Downscale CVPixelBuffer with metal

SampleBufferDelegate.swift

import AVFoundation
import MetalPerformanceShaders

final class SamplebufferDelegate: NSObject, MTLTextureSource, AVCaptureVideoDataOutputSampleBufferDelegate {
    var consumer: MTLTextureConsumer?

    private let device: MTLDevice = MTLCreateSystemDefaultDevice()!
    private var textureCache: CVMetalTextureCache!
    private let texAttributes = [kCVMetalTextureUsage as String: [MTLTextureUsage.shaderRead],
                                 kCVMetalTextureStorageMode as String: [MTLStorageMode.shared]] as CFDictionary

    override init() {
        super.init()
        guard CVMetalTextureCacheCreate(kCFAllocatorDefault, nil, device, nil, &textureCache) == kCVReturnSuccess else {
            fatalError("Can't create texture cache")
        }
    }

    func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
        guard let consumer = consumer else { return }
        guard let scaledSampleBuffer = self.buffer(with: sampleBuffer) else { return }
        print(scaledSampleBuffer)
    }

    func captureOutput(_ output: AVCaptureOutput, didDrop sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) {
        print("did drop sample buffer, alarm!")
    }
}

private extension SamplebufferDelegate {
    func buffer(with sampleBuffer: CMSampleBuffer) -> CVPixelBuffer? {
        autoreleasepool {
            guard let imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer) else { return nil }
            let width = CVPixelBufferGetWidth(imageBuffer)
            let height = CVPixelBufferGetHeight(imageBuffer)

            var cvMetalTextureOut: CVMetalTexture?
            let result = CVMetalTextureCacheCreateTextureFromImage(kCFAllocatorDefault,
                                                                   textureCache,
                                                                   imageBuffer,
                                                                   texAttributes,
                                                                   .bgra8Unorm,
                                                                   width,
                                                                   height,
                                                                   0,
                                                                   &cvMetalTextureOut)
            guard result == kCVReturnSuccess else { return nil }
            guard let tex = cvMetalTextureOut.flatMap(CVMetalTextureGetTexture) else { return nil }
            let scaleKernel = MPSImageLanczosScale(device: device)
            let transformFactor: MPSScaleTransform = .init(scaleX: 0.5, scaleY: 0.5, translateX: 0, translateY: 0)
            withUnsafePointer(to: transformFactor) { scaleKernel.scaleTransform = $0 }
            let descriptor: MTLTextureDescriptor = .init()
            descriptor.width = tex.width / 2
            descriptor.width = tex.height / 2
            descriptor.pixelFormat = tex.pixelFormat
            guard let scaledTexture = device.makeTexture(descriptor: descriptor) else { return nil }
            guard let commandBuffer = device.makeCommandQueue()?.makeCommandBuffer() else { return nil }
            scaleKernel.encode(commandBuffer: commandBuffer, sourceTexture: tex, destinationTexture: scaledTexture)
            commandBuffer.waitUntilCompleted()
            var outPixelbuffer: CVPixelBuffer?
            if let datas = scaledTexture.buffer?.contents() {
                CVPixelBufferCreateWithBytes(kCFAllocatorDefault,
                                             scaledTexture.width,
                                             scaledTexture.height,
                                             kCVPixelFormatType_64RGBAHalf,
                                             datas,
                                             scaledTexture.bufferBytesPerRow,
                                             nil,
                                             nil,
                                             nil,
                                             &outPixelbuffer);
            }
            return  outPixelbuffer
        }
    }
}