我必须在视频中应用不透明度。我必须在一秒钟的视频结束前应用它。我正在使用“firstInstruction”来获取视频的总时长。然而，当我调用“firstInstruction.setOpacityRamp”方法时，我无法减去第二个..

    let mainInstruction = AVMutableVideoCompositionInstruction()
    mainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, CMTimeAdd(firstAsset.duration, secondAsset.duration))
    let firstInstruction = VideoHelper.videoCompositionInstruction(firstTrack, asset: firstAsset)
    firstInstruction.setOpacityRamp(fromStartOpacity: 1, toEndOpacity: 0.1, timeRange: mainInstruction.timeRange)

最佳答案

我会使用三个指令来应用交叉淡入淡出:

1. 仅显示第一个视频轨道的“直通”指令，直到第一个 Assets 结束前一秒。
2. 同时显示第一个视频轨道的最后一秒和第二个视频轨道的第一秒的交叉淡入淡出指令，带有不透明度斜坡。
3. 仅显示第二个视频轨道的“直通”指令，从一秒开始进入第二个视频轨道。

所以，首先，让我们了解一下轨道:

import AVFoundation
import CoreVideo

func crossFade(asset0: AVAsset, asset1: AVAsset, crossFadeDuration: CMTime, to outputUrl: URL) throws {
    guard
        let asset0Track = asset0.tracks(withMediaType: .video).first,
        let asset1Track = asset1.tracks(withMediaType: .video).first,
        case let composition = AVMutableComposition(),
        case let compositionTrack0Id = composition.unusedTrackID(),
        let compositionTrack0 = composition.addMutableTrack(
            withMediaType: .video, preferredTrackID: compositionTrack0Id),
        case let compositionTrack1Id = composition.unusedTrackID(),
        let compositionTrack1 = composition.addMutableTrack(
            withMediaType: .video, preferredTrackID: compositionTrack1Id)
        else { return }

现在让我们计算我们需要的所有时间。首先，组合中 asset0Track 的整个范围，包括直通和交叉淡入淡出期间:

    // When does asset0Track start, in the composition?
    let asset0TrackStartTime = CMTime.zero

    // When does asset0Track end, in the composition?
    let asset0TrackEndTime = asset0TrackStartTime + asset0Track.timeRange.duration

接下来，交叉淡入淡出的时间范围:

    // When does the cross-fade end, in the composition?
    // It should end exactly at the end of asset0's video track.
    let crossFadeEndTime = asset0TrackEndTime

    // When does the cross-fade start, in the composition?
    let crossFadeStartTime = crossFadeEndTime - crossFadeDuration

    // What is the entire time range of the cross-fade, in the composition?
    let crossFadeTimeRange = CMTimeRangeMake(
        start: crossFadeStartTime,
        duration: crossFadeDuration)

接下来，合成中 asset1Track 的整个范围，包括交叉淡入淡出和直通期间:

    // When does asset1Track start, in the composition?
    // It should start exactly at the start of the cross-fade.
    let asset1TrackStartTime = crossFadeStartTime

    // When does asset1Track end, in the composition?
    let asset1TrackEndTime = asset1TrackStartTime + asset1Track.timeRange.duration

最后，两个传递时间范围:

    // What is the time range during which only asset0 is visible, in the composition?
    let compositionTrack0PassThroughTimeRange = CMTimeRangeMake(
        start: asset0TrackStartTime,
        duration: crossFadeStartTime - asset0TrackStartTime)

    // What is the time range during which only asset1 is visible, in the composition?
    let compositionTrack1PassThroughTimeRange = CMTimeRangeMake(
        start: crossFadeEndTime,
        duration: asset1TrackEndTime - crossFadeEndTime)

现在我们可以将输入轨道插入合成的轨道中:

    // Put asset0Track into compositionTrack0.
    try compositionTrack0.insertTimeRange(
        asset0Track.timeRange,of: asset0Track, at: asset0TrackStartTime)

    // Put asset1Track into compositionTrack1.
    try compositionTrack1.insertTimeRange(
        asset1Track.timeRange, of: asset1Track, at: asset1TrackStartTime)

