老生常谈：视频的合成
请勿转载~
ps： 只讲解其中的关键点和我自己在使用过程遇到的坑，如果有什么问题请在简书留言。
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- (IBAction)mergeVideo:(id)sender
{
if (YES) {
/* 合成视频套路就是下面几条，跟着走就行了，具体函数意思自行google
1.不用说，肯定加载。用ASSET
2.这里不考虑音轨，所以只获取video信息。用track 获取asset里的视频信息，一共两个track,一个track是你自己拍的视频，第二个track是特效视频,因为两个视频需要同时播放，所以起始时间相同，都是timezero,时长自然是你自己拍的视频时长。然后把两个track都放到mixComposition里。
3.第三步就是最重要的了。instructionLayer,看字面意思也能看个七七八八了。架构图层，就是告诉系统，等下合成视频，视频大小，方向，等等。这个地方就是合成视频的核心。我们只需要更改透明度就行了，把特效track的透明度改一下，让他能显示底下你自己拍的视屏图层就行了。
4.
**/
NSLog(@"First Asset = %@",firstAsset);
// 1
secondAsset = [AVAsset assetWithURL:videoURL];
NSString *path = [[NSBundle mainBundle] pathForResource:@"rain" ofType:@"mp4"];
NSLog(@"path is %@",path);
//firstAsset = [AVURLAsset URLAssetWithURL:[NSURL fileURLWithPath:path] options:nil];
firstAsset = [AVURLAsset URLAssetWithURL:[NSURL fileURLWithPath:path] options:nil];
NSLog(@"second Asset = %@",secondAsset);
NSLog(@"firstAsset==%f,%f",firstAsset.naturalSize.width,firstAsset.naturalSize.height);
NSLog(@"secondAsset==%f,%f",secondAsset.naturalSize.width,secondAsset.naturalSize.height);
//second Video
//secondAsset = [AVAsset assetWithURL:videoTwoURL];
}
if (firstAsset&&secondAsset) {
// 2.
CGSize targetSize = CGSizeMake(640, 480);
mixComposition = [[AVMutableComposition alloc] init];
AVMutableCompositionTrack *firstTrack =
[mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo
preferredTrackID:kCMPersistentTrackID_Invalid];
[firstTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero,CMTimeAdd(firstAsset.duration, CMTimeMakeWithSeconds(5.0f, 600)))
ofTrack:[[firstAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0]
atTime:CMTimeMakeWithSeconds(0.0f, 30)
error:nil];
[firstTrack scaleTimeRange:CMTimeRangeMake(kCMTimeZero, firstAsset.duration) toDuration:CMTimeAdd(firstAsset.duration, CMTimeMakeWithSeconds(5.0f, 600))];
AVMutableCompositionTrack *secondTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
[secondTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero,secondAsset.duration)
ofTrack:[[secondAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0]
atTime:kCMTimeZero
error:nil];
//CGAffineTransformMake(a,b,c,d,tx,ty) ad缩放 bc 旋转tx,ty位移
//3.
