pixelation artifact when exporting video using Metal

Question

I have a very strange bug and only happens in random devices. I'm using avassetwriter to export a movie and drawing/shader with Metal, but i get some colored pixelated artifacts in the animated mp4 movie.

video: https://drive.google.com/file/d/1g6KyL18JqclOW1kVQkif3o3zaqylYaLk/view?usp=sharing

In the image/frame you can see colored artifact pixels, this usually happens when the line moves(animated line)

Any idea? thankyou

init

    init?(outputURL url: URL, size: CGSize) {

    do {
        assetWriter = try AVAssetWriter(outputURL: url, fileType: .mp4)
    } catch let error{
        print(error)
        return nil
    }
    let compressionProperties = NSDictionary(dictionary: [
        AVVideoAverageBitRateKey:ClipSettings.bitrate
        ])

    let outputSettings: [String: Any]
    if #available(iOS 11.0, *),AVAssetExportSession.allExportPresets().contains(AVAssetExportPresetHEVCHighestQuality) {
        outputSettings = [ AVVideoCodecKey : AVVideoCodecType.hevc,
                           AVVideoWidthKey : size.width,
                           AVVideoHeightKey : size.height,
                           AVVideoCompressionPropertiesKey:compressionProperties
        ]
    } else {
        // Fallback on earlier versions
        outputSettings = [ AVVideoCodecKey : AVVideoCodecType.h264,
                           AVVideoWidthKey : size.width,
                           AVVideoHeightKey : size.height,
                           AVVideoCompressionPropertiesKey:compressionProperties
        ]
    }

    assetWriterVideoInput = AVAssetWriterInput(mediaType: .video, outputSettings: outputSettings)
    assetWriterVideoInput.expectsMediaDataInRealTime = false


    let sourcePixelBufferAttributes: [String: Any] = [
        kCVPixelBufferPixelFormatTypeKey as String : kCVPixelFormatType_32BGRA,
        kCVPixelBufferWidthKey as String : size.width,
        kCVPixelBufferHeightKey as String : size.height
    ]

    assetWriterPixelBufferInput = AVAssetWriterInputPixelBufferAdaptor(assetWriterInput: assetWriterVideoInput,
                                                                       sourcePixelBufferAttributes: sourcePixelBufferAttributes)


    if assetWriter.canAdd(assetWriterVideoInput){
        assetWriter.add(assetWriterVideoInput)

    }else{
        print("add failed")
    }

this is how write a frame from metal texture

    func writeFrame(forTexture texture: MTLTexture, time: TimeInterval, podSticker: PodStickerView) {
    if !isRecording {
        return
    }
    let fps: Int32 = 60
    let intervalDuration = CFTimeInterval(1.0 / Double(fps))
    let timescale: Float = 600
    let kTimescale: Int32 = Int32(timescale)
    let frameDuration = CMTimeMake(
        value: Int64( floor(timescale / Float(fps)) ),
        timescale: kTimescale
    )

    var waitTime = 300.0 //fixes dropped frames
    while !assetWriterVideoInput.isReadyForMoreMediaData {
        let waitIntervale: TimeInterval = 0.001 * waitTime
        let maxDate = Date(timeIntervalSinceNow: waitIntervale)
        RunLoop.current.run(until: maxDate)
        waitTime += 200.0 // add 200ms every time

    }

    guard let pixelBufferPool = assetWriterPixelBufferInput.pixelBufferPool else {
        print("Pixel buffer asset writer input did not have a pixel buffer pool available; cannot retrieve frame")
        return
    }

    var maybePixelBuffer: CVPixelBuffer? = nil
    let status  = CVPixelBufferPoolCreatePixelBuffer(nil, pixelBufferPool, &maybePixelBuffer)
    if status != kCVReturnSuccess {
        print("Could not get pixel buffer from asset writer input; dropping frame...")
        return
    }

    guard let pixelBuffer = maybePixelBuffer else { return }

    CVPixelBufferLockBaseAddress(pixelBuffer, [])
    let pixelBufferBytes = CVPixelBufferGetBaseAddress(pixelBuffer)!

    // Use the bytes per row value from the pixel buffer since its stride may be rounded up to be 16-byte aligned
    let bytesPerRow = CVPixelBufferGetBytesPerRow(pixelBuffer)
    let region = MTLRegionMake2D(0, 0, texture.width, texture.height)

    texture.getBytes(pixelBufferBytes, bytesPerRow: bytesPerRow, from: region, mipmapLevel: 0)

    let presentationTime = CMTimeMultiply(frameDuration, multiplier:  Int32(frameNumber))
    Engine.renderTime = presentationTime.seconds



        //write video
        self.assetWriterPixelBufferInput.append(pixelBuffer, withPresentationTime: presentationTime)
        CVPixelBufferUnlockBaseAddress(pixelBuffer,[])

// } }

Answer 1

I had to change the usage of the MTLTexture from .shaderRead to .shaderWrite.

according to apple "In iOS devices with GPU family 5, Metal doesn’t apply lossless compression to the given texture if you set this option."