Question

我想阅读时间码音轨的时间价值。有一个来自Apple的优秀文档（参见Technical Note 2310）但它是用Objective C编写的。

我已经将核心逻辑翻译成了Swift 3.它的工作方式与 ObjC版本，表示来自时间码的CMSampleBuffer 轨道被读取并转换为CMBlockBuffer。我失败了创建数据指针CMBlockBufferGetDataPointer（在 timecodeFrame() func），这意味着原始数据总是如此给我0帧。所以它归结为这个问题，我该怎么做正确处理原始数据？

import Foundation
import AVFoundation
import CoreMedia

let movie = URL(fileURLWithPath: "videoWithTimecodeTrack.mov")
let asset = AVAsset(url: movie)

asset.loadValuesAsynchronously(forKeys: ["tracks"]) {

    var error: NSError?
    guard asset.statusOfValue(forKey: "tracks", error: &error) == AVKeyValueStatus.loaded
        else { if let error = error { return print(error) } }

    readStartTimecode(asset: asset)
}


func readStartTimecode(ofAsset asset: AVAsset) {

    let timecodeTracks = asset.tracks(withMediaType: AVMediaTypeTimecode)
    guard let timecodeTrack = timecodeTracks.first,
        let assetReader = try? AVAssetReader(asset: asset) else { return }

    let readerOutput = AVAssetReaderTrackOutput(track: timecodeTrack, outputSettings: nil)
    assetReader.add(readerOutput)
    guard assetReader.startReading() else { return }

    while let sampleBuffer = readerOutput.copyNextSampleBuffer() {
        if let frame = timecodeFrame(sampleBuffer: sampleBuffer) {
            print("timecodeFrame: \(frame)")
        }
    }
}

func timecodeFrame(sampleBuffer: CMSampleBuffer) -> UInt32? {

    guard let blockBuffer = CMSampleBufferGetDataBuffer(sampleBuffer),
        let formatDescription = CMSampleBufferGetFormatDescription(sampleBuffer)
        else { return nil }

    var rawData: UnsafeMutablePointer<Int8>? = nil
    var length: Int = 0
    var totalLength: Int = 0

    let status = CMBlockBufferGetDataPointer(blockBuffer, 0, &length, &totalLength, &rawData)
    guard status == kCMBlockBufferNoErr,
        let frameRead = rawData?.pointee
        else { return nil }

    let type = CMFormatDescriptionGetMediaSubType(formatDescription)

    if type == kCMTimeCodeFormatType_TimeCode32 {
        let frame = UInt32(frameRead)
        let bigFrame = CFSwapInt32BigToHost(frame)
        print("kCMTimeCodeFormatType_TimeCode32: \(bigFrame)")
    }
    if type == kCMTimeCodeFormatType_TimeCode64 {
        print("kCMTimeCodeFormatType_TimeCode64")
        // todo
    }
    return nil
}

编辑：数据指针检索的Objective C版本如下所示：

size_t length = 0;
size_t totalLength = 0;
char *rawData = NULL;

CMBlockBufferGetDataPointer(blockBuffer, 0, &length, &totalLength, &rawData);
if (status == kCMBlockBufferNoErr) {
    int32_t *frameNumberRead = (int32_t *)rawData;
    (int)Endian32_Swap(*frameNumberRead)]
}

Answer 1

解决方案是不转换Int8数据，如UInt32(rawData.pointee)，而是将UnsafeMutablePointer<Int8>指针的内存作为不同类型（临时）访问。这看起来像这样：

if let frames = rawData?.withMemoryRebound(to: UInt32.self, capacity: 1, { CFSwapInt32BigToHost($0.pointee) }) {
    return frames
}

完整的功能如下所示：

func timecodeFrame(sampleBuffer: CMSampleBuffer) -> UInt32? {

    guard let blockBuffer = CMSampleBufferGetDataBuffer(sampleBuffer),
        let formatDescription = CMSampleBufferGetFormatDescription(sampleBuffer)
        else { return nil }

    var rawData: UnsafeMutablePointer<Int8>? = nil
    var length: Int = 0
    var totalLength: Int = 0

    let status = CMBlockBufferGetDataPointer(blockBuffer, 0, &length, &totalLength, &rawData)
    guard status == kCMBlockBufferNoErr else { return nil }

    let type = CMFormatDescriptionGetMediaSubType(formatDescription)

    if type == kCMTimeCodeFormatType_TimeCode32 {
        if let frames = rawData?.withMemoryRebound(to: UInt32.self, capacity: 1, { CFSwapInt32BigToHost($0.pointee) }) {
            return frames
        }
    }
    if type == kCMTimeCodeFormatType_TimeCode64 {
        if let frames = rawData?.withMemoryRebound(to: UInt64.self, capacity: 1, { CFSwapInt64BigToHost($0.pointee) }) {
            return UInt32(frames)
        }
    }
    return nil
}

我希望这对想要阅读视频时间码轨道的开始时间码的其他人有用。

如何使用AVAsset和Swift 3读取时间码轨道？

1 个答案: