanceToCenter);
    
    return sourceColor * vignetteFactor;
}

步骤2：Swift端集成Metal与RealityKit

import RealityKit
import MetalKit

class VignettePostProcessor: NSObject, ARSessionDelegate {
    private let arView: ARView
    private let device: MTLDevice
    private let commandQueue: MTLCommandQueue
    private var pipelineState: MTLRenderPipelineState!
    
    // 可动态调整的暗角参数
    var vignetteStrength: Float = 0.7
    var vignetteSize: Float = 0.8

    init(arView: ARView) {
        self.arView = arView
        self.device = MTLCreateSystemDefaultDevice()!
        self.commandQueue = device.makeCommandQueue()!
        super.init()
        setupMetalPipeline()
        arView.session.delegate = self
    }

    private func setupMetalPipeline() {
        guard let library = device.makeDefaultLibrary(),
              let vertexFunc = library.makeFunction(name: "vertexShader"),
              let fragmentFunc = library.makeFunction(name: "fragmentShader") else {
            fatalError("Failed to load Metal functions")
        }

        let pipelineDescriptor = MTLRenderPipelineDescriptor()
        pipelineDescriptor.vertexFunction = vertexFunc
        pipelineDescriptor.fragmentFunction = fragmentFunc
        pipelineDescriptor.colorAttachments[0].pixelFormat = .bgra8Unorm

        do {
            pipelineState = try device.makeRenderPipelineState(descriptor: pipelineDescriptor)
        } catch {
            fatalError("Failed to create pipeline state: \(error)")
        }
    }

    func renderer(_ renderer: SCNSceneRenderer, willRenderScene scene: SCNScene, atTime time: TimeInterval) {
        guard let commandBuffer = commandQueue.makeCommandBuffer(),
              let renderPassDescriptor = arView.currentRenderPassDescriptor else { return }

        let encoder = commandBuffer.makeRenderCommandEncoder(descriptor: renderPassDescriptor)!
        encoder.setRenderPipelineState(pipelineState)

        // 传递动态参数到着色器
        var strength = vignetteStrength
        var size = vignetteSize
        encoder.setFragmentBytes(&strength, length: MemoryLayout<Float>.stride, index: 0)
        encoder.setFragmentBytes(&size, length: MemoryLayout<Float>.stride, index: 1)

        // 绘制全屏四边形覆盖视图
        encoder.drawPrimitives(type: .triangleStrip, vertexStart: 0, vertexCount: 4)

        encoder.endEncoding()
        commandBuffer.commit()
    }
}

// 使用方式
let arView = ARView(frame: .zero)
let postProcessor = VignettePostProcessor(arView: arView)
// 动态调整参数示例：postProcessor.vignetteStrength = 0.5

方案对比 #

• 全屏Plane方案：实现成本极低，无需Metal知识，适合静态暗角需求；缺点是参数调整需重新生成纹理，灵活性有限。
• Metal后期处理方案：支持实时调整暗角参数，可扩展实现更复杂的后期效果；缺点是需要具备基础Metal知识，代码量较大。

注意事项 #

• Plane方案中，需根据相机FOV调整平面尺寸与位置，避免暗角边缘裁切或未覆盖全屏。
• Metal方案中，需确保RealityKit渲染与Metal命令队列同步，避免帧丢失或卡顿。

内容的提问来源于stack exchange，提问作者闪电狮