Swift实现MP4视频导出时修改帧率与码率的方法

要自定义导出MP4的帧率和码率，仅靠AVAssetExportSession的系统预设无法实现精细控制，需要改用AVAssetWriter手动配置编码参数。以下是替换后的完整实现：

func convertMOVtoMP4WithCustomParams(inputURL: URL, outputURL: URL, targetFrameRate: Int, targetBitRate: Int, completion: @escaping (Bool) -> Void) {
    let asset = AVURLAsset(url: inputURL)
    
    // 获取视频轨和音频轨
    guard let videoTrack = asset.tracks(withMediaType: .video).first,
          let audioTrack = asset.tracks(withMediaType: .audio).first else {
        completion(false)
        return
    }
    
    // 初始化AVAssetWriter
    guard let writer = try? AVAssetWriter(outputURL: outputURL, fileType: .mp4) else {
        completion(false)
        return
    }
    
    // 配置视频输入参数
    let videoSettings: [String: Any] = [
        AVVideoCodecKey: AVVideoCodecType.h264,
        AVVideoWidthKey: videoTrack.naturalSize.width,
        AVVideoHeightKey: videoTrack.naturalSize.height,
        AVVideoAverageBitRateKey: targetBitRate, // 平均码率，单位bps
        AVVideoExpectedFrameRateKey: targetFrameRate, // 目标帧率
        AVVideoCompressionPropertiesKey: [
            AVVideoProfileLevelKey: AVVideoProfileLevelH264BaselineAutoLevel,
            AVVideoMaxKeyFrameIntervalKey: targetFrameRate * 2 // 关键帧间隔
        ]
    ]
    
    let videoInput = AVAssetWriterInput(mediaType: .video, outputSettings: videoSettings)
    videoInput.expectsMediaDataInRealTime = false
    videoInput.transform = videoTrack.preferredTransform
    
    // 配置音频输入参数
    let audioSettings: [String: Any] = [
        AVFormatIDKey: kAudioFormatMPEG4AAC,
        AVNumberOfChannelsKey: 2,
        AVSampleRateKey: 44100,
        AVEncoderBitRateKey: 128000 // 音频码率，128kbps
    ]
    
    let audioInput = AVAssetWriterInput(mediaType: .audio, outputSettings: audioSettings)
    audioInput.expectsMediaDataInRealTime = false
    
    // 添加输入到writer
    if writer.canAdd(videoInput) {
        writer.add(videoInput)
    }
    if writer.canAdd(audioInput) {
        writer.add(audioInput)
    }
    
    // 开始写入
    let reader = AVAssetReader(asset: asset)!
    let videoOutput = AVAssetReaderTrackOutput(track: videoTrack, outputSettings: [kCVPixelBufferPixelFormatTypeKey as String: kCVPixelFormatType_32BGRA])
    let audioOutput = AVAssetReaderTrackOutput(track: audioTrack, outputSettings: nil)
    
    if reader.canAdd(videoOutput) {
        reader.add(videoOutput)
    }
    if reader.canAdd(audioOutput) {
        reader.add(audioOutput)
    }
    
    writer.startWriting()
    writer.startSession(atSourceTime: .zero)
    reader.startReading()
    
    // 处理视频数据写入
    videoInput.requestMediaDataWhenReady(on: DispatchQueue.global()) {
        while videoInput.isReadyForMoreMediaData {
            if let sampleBuffer = videoOutput.copyNextSampleBuffer() {
                videoInput.append(sampleBuffer)
            } else {
                videoInput.markAsFinished()
                break
            }
        }
    }
    
    // 处理音频数据写入
    audioInput.requestMediaDataWhenReady(on: DispatchQueue.global()) {
        while audioInput.isReadyForMoreMediaData {
            if let sampleBuffer = audioOutput.copyNextSampleBuffer() {
                audioInput.append(sampleBuffer)
            } else {
                audioInput.markAsFinished()
                break
            }
        }
    }
    
    // 监听写入完成
    writer.finishWriting {
        completion(writer.status == .completed)
    }
}

关键参数说明 #

• 目标帧率：通过AVVideoExpectedFrameRateKey设置，比如传入30就代表30帧每秒，可根据需求调整为24、60等数值。
• 目标码率：AVVideoAverageBitRateKey控制视频平均码率，单位是bps（比特每秒），比如1000000代表1Mbps，数值越小视频体积越小，但画质会相应降低。
• 音频码率：音频部分的AVEncoderBitRateKey可按需调整，128kbps是常见的平衡音质和体积的取值。

调用示例：

// 转成30帧、1Mbps码率的MP4
convertMOVtoMP4WithCustomParams(
    inputURL: movFileURL,
    outputURL: mp4FileURL,
    targetFrameRate: 30,
    targetBitRate: 1000000
) { success in
    print(success ? "转换成功" : "转换失败")
}

内容的提问来源于stack exchange，提问作者Wahab Khan Jadon