AVAudioMix 音频回避不起作用

Question

我正在尝试对我构建的 AVMutableComposition 进行音频编辑。

        var commentaryTimeRange = CMTimeRange(start: commentaryItem.startTimeInTimeline, duration: commentaryItem.timeRange.duration)
        if CMTimeCompare(CMTimeRangeGetEnd(commentaryTimeRange), composition.duration) == 1 {
            commentaryTimeRange.duration = CMTimeSubtract(composition.duration, commentaryTimeRange.start);
            commentaryItem.timeRange = commentaryTimeRange
        }
        // Add the commentary track
        let compositionCommentaryTrack = composition.addMutableTrack(withMediaType: AVMediaTypeAudio, preferredTrackID: kCMPersistentTrackID_Invalid)
        let track = commentaryItem.asset.tracks(withMediaType: AVMediaTypeAudio).first!
        try! compositionCommentaryTrack.insertTimeRange(CMTimeRange(start: kCMTimeZero, duration:commentaryTimeRange.duration), of: track, at: commentaryTimeRange.start)

        let tracksToDuck = composition.tracks(withMediaType: AVMediaTypeAudio)
        var trackMixArray = [AVMutableAudioMixInputParameters]()
        let rampDuration = CMTime(seconds: 1, preferredTimescale: 2)
        for track in tracksToDuck {
            let trackMix = AVMutableAudioMixInputParameters(track: track)
            trackMix.setVolumeRamp(fromStartVolume: 1.0, toEndVolume: 0.2, timeRange: CMTimeRange(start: CMTimeSubtract(commentaryTimeRange.start, rampDuration), duration: CMTimeSubtract(commentaryTimeRange.duration, rampDuration)))
            trackMix.setVolumeRamp(fromStartVolume: 0.2, toEndVolume: 1.0, timeRange: CMTimeRange(start: CMTimeRangeGetEnd(commentaryTimeRange), duration: rampDuration))
            trackMixArray.append(trackMix)
        }
        let audioMix = AVMutableAudioMix()
        audioMix.inputParameters = trackMixArray

基本上我是想通过降低原始音量在视频轨道上添加评论。
音频在输出中正确混合，但音频指令似乎被忽略了。
当然，audiomix 被传递给 AVPlayerItem，通过调试我可以看到所有指令都在那里并正确传递给它。

 func makePlayable() -> AVPlayerItem {
        let playerItem = AVPlayerItem(asset: composition.copy() as! AVAsset, automaticallyLoadedAssetKeys: NewsPlayerViewController.assetKeysRequiredToPlay)
        playerItem.videoComposition = videoComposition
        playerItem.audioMix = audioMix?.copy() as! AVAudioMix?
        if let overlayLayer = overlayLayer {
            let syncLayer = AVSynchronizedLayer(playerItem: playerItem)
            syncLayer.addSublayer(overlayLayer)
            playerItem.syncLayer = syncLayer
        }
        return playerItem
 }

我找到了一些答案，指出缺少曲目标识符的原因，或者有一个曲目和没有一个曲目之间的某种不匹配。
我的作品不使用任何曲目 ID，而且 Apple 的 AVEdit 示例代码不使用它们并且可以正常工作。

Answer 1

解决方案是在添加评论曲目之前简单地计算要闪避的曲目数。

      let tracksToDuck = composition.tracks(withMediaType: AVMediaTypeAudio)// <- MOVE HERE, AT THE TOP

        var commentaryTimeRange = CMTimeRange(start: commentaryItem.startTimeInTimeline, duration: commentaryItem.timeRange.duration)
        if CMTimeCompare(CMTimeRangeGetEnd(commentaryTimeRange), composition.duration) == 1 {
            commentaryTimeRange.duration = CMTimeSubtract(composition.duration, commentaryTimeRange.start);
            commentaryItem.timeRange = commentaryTimeRange
        }
        // Add the commentary track
        let compositionCommentaryTrack = composition.addMutableTrack(withMediaType: AVMediaTypeAudio, preferredTrackID: kCMPersistentTrackID_Invalid)
        let track = commentaryItem.asset.tracks(withMediaType: AVMediaTypeAudio).first!
        try! compositionCommentaryTrack.insertTimeRange(CMTimeRange(start: kCMTimeZero, duration:commentaryTimeRange.duration), of: track, at: commentaryTimeRange.start)

        var trackMixArray = [AVMutableAudioMixInputParameters]()
        let rampDuration = CMTime(seconds: 1, preferredTimescale: 2)
        for track in tracksToDuck {
            let trackMix = AVMutableAudioMixInputParameters(track: track)
            trackMix.setVolumeRamp(fromStartVolume: 1.0, toEndVolume: 0.2, timeRange: CMTimeRange(start: CMTimeSubtract(commentaryTimeRange.start, rampDuration), duration: CMTimeSubtract(commentaryTimeRange.duration, rampDuration)))
            trackMix.setVolumeRamp(fromStartVolume: 0.2, toEndVolume: 1.0, timeRange: CMTimeRange(start: CMTimeRangeGetEnd(commentaryTimeRange), duration: rampDuration))
            trackMixArray.append(trackMix)
        }
        let audioMix = AVMutableAudioMix()
        audioMix.inputParameters = trackMixArray

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