SwiftでMetalを使用して色加算をしよう

ブレンドモードで色加算を実現する覚書
「SwiftでMetalを使用してテクスチャを表示しよう」を参照
https://qiita.com/sanoh/items/98f52e3e71351bbcbc89
■Setp1：MetalRenderer.mを編集
---
色加算のブレンドモードを設定します。

```swift:MetalRenderer.swift
import MetalKit

let maxBuffersInFlight = 3

class MetalRenderer: NSObject, MTKViewDelegate {
    let _device: MTLDevice
    let _command_queue: MTLCommandQueue
    let _in_flight_semaphore = DispatchSemaphore(value: maxBuffersInFlight)
    
    let _buffer_position : MTLBuffer!
    let _render_pipeline_state: MTLRenderPipelineState
    
    let _color_image: MTLTexture
    init?(metalKitView: MTKView) {
        let vertices_array:[Float] =
            [
                +0.0, +1.0, 0.0, 0.0,
                -1.0, -1.0, 1.0, 0.0,
                +1.0, -1.0, 1.0, 1.0,
                ]
        _device = metalKitView.device!
        metalKitView.colorPixelFormat = MTLPixelFormat.bgra8Unorm
        _command_queue = _device.makeCommandQueue()!
        //  頂点情報の登録
        let size = vertices_array.count * MemoryLayout<Float>.size
        _buffer_position = _device.makeBuffer(bytes: vertices_array, length: size)
        //  シェーダーの登録
        guard let library = _device.makeDefaultLibrary() else { fatalError() }
        let descriptor = MTLRenderPipelineDescriptor()
        descriptor.vertexFunction=library.makeFunction(name: "myVertexShader")
        descriptor.fragmentFunction=library.makeFunction(name: "myFragmentShader")
        descriptor.colorAttachments[0].pixelFormat = metalKitView.colorPixelFormat
        //  ブレンドモードの設定（色加算）＜新規＞
        descriptor.colorAttachments[0].isBlendingEnabled = true
        descriptor.colorAttachments[0].rgbBlendOperation = .add
        descriptor.colorAttachments[0].alphaBlendOperation = .add
        descriptor.colorAttachments[0].sourceRGBBlendFactor = .one
        descriptor.colorAttachments[0].sourceAlphaBlendFactor = .one
        descriptor.colorAttachments[0].destinationRGBBlendFactor = .one
        descriptor.colorAttachments[0].destinationAlphaBlendFactor = .one

        _render_pipeline_state = try! _device.makeRenderPipelineState(descriptor: descriptor)
        //  テクスチャ読込+登録
        let textureLoader = MTKTextureLoader(device: _device)
        let textureLoaderOptions = [
            MTKTextureLoader.Option.textureUsage: NSNumber(value: MTLTextureUsage.shaderRead.rawValue),
            MTKTextureLoader.Option.textureStorageMode: NSNumber(value: MTLStorageMode.`private`.rawValue)
        ]
        _color_image    = try! textureLoader.newTexture(name: "colorImage",
                                                        scaleFactor: 1.0,
                                                        bundle: nil,
                                                        options: textureLoaderOptions)
        super.init()
    }
    
    func draw(in view: MTKView) {
        _ = _in_flight_semaphore.wait(timeout: DispatchTime.distantFuture)
        if let command_buffer = _command_queue.makeCommandBuffer() {
            let semaphore = _in_flight_semaphore
            command_buffer.addCompletedHandler { (_ command_buffer)-> Swift.Void in
                semaphore.signal()
            }
            let render_pass_descriptor = view.currentRenderPassDescriptor
            render_pass_descriptor?.colorAttachments[0].loadAction=MTLLoadAction.clear
            render_pass_descriptor?.colorAttachments[0].storeAction=MTLStoreAction.store
            render_pass_descriptor?.colorAttachments[0].clearColor=MTLClearColorMake(1.0, 0.0, 0.0, 1.0) // 背景色
            if let render_pass_descriptor = render_pass_descriptor {
                if let render_encoder = command_buffer.makeRenderCommandEncoder(descriptor: render_pass_descriptor) {
                    //  テクスチャの設定
                    render_encoder.setFragmentTexture(_color_image,
                                                      index: TextureIndex.color.rawValue)
                    //  モデルの設定
                    render_encoder.setRenderPipelineState(_render_pipeline_state)
                    render_encoder.setVertexBuffer(_buffer_position,
                                                   offset: 0,
                                                   index: 0)
                    render_encoder.drawPrimitives(type: MTLPrimitiveType.triangle,
                                                  vertexStart: 0,
                                                  vertexCount: 3)
                    
                    render_encoder.endEncoding()
                    if let drawable = view.currentDrawable {
                        command_buffer.present(drawable)
                    }
                }
            }
            command_buffer.commit()
        }
    }
    
    func mtkView(_ view: MTKView, drawableSizeWillChange size: CGSize) {
    }
}
```
■解説
なにも設定しないと、アルファは「false」の設定

```swift
        descriptor.colorAttachments[0].isBlendingEnabled = true
        descriptor.colorAttachments[0].rgbBlendOperation = .add
        descriptor.colorAttachments[0].alphaBlendOperation = .add
        descriptor.colorAttachments[0].sourceRGBBlendFactor = .one
        descriptor.colorAttachments[0].sourceAlphaBlendFactor = .one
        descriptor.colorAttachments[0].destinationRGBBlendFactor = .one
        descriptor.colorAttachments[0].destinationAlphaBlendFactor = .one
```
■Step2:実行
---
iOsは実機で実行してくださいMacの場合は以下のようになります。
もとはこれが
![スクリーンショット 2018-11-30 12.30.01.png](https://qiita-image-store.s3.amazonaws.com/0/122121/6861de64-1368-10ef-ccf8-15a30a138dc8.png)

こうなります
![スクリーンショット 2018-12-03 18.28.37.png](https://qiita-image-store.s3.amazonaws.com/0/122121/1cda1fa4-7fc3-5ea4-97c1-6cb5c5189599.png)