[MetalKit]Using ARKit with Metal part 2使用ARKit與Metal 2

本系列文章是對 http://metalkit.org 上面MetalKit內容的全面翻譯和學習.

MetalKit系統文章目錄

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正如上次強調的那樣,在ARKit應用中共有三個層級:渲染,追蹤和場景理解.上次我們詳細分析瞭如何用Metal在自定義view中實現渲染. ARKit使用了視覺慣性里程計來精確追蹤周圍的世界,並將相機感測器資料和CoreMotion資料結合起來.即使當我們在運動時,也無需額外調校來保持畫面穩定.本文中,我們關注場景理解-用平面檢測,點選測試和光線估計來描述場景屬性的方法.ARKit能分析相機中的場景,並找到類似地板那樣的水平面.首先,我們需要啟用平面檢測功能(預設是off),只需在執行會話配置羊新增一行:

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    let configuration = ARWorldTrackingConfiguration()
    configuration.planeDetection = .horizontal
    session.run(configuration)
}

注意:當前API版本下,只能檢測水平面

ARSessionObserver代理方法對處理會話錯誤,追蹤改變和打斷非常有用:

func session(_ session: ARSession, didFailWithError error: Error) {}
func session(_ session: ARSession, cameraDidChangeTrackingState camera: ARCamera) {}
func session(_ session: ARSession, didOutputAudioSampleBuffer audioSampleBuffer: CMSampleBuffer) {}
func sessionWasInterrupted(_ session: ARSession) {}
func sessionInterruptionEnded(_ session: ARSession) {}

還有另一些代理方法屬於ARSessionDelegate協議(屬於ARSessionObserver擴充套件)能讓我們處理錨點.在第一個裡面寫上print():

func session(_ session: ARSession, didAdd anchors: [ARAnchor]) {
    print(anchors)
}
func session(_ session: ARSession, didRemove anchors: [ARAnchor]) {}
func session(_ session: ARSession, didUpdate anchors: [ARAnchor]) {}
func session(_ session: ARSession, didUpdate frame: ARFrame) {}

讓我們進入到Renderer.swift裡面.首先,建立一些需要的類屬性.這些變數將幫助我們建立並在螢幕上顯示一個除錯用的平面:

var debugUniformBuffer: MTLBuffer!
var debugPipelineState: MTLRenderPipelineState!
var debugDepthState: MTLDepthStencilState!var debugMesh: MTKMesh!
var debugUniformBufferOffset: Int = 0
var debugUniformBufferAddress: UnsafeMutableRawPointer!
var debugInstanceCount: Int = 0

下一步,在setupPipeline()我們建立緩衝器:

debugUniformBuffer = device.makeBuffer(length: anchorUniformBufferSize, options: .storageModeShared)

我們需要給我們的平面建立新的頂點及片段函式,還有新的渲染管線和深度模板狀態.在建立命令佇列的前面新增幾行:

let debugGeometryVertexFunction = defaultLibrary.makeFunction(name: "vertexDebugPlane")!
let debugGeometryFragmentFunction = defaultLibrary.makeFunction(name: "fragmentDebugPlane")!
anchorPipelineStateDescriptor.vertexFunction =  debugGeometryVertexFunction
anchorPipelineStateDescriptor.fragmentFunction = debugGeometryFragmentFunction
do { try debugPipelineState = device.makeRenderPipelineState(descriptor: anchorPipelineStateDescriptor)
} catch let error { print(error) }
debugDepthState = device.makeDepthStencilState(descriptor: anchorDepthStateDescriptor)

下一步,在setupAssets()裡我們需要建立一個新的Model I/O平面網格,並用它建立Metal網格.在函式的末尾新增幾行:

mdlMesh = MDLMesh(planeWithExtent: vector3(0.1, 0.1, 0.1), segments: vector2(1, 1), geometryType: .triangles, allocator: metalAllocator)
mdlMesh.vertexDescriptor = vertexDescriptor
do { try debugMesh = MTKMesh(mesh: mdlMesh, device: device)
} catch let error { print(error) }

下一步,在updateBufferStates()中我們需要更新平面所在緩衝器的地址.新增下面幾行:

debugUniformBufferOffset = alignedInstanceUniformSize * uniformBufferIndex
debugUniformBufferAddress = debugUniformBuffer.contents().advanced(by: debugUniformBufferOffset)

下一步,在updateAnchors()中我們需要更新變換矩陣和錨點數.在迴圈之前新增下面幾行:

let count = frame.anchors.filter{ $0.isKind(of: ARPlaneAnchor.self) }.count
debugInstanceCount = min(count, maxAnchorInstanceCount - (anchorInstanceCount - count))

然後,在迴圈中用下面幾行替換最後的三行:

if anchor.isKind(of: ARPlaneAnchor.self) {
    let transform = anchor.transform * rotationMatrix(rotation: float3(0, 0, Float.pi/2))
    let modelMatrix = simd_mul(transform, coordinateSpaceTransform)
    let debugUniforms = debugUniformBufferAddress.assumingMemoryBound(to: InstanceUniforms.self).advanced(by: index)
    debugUniforms.pointee.modelMatrix = modelMatrix
} else {
    let modelMatrix = simd_mul(anchor.transform, coordinateSpaceTransform)
    let anchorUniforms = anchorUniformBufferAddress.assumingMemoryBound(to: InstanceUniforms.self).advanced(by: index)
    anchorUniforms.pointee.modelMatrix = modelMatrix
}