这就是我们需要为 AVMutableComposition 做的所有事情。但是我们还需要制作一个AVMutableVideoComposition:

    let videoComposition = AVMutableVideoComposition()
    videoComposition.frameDuration =
        min(asset0Track.minFrameDuration, asset1Track.minFrameDuration)
    videoComposition.renderSize = CGSize(
        width: max(asset0Track.naturalSize.width, asset1Track.naturalSize.width),
        height: max(asset0Track.naturalSize.height, asset1Track.naturalSize.height))

我们需要设置视频合成的指令。第一条指令是在适当的时间范围内仅通过 compositionTrack0:

    // I'm using a helper function defined below.
    let compositionTrack0PassThroughInstruction = AVMutableVideoCompositionInstruction.passThrough(
        trackId: compositionTrack0Id, timeRange: compositionTrack0PassThroughTimeRange)

第二个指令是交叉淡入淡出，所以比较复杂。它需要两个子指令，一个用于交叉淡入淡出中的每一层。每一层指令，以及整体交叉淡入淡出指令，使用相同的时间范围:

    let crossFadeLayer0Instruction = AVMutableVideoCompositionLayerInstruction()
    crossFadeLayer0Instruction.trackID = compositionTrack0Id
    crossFadeLayer0Instruction.setOpacityRamp(fromStartOpacity: 1, toEndOpacity: 0, timeRange: crossFadeTimeRange)

    let crossFadeLayer1Instruction = AVMutableVideoCompositionLayerInstruction()
    crossFadeLayer1Instruction.trackID = compositionTrack1Id
    crossFadeLayer1Instruction.setOpacityRamp(fromStartOpacity: 0, toEndOpacity: 1, timeRange: crossFadeTimeRange)

    let crossFadeInstruction = AVMutableVideoCompositionInstruction()
    crossFadeInstruction.timeRange = crossFadeTimeRange
    crossFadeInstruction.layerInstructions = [crossFadeLayer0Instruction, crossFadeLayer1Instruction]

第三条指令是在适当的时间范围内仅通过 compositionTrack1:

    let compositionTrack1PassThroughInstruction = AVMutableVideoCompositionInstruction.passThrough(
        trackId: compositionTrack1Id, timeRange: compositionTrack1PassThroughTimeRange)

现在我们已经有了所有三个指令，我们可以将它们提供给视频组合:

    videoComposition.instructions = [compositionTrack0PassThroughInstruction, crossFadeInstruction, compositionTrack1PassThroughInstruction]

现在我们可以一起使用composition和videoComposition，例如导出一个新的电影文件:

    let export = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetMediumQuality)!
    export.outputURL = outputUrl
    export.videoComposition = videoComposition
    export.exportAsynchronously {
        exit(0)
    }
}

这是我用来创建传递指令的助手:

extension AVMutableVideoCompositionInstruction {
    static func passThrough(trackId: CMPersistentTrackID, timeRange: CMTimeRange) -> AVMutableVideoCompositionInstruction {
        let layerInstruction = AVMutableVideoCompositionLayerInstruction()
        layerInstruction.trackID = trackId

        let instruction = AVMutableVideoCompositionInstruction()
        instruction.timeRange = timeRange
        instruction.layerInstructions = [layerInstruction]

        return instruction
    }
}

这是我的测试代码。我使用 macOS 命令行应用程序进行测试:

let asset0 = AVURLAsset(url: URL(fileURLWithPath: "/tmp/asset0.mp4"))
let asset1 = AVURLAsset(url: URL(fileURLWithPath: "/tmp/asset1.mp4"))

let outputUrl = URL(fileURLWithPath: "/tmp/output.mp4")
try! crossFade(asset0: asset0, asset1: asset1, crossFadeDuration: CMTimeMake(value: 1, timescale: 1), to: outputUrl)

dispatchMain()

结果:

请注意，由于 Stack Overflow 对图像文件大小的限制，我不得不将动画制作得很小且颜色很浅。

输入视频由 Jeffrey Beach 提供.

关于ios - 计算视频之间淡入淡出的时间，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/54119791/