Question

我正在使用这个Swift类（原来在这个问题的答案中显示： Capture Metal MTKView as Movie in realtime?）尝试将我的Metal app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。帧的大小正确，我的像素格式正确（bgra8Unorm）。任何人都知道为什么它可能不起作用？

我在呈现之前调用writeFrame函数并提交当前drawable，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()      
        }

我最初得到了一个错误，我的MetalKitView图层是＆＃39; framebufferOnly＆＃39;。所以我在尝试录制之前将其设置为false。摆脱了错误，但帧都是黑色的。我也尝试在程序的最开始将它设置为false，但我得到了相同的结果。

我也尝试过使用＆＃39; addCompletedHandler＆＃39;而不是＆＃39; addScheduledHandler＆＃39;，但这给了我错误＆＃34; [CAMetalLayerDrawable texture]不应该在已经呈现这个drawable之后被调用。改为获得nextDrawable。＆＃34;

感谢您的任何建议！

编辑：我在@Idogy的帮助下解决了这个问题。测试显示原始版本适用于iOS但不适用于Mac。他说，因为我有一个NVIDIA GPU，所以帧缓冲区是私有的。所以我不得不在纹理上添加一个带有同步调用的blitCommandEncoder，然后它就开始工作了。像这样：

   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()    
    }

Answer 1

我相信你过早地编写你的帧 - 通过在你的渲染循环中调用writeFrame，你实际上是在它仍然是空的时候捕获drawable（GPU只是没有＆＃t; t渲染它。）

请记住，在致电commmandBuffer.commit()之前，GPU甚至开始渲染你的框架。在尝试抓取结果帧之前，您需要等待GPU完成渲染。该序列有点令人困惑，因为您在调用present()之前还调用commit()，但这不是运行时的实际操作顺序。那个present调用只是告诉Metal安排一次调用，一旦GPU完成呈现，就将你的帧呈现给屏幕。

您应该在完成处理程序中调用writeFrame（使用commandBuffer.addCompletedHandler()）。那应该照顾好这个。

更新：虽然上面的答案是正确的，但它只是部分的。由于OP使用带有私有VRAM的独立GPU，因此CPU无法看到渲染目标像素。该问题的解决方案是添加MTLBlitCommandEncoder，并使用synchronize()方法确保渲染的像素从GPU的VRAM复制回RAM。

尝试使用AVFoundation AVAssetWriter将Metal框架写入Quicktime文件时的所有黑框

1 个答案: