iOS 高级视频编辑：深入探索 AVAssetReader 和 AVAssetWriter

掌控视频编辑的利器：揭秘 AVAssetReader 和 AVAssetWriter

在视频无处不在的移动时代，作为 iOS 开发者，精通高级视频编辑技术至关重要。AVAssetReader 和 AVAssetWriter 作为两大核心框架，将助力我们深入操纵媒体数据，实现丰富的编辑和转换功能。

AVAssetReader：窥探媒体数据的秘密

AVAssetReader 宛如一把钥匙，让我们窥探媒体数据的内部构造。通过与 AVAsset 关联，我们可以深入了解视频格式、元数据和音频流的奥秘。这种访问权限让我们得以施展妙手，例如：

• 提取视频帧： 逐帧抓取视频数据，开启图像处理或视频分析之旅。
• 解析音轨： 剥离音频信息，单独导出为文件，让声音自由翱翔。
• 检阅元数据： 尽情探索视频标题、创作者等信息，为其注入新的生命力。

AVAssetWriter：重塑媒体数据的艺术

AVAssetWriter 是 AVAssetReader 的孪生兄弟，赋予我们改写媒体数据的能力。同样地，我们首先建立一个 AVAssetWriter 对象，再与 AVAsset 携手合作。随后，AVAssetWriterInput 和 AVAssetWriterOutput 对象闪亮登场，等待我们输入和输出新的视频和音频数据。这些数据可由外部来源提供，亦可通过我们自己的代码生成。

依托 AVAssetWriter，我们可以大展身手：

• 转码视频： 挥洒自如地将视频从一种格式转化为另一种，例如将 H.264 幻化为 H.265。
• 裁剪视频： 挥舞剪刀，去除视频中多余的部分，例如广告或片头片尾，留下精华。
• 合并视频： 将多个视频片段无缝衔接，组成一部宏大的史诗。
• 添加水印： 在视频画布上留下印记，叠加文本或图像水印，彰显个性。

高级视频编辑任务的舞台

AVAssetReader 和 AVAssetWriter 携手共舞，为我们搭建起高级视频编辑任务的广阔舞台，让我们尽情施展才华：

• 快进或慢放： 通过调整帧速率或时间尺速率，让时间在手中自由流淌。
• 滤镜与特效： 借助 Core Image 或其他框架，为视频帧披上色彩斑斓的外衣。
• 稳定视频： 抹平视频中的抖动和晃动，呈现稳如泰山的画面。
• 交互式视频： 赋予用户选择不同故事情节或结局的权力，打造身临其境的体验。

结语

AVAssetReader 和 AVAssetWriter，这对黄金搭档，为 iOS 开发者开启了媒体数据的无限可能。熟练掌握这些框架，我们将突破传统视频编辑的桎梏，释放创造力的洪流。

常见问题解答

1. 如何使用 AVAssetReader 提取视频帧？

import AVFoundation

let asset = AVAsset(url: URL(fileURLWithPath: "path/to/video.mp4"))
let reader = try! AVAssetReader(asset: asset)
let outputSettings: [String: Any] = [
    kCVPixelBufferPixelFormatTypeKey as String: kCVPixelFormatType_32BGRA
]
let output = AVAssetReaderTrackOutput(track: asset.tracks[0], outputSettings: outputSettings)
reader.add(output)

reader.startReading()

while reader.status == .reading {
    if let sampleBuffer = output.copyNextSampleBuffer() {
        // Do something with the video frame
    }
}

2. 如何使用 AVAssetWriter 将视频转码为 H.265？

import AVFoundation

let inputURL = URL(fileURLWithPath: "path/to/input.mp4")
let outputURL = URL(fileURLWithPath: "path/to/output.mp4")

let asset = AVAsset(url: inputURL)
let reader = try! AVAssetReader(asset: asset)
let outputSettings: [String: Any] = [
    AVVideoCodecKey: AVVideoCodecType.hevc,
    AVVideoWidthKey: 1280,
    AVVideoHeightKey: 720
]
let output = AVAssetWriterInput(mediaType: .video, outputSettings: outputSettings)
output.transform = asset.tracks[0].preferredTransform

let writer = try! AVAssetWriter(outputURL: outputURL, fileType: .mp4)
writer.add(output)

reader.startReading()
writer.startWriting()

while reader.status == .reading {
    if let sampleBuffer = output.copyNextSampleBuffer() {
        writer.append(sampleBuffer)
    }
}

reader.finishReading()
writer.finishWriting()

3. 如何使用 AVAssetWriter 给视频添加水印？

import AVFoundation

let inputURL = URL(fileURLWithPath: "path/to/input.mp4")
let outputURL = URL(fileURLWithPath: "path/to/output.mp4")

let asset = AVAsset(url: inputURL)
let reader = try! AVAssetReader(asset: asset)
let videoTrack = asset.tracks(withMediaType: .video)[0]
let outputSettings: [String: Any] = [
    AVVideoCodecKey: videoTrack.codecType,
    AVVideoWidthKey: videoTrack.naturalSize.width,
    AVVideoHeightKey: videoTrack.naturalSize.height
]
let videoOutput = AVAssetWriterInput(mediaType: .video, outputSettings: outputSettings)

let watermarkImage = UIImage(named: "watermark.png")!
let watermarkFilter = CIFilter(name: "CISourceOverCompositing")!
watermarkFilter.setValue(CIImage(image: watermarkImage), forKey: "inputImage")

let composition = AVVideoComposition(asset: asset) { request in
    let watermarkLayer = CALayer()
    watermarkLayer.contents = watermarkImage.cgImage
    watermarkLayer.frame = CGRect(x: 100, y: 100, width: watermarkImage.size.width, height: watermarkImage.size.height)

    let transform = request.sourceImage.transform
        .translatedBy(x: watermarkLayer.frame.origin.x, y: watermarkLayer.frame.origin.y)
        .scaledBy(x: watermarkLayer.frame.width / request.sourceImage.extent.width,
                  y: watermarkLayer.frame.height / request.sourceImage.extent.height)

    watermarkFilter.setValue(request.sourceImage, forKey: "inputBackgroundImage")
    let watermarkImage = watermarkFilter.outputImage?.transformed(by: transform)

    request.context?.draw(watermarkImage!, over: request.sourceImage.extent)
}

let watermarkLayer = AVVideoCompositionLayerInstruction(assetTrack: videoTrack, compositionInstruction: composition)

let writer = try! AVAssetWriter(outputURL: outputURL, fileType: .mp4)
writer.add(videoOutput)
writer.add(composition)

reader.startReading()
writer.startWriting()

while reader.status == .reading {
    if let sampleBuffer = output.copyNextSampleBuffer() {
        writer.append(sampleBuffer)
    }
}

reader.finishReading()
writer.finishWriting()

4. 如何使用 AVAssetWriter 裁剪视频？

import AVFoundation

let inputURL = URL(fileURLWithPath: "path/to/input.mp4")
let outputURL = URL(fileURLWithPath: "path/to/output.mp4")

let asset = AVAsset(url: inputURL)
let startTime = CMTime(seconds: 10, preferredTimescale: 600)
let endTime = CMTime(seconds: 20, preferredTimescale: 600)
let timeRange = CMTimeRange(start: startTime, duration: endTime - startTime)

let reader = try! AVAssetReader(asset: asset)
let videoTrack = asset.tracks(withMediaType: .video)[0]
let outputSettings: [String: Any] = [
    AVVideoCodecKey: videoTrack.codecType,
    AVVideoWidthKey: videoTrack.naturalSize.width,
    AVVideoHeightKey: videoTrack.naturalSize.height
]
let videoOutput = AVAssetWriterInput(mediaType: .video, outputSettings: outputSettings)
videoOutput.timeRange = timeRange

let writer = try! AVAssetWriter(outputURL: outputURL, fileType: .mp4)
writer.add(videoOutput)

reader.startReading()
writer.startWriting()

while reader.status == .reading {
    if let sampleBuffer = output.copyNextSampleBuffer() {
        if sampleBuffer.presentationTimeStamp >= startTime && sampleBuffer.presentationTimeStamp < endTime {
            writer.append(sampleBuffer)
        }
    }
}