尝试使用AVFoundation AVAssetWriter将金属帧写入Quicktime文件时出现的所有黑色帧

我正在使用这个Swift类(最初显示在这个问题的答案中: Capture Metal MTKView as Movie in realtime? )尝试将我的金属应用程序框架录制到电影文件中。

class MetalVideoRecorder {
    var isRecording = false
    var recordingStartTime = TimeInterval(0)

    private var assetWriter: AVAssetWriter
    private var assetWriterVideoInput: AVAssetWriterInput
    private var assetWriterPixelBufferInput: AVAssetWriterInputPixelBufferAdaptor

    init?(outputURL url: URL, size: CGSize) {
        do {
            assetWriter = try AVAssetWriter(outputURL: url, fileType: AVFileTypeAppleM4V)
        } catch {
            return nil
        }

        let outputSettings: [String: Any] = [ AVVideoCodecKey : AVVideoCodecH264,
            AVVideoWidthKey : size.width,
            AVVideoHeightKey : size.height ]

        assetWriterVideoInput = AVAssetWriterInput(mediaType: AVMediaTypeVideo, outputSettings: outputSettings)
        assetWriterVideoInput.expectsMediaDataInRealTime = true

        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)

        assetWriter.add(assetWriterVideoInput)
    }

    func startRecording() {
        assetWriter.startWriting()
        assetWriter.startSession(atSourceTime: kCMTimeZero)

        recordingStartTime = CACurrentMediaTime()
        isRecording = true
    }

    func endRecording(_ completionHandler: @escaping () -> ()) {
        isRecording = false

        assetWriterVideoInput.markAsFinished()
        assetWriter.finishWriting(completionHandler: completionHandler)
    }

    func writeFrame(forTexture texture: MTLTexture) {
        if !isRecording {
            return
        }

        while !assetWriterVideoInput.isReadyForMoreMediaData {}

        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 frameTime = CACurrentMediaTime() - recordingStartTime
        let presentationTime = CMTimeMakeWithSeconds(frameTime, 240)
        assetWriterPixelBufferInput.append(pixelBuffer, withPresentationTime: presentationTime)

        CVPixelBufferUnlockBaseAddress(pixelBuffer, [])
    }
}

我没有看到任何错误,但生成的Quicktime文件中的帧都是黑色的。帧大小正确,像素格式正确(bgra8Unorm)。有人知道为什么它可能不起作用吗?

我在呈现并提交当前可绘制文件之前调用WriteName函数,如下所示:

        if let drawable = view.currentDrawable {

            if BigVideoWriter != nil && BigVideoWriter!.isRecording {
                commandBuffer.addCompletedHandler { commandBuffer in
                    BigVideoWriter?.writeFrame(forTexture: drawable.texture)
                }
            }

            commandBuffer.present(drawable)
            commandBuffer.commit()      
        }

最初我确实遇到了一个错误,我的MetalKitView层是“framebufferOnly”。所以我在尝试录制之前将其设置为false。这消除了错误,但帧都是黑色的。在程序开始时,我还尝试将其设置为false,但得到了相同的结果。

我还尝试使用'addCompletedHandler'而不是'addScheduledHandler',但这给了我一个错误“[CAMetalLayerDrawable texture]不应该在已经呈现了这个drawable之后被调用。取而代之的是下一个drawable。 ".

谢谢你的建议!

编辑:我在@Idogy的帮助下解决了这个问题。测试显示,最初的版本可以在iOS上运行,但不能在Mac上运行。他说,因为我有一个NVIDIA GPU,帧缓冲区是私有的。因此,我必须添加一个blitCommandEncoder,对纹理进行同步调用,然后它开始工作。这样地:

   if let drawable = view.currentDrawable {

        if BigVideoWriter != nil && BigVideoWriter!.isRecording {
 #if ISMAC
            if let blitCommandEncoder = commandBuffer.makeBlitCommandEncoder() {
                blitCommandEncoder.synchronize(resource: drawable.texture)
                blitCommandEncoder.endEncoding()
            }
 #endif
            commandBuffer.addCompletedHandler { commandBuffer in
                BigVideoWriter?.writeFrame(forTexture: drawable.texture)
            }
        }

        commandBuffer.present(drawable)
        commandBuffer.commit()    
    }