我們必須將平面繞z軸旋轉90度,來讓它呈水平狀態.注意,我們使用了一個自定義方法,名為rotationMatrix(),讓我們來定義它.我們在以前的文章中,當第一次介紹3D矩陣時就見過了這個矩陣:

func rotationMatrix(rotation: float3) -> float4x4 {
    var matrix: float4x4 = matrix_identity_float4x4
    let x = rotation.x
    let y = rotation.y
    let z = rotation.z
    matrix.columns.0.x = cos(y) * cos(z)
    matrix.columns.0.y = cos(z) * sin(x) * sin(y) - cos(x) * sin(z)
    matrix.columns.0.z = cos(x) * cos(z) * sin(y) + sin(x) * sin(z)
    matrix.columns.1.x = cos(y) * sin(z)
    matrix.columns.1.y = cos(x) * cos(z) + sin(x) * sin(y) * sin(z)
    matrix.columns.1.z = -cos(z) * sin(x) + cos(x) * sin(y) * sin(z)
    matrix.columns.2.x = -sin(y)
    matrix.columns.2.y = cos(y) * sin(x)
    matrix.columns.2.z = cos(x) * cos(y)
    matrix.columns.3.w = 1.0
    return matrix
}

下一步,在drawAnchorGeometry()中我們需要確保我們在繪製之前至少有一個錨點.將第一行替換為下面這行:

guard anchorInstanceCount - debugInstanceCount > 0 else { return }

下一步,讓我們建立drawDebugGeometry()函式來繪製我們的平面.它非常類似於錨點繪製函式:

func drawDebugGeometry(renderEncoder: MTLRenderCommandEncoder) {
    guard debugInstanceCount > 0 else { return }
    renderEncoder.pushDebugGroup("DrawDebugPlanes")
    renderEncoder.setCullMode(.back)
    renderEncoder.setRenderPipelineState(debugPipelineState)
    renderEncoder.setDepthStencilState(debugDepthState)
    renderEncoder.setVertexBuffer(debugUniformBuffer, offset: debugUniformBufferOffset, index: 2)
    renderEncoder.setVertexBuffer(sharedUniformBuffer, offset: sharedUniformBufferOffset, index: 3)
    renderEncoder.setFragmentBuffer(sharedUniformBuffer, offset: sharedUniformBufferOffset, index: 3)
    for bufferIndex in 0..<debugMesh.vertexBuffers.count {
        let vertexBuffer = debugMesh.vertexBuffers[bufferIndex]
        renderEncoder.setVertexBuffer(vertexBuffer.buffer, offset: vertexBuffer.offset, index:bufferIndex)
    }
    for submesh in debugMesh.submeshes {
        renderEncoder.drawIndexedPrimitives(type: submesh.primitiveType, indexCount: submesh.indexCount, indexType: submesh.indexType, indexBuffer: submesh.indexBuffer.buffer, indexBufferOffset: submesh.indexBuffer.offset, instanceCount: debugInstanceCount)
    }
    renderEncoder.popDebugGroup()
}

在Renderer中,還有一件需要完成,就是-在update()中結束編碼前,呼叫這個函式:

drawDebugGeometry(renderEncoder: renderEncoder)

最後,讓我們進入Shaders.metal檔案中.我們需要一個新的結構體,只包含從頂點描述符中傳遞過來的頂點位置:

typedef struct {
    float3 position [[attribute(0)]];
} DebugVertex;

在頂點著色器中我們用模型-檢視矩陣來更新頂點位置:

vertex float4 vertexDebugPlane(DebugVertex in [[ stage_in]],
                               constant SharedUniforms &sharedUniforms [[ buffer(3) ]],
                               constant InstanceUniforms *instanceUniforms [[ buffer(2) ]],
                               ushort vid [[vertex_id]],
                               ushort iid [[instance_id]]) {
    float4 position = float4(in.position, 1.0);
    float4x4 modelMatrix = instanceUniforms[iid].modelMatrix;
    float4x4 modelViewMatrix = sharedUniforms.viewMatrix * modelMatrix;
    float4 outPosition = sharedUniforms.projectionMatrix * modelViewMatrix * position;
    return outPosition;
}

最後,在片段著色器中,我們給平面一個顯眼在顏色以便於在檢視中觀察到它:

fragment float4 fragmentDebugPlane() {
    return float4(0.99, 0.42, 0.62, 1.0);
}

如果你執行應用,當檢測到平面時,你將看到新增了一個矩形,像這樣:

接下來要做的是當我們檢測到更多或從先前檢測到的平面上移開時,更新/移除平面.別一個代理方法能幫助我們實現這個效果.接下來,我們將研究碰撞和物理效果.只是對以後的思考.

我要感謝Caroline為本文構造了平面檢測.

原始碼source code已釋出在Github上.
下次見!