AVMutableComposition 输出冻结在第一个视频的最后一帧

Question

我正在尝试使用 AVMutableComposition 将多个剪辑（视频）合并为一个，我已经成功地完成了这项工作以及旋转和平移每条指令，但是，仍然存在一个问题。当第一个剪辑完成时，输出冻结在它的最后一帧（第一个剪辑的最后一帧）；只有当有另一个剪辑可见时才会发生这种情况，例如，如果我将第二个和第三个剪辑的不透明度在 CMTime.Zero 处设置为 0，第一个在 firstClip.Duration 处设置为 0，则结果将是一个显示第一个剪辑视频的视频，一旦完成，它就会显示黑色背景。剪辑的音频完美无缺。

这是我的代码：

        public void TESTING()
        {
            //microphone
            AVCaptureDevice microphone = AVCaptureDevice.DefaultDeviceWithMediaType(AVMediaType.Audio);

            AVMutableComposition mixComposition = AVMutableComposition.Create();
            AVVideoCompositionLayerInstruction[] Instruction_Array = new AVVideoCompositionLayerInstruction[Clips.Count];

            foreach (string clip in Clips)
            {
                var asset = AVUrlAsset.FromUrl(new NSUrl(clip, false)) as AVUrlAsset;
                #region HoldVideoTrack
                
                //This range applies to the video, not to the mixcomposition
                CMTimeRange range = new CMTimeRange()
                {
                    Start = CMTime.Zero,
                    Duration = asset.Duration
                };

                var duration = mixComposition.Duration;
                NSError error;

                AVMutableCompositionTrack videoTrack = mixComposition.AddMutableTrack(AVMediaType.Video, 0);
                AVAssetTrack assetVideoTrack = asset.TracksWithMediaType(AVMediaType.Video)[0];
                videoTrack.InsertTimeRange(range, assetVideoTrack, duration, out error);
                videoTrack.PreferredTransform = assetVideoTrack.PreferredTransform;

                if (microphone != null)
                {
                    AVMutableCompositionTrack audioTrack = mixComposition.AddMutableTrack(AVMediaType.Audio, 0);
                    AVAssetTrack assetAudioTrack = asset.TracksWithMediaType(AVMediaType.Audio)[0];
                    audioTrack.InsertTimeRange(range, assetAudioTrack, duration, out error);
                }
                #endregion

                AVAssetTrack videoTrackWithMediaType = mixComposition.TracksWithMediaType(AVMediaType.Video)[0];

                var instruction = AVMutableVideoCompositionLayerInstruction.FromAssetTrack(videoTrackWithMediaType);
                
                #region Instructions
                int counter = Clips.IndexOf(clip);
                Instruction_Array[counter] = TestingInstruction(asset, mixComposition.Duration, videoTrackWithMediaType);
                #endregion
            }

            // 6
            AVMutableVideoCompositionInstruction mainInstruction = AVMutableVideoCompositionInstruction.Create() as AVMutableVideoCompositionInstruction;

            CMTimeRange rangeIns = new CMTimeRange()
            {
                Start = new CMTime(0, 0),
                Duration = mixComposition.Duration
            };
            mainInstruction.TimeRange = rangeIns;
            mainInstruction.LayerInstructions = Instruction_Array;

            var mainComposition = AVMutableVideoComposition.Create();
            mainComposition.Instructions = new AVVideoCompositionInstruction[1] { mainInstruction };
            mainComposition.FrameDuration = new CMTime(1, 30);
            mainComposition.RenderSize = new CGSize(mixComposition.NaturalSize.Height, mixComposition.NaturalSize.Width);

            finalVideo_path = NSUrl.FromFilename(Path.Combine(Path.GetTempPath(), "Whole2.mov"));
            if (File.Exists(Path.GetTempPath() + "Whole2.mov"))
            {
                File.Delete(Path.GetTempPath() + "Whole2.mov");
            }

            //... export video ...
            AVAssetExportSession exportSession = new AVAssetExportSession(mixComposition, AVAssetExportSessionPreset.HighestQuality)
            {
                OutputUrl = NSUrl.FromFilename(Path.Combine(Path.GetTempPath(), "Whole2.mov")),
                OutputFileType = AVFileType.QuickTimeMovie,
                ShouldOptimizeForNetworkUse = true,
                VideoComposition = mainComposition
            };
            exportSession.ExportAsynchronously(_OnExportDone);

            FinalVideo = Path.Combine(Path.GetTempPath(), "Whole2.mov");
        }

        private AVMutableVideoCompositionLayerInstruction TestingInstruction(AVAsset asset, CMTime currentTime, AVAssetTrack mixComposition_video_Track)
        {
            var instruction = AVMutableVideoCompositionLayerInstruction.FromAssetTrack(mixComposition_video_Track);

            var startTime = CMTime.Subtract(currentTime, asset.Duration);

            //NaturalSize.Height is passed as a width parameter because IOS stores the video recording horizontally 
            CGAffineTransform translateToCenter = CGAffineTransform.MakeTranslation(mixComposition_video_Track.NaturalSize.Height, 0);
            //Angle in radiants, not in degrees
            CGAffineTransform rotate = CGAffineTransform.Rotate(translateToCenter, (nfloat)(Math.PI / 2));

            instruction.SetTransform(rotate, (CMTime.Subtract(currentTime, asset.Duration)));

            instruction.SetOpacity(1, startTime);
            instruction.SetOpacity(0, currentTime);

            return instruction;
        }
    }

有人知道怎么解决吗？

如果您需要更多信息，我会在看到您的请求后立即提供。谢谢大家的宝贵时间，祝你有美好的一天。 (:

Answer 1

我相信我已经解决了您代码中的问题。您只是在第一条轨道上创建说明。看看这里的这两行：

AVAssetTrack videoTrackWithMediaType = mixComposition.TracksWithMediaType(AVMediaType.Video)[0];

var instruction = AVMutableVideoCompositionLayerInstruction.FromAssetTrack(videoTrackWithMediaType);

AVMutableComposition.tracksWithMediaType 获取一个轨道数组，因此在第一行末尾 [0]，仅抓取组合中的第一条轨道，即第一个视频。当您循环播放时，您只是在多次为第一个视频创建说明。

你的代码和我对 Xamarin 的不熟悉让我感到困惑，但我相信你可以这样做并且它应该可以工作：

更改这些行：

AVAssetTrack videoTrackWithMediaType =     mixComposition.TracksWithMediaType(AVMediaType.Video)[0];

var instruction = AVMutableVideoCompositionLayerInstruction.FromAssetTrack(videoTrackWithMediaType);
                
#region Instructions
int counter = Clips.IndexOf(clip);
Instruction_Array[counter] = TestingInstruction(asset, mixComposition.Duration, videoTrackWithMediaType);
#endregion

为此：

var instruction = AVMutableVideoCompositionLayerInstruction.FromAssetTrack(videoTrack);
                
#region Instructions
int counter = Clips.IndexOf(clip);
Instruction_Array[counter] = TestingInstruction(asset, mixComposition.Duration, videoTrack);
#endregion

我在这里所做的只是删除您创建的 videoTracksWithMediaType 变量，并改用 videoTrack。无需获取相应的轨道，因为您已经创建了它，并且在创建指令时仍然可以在您所在的代码块中访问它。

AVMutableComposition 输出冻结在第一个视频的最后一帧

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