AVMutableVideoCompositionInstruction *mainInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
mainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero,secondAsset.duration);
//第一个视频的架构层
AVMutableVideoCompositionLayerInstruction *firstlayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:firstTrack];
[firstlayerInstruction setOpacity:0 atTime:CMTimeAdd(firstAsset.duration, CMTimeMakeWithSeconds(5.0f, 600))];
//[firstlayerInstruction setTransform:[self layerTrans:firstAsset withTargetSize:targetSize] atTime:kCMTimeZero];
// [firstlayerInstruction setOpacityRampFromStartOpacity:1.0 toEndOpacity:0.0 timeRange:CMTimeRangeMake(kCMTimeZero, firstAsset.duration)];
// 第二个视频的架构层
CGRect test = CGRectMake(0, 0, 300, 300);
[firstlayerInstruction setCropRectangle:test atTime:kCMTimeZero];//展示整个layer中某一位置和大小的视图
AVMutableVideoCompositionLayerInstruction *secondlayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:secondTrack];
[secondlayerInstruction setTransform:[self layerTrans:secondAsset withTargetSize:targetSize] atTime:kCMTimeZero];
// [secondlayerInstruction setOpacityRampFromStartOpacity:0.0 toEndOpacity:1.0 timeRange:CMTimeRangeMake(kCMTimeZero, CMTimeMake(600, 600))];
// 这个地方你把数组顺序倒一下，视频上下位置也跟着变了。
mainInstruction.layerInstructions = [NSArray arrayWithObjects:firstlayerInstruction,secondlayerInstruction, nil];
mainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, secondAsset.duration);
mainComposition = [AVMutableVideoComposition videoComposition];
mainComposition.instructions = [NSArray arrayWithObjects:mainInstruction,nil];
mainComposition.frameDuration = CMTimeMake(1, 30);
mainComposition.renderSize = CGSizeMake(640, 480);
// 导出路径
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *myPathDocs = [documentsDirectory stringByAppendingPathComponent:
[NSString stringWithFormat:@"mergeVideo.mov"]];
NSFileManager *fileManager = [NSFileManager defaultManager];
[fileManager removeItemAtPath:myPathDocs error:NULL];
NSURL *url = [NSURL fileURLWithPath:myPathDocs];
// NSLog(@"URL:- %@", [url description]);
//导出
AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetHighestQuality];
exporter.outputURL = url;
exporter.outputFileType = AVFileTypeQuickTimeMovie;
exporter.shouldOptimizeForNetworkUse = YES;
exporter.videoComposition = mainComposition;
NSLog(@"%.0f",mixComposition.duration.value/mixComposition.duration.timescale + 0.0f);
AVPlayerItem *playerItem = [AVPlayerItem playerItemWithAsset:exporter.asset];
playerItem.videoComposition = exporter.videoComposition;
AVPlayer *player = [AVPlayer playerWithPlayerItem:playerItem];
AVPlayerLayer *playerLayer = [AVPlayerLayer playerLayerWithPlayer:player];
playerLayer.frame = self.view.layer.bounds;
playerLayer.videoGravity = AVLayerVideoGravityResizeAspect;
[self.view.layer addSublayer:playerLayer];
[NSTimer scheduledTimerWithTimeInterval:0.1 target:self selector:@selector(playItem:) userInfo:@{@"item":player} repeats:NO];
[exporter exportAsynchronouslyWithCompletionHandler:^{
dispatch_async(dispatch_get_main_queue(), ^{
[self exportDidFinish:exporter];
});
}];
}else {
UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"出错!" message:@"选择视频"
delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil];
[alert show];
}
}
上述代码里面包含了多视频合成以及背景音频的合成：
1.视频合成最主要的还是依靠AVMutableComposition，其属性AVMutableVideoComposition包含着视频片段的所有信息，AVMutableAudioMix则包含了所有音频片段的信息。
- 如上所示，如果只是简单的视频合成，加一段或者不加音频，则只需要依次创建AVMutableComposition，AVMutableVideoComposition，AVMutableCompositionTrack，AVMutableVideoCompositionLayerInstruction。
- 上述代码就是前一篇文正提到的N久远的代码，好像是个印度人写的(也包含了一些我我之前测试用的片段)，反正仔细看也能很清楚的发现代码顺序很别扭。 不过其中有很多启发性质的方法，比如scaleTimeRange:CMTimeRangeMake，之类的，说明视频片段被加到AVMutableCompositionTrack上之后其实是可以伸缩的，仔细看文档还可以发现很多细分的方法，插入，删除之类的。
- AVMutableVideoCompositionLayerInstruction是和track对应的，其实两个可以理解为track-avplayer,layerInstruction--AVPlayerlayer,仔细去看layer文档，就能找到layer的一些简单变换，伸缩，透明度之类的。
- 值得一提的是，AVMutableVideoComposition中插入AVMutableCompositionTrack是有数量限制的，15个位上限。所以如果你想要多短视频合成的话，最好优先考虑清楚每个视频的表现逻辑，然后根据需要确定创建多少个track,最后按照时间以及变换规则对layer进行变换。
- AVAssetExportSession即导出，有多重格式可选择，不过貌似就只能选AVFileTypeQuickTimeMovie，如果你的源视频来自网络则需要把shouldOptimizeForNetworkUse设置为yes.
- 绝大多数的导出异常都是mainInstruction出现问题
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NSURL *videoURL;
AVURLAsset *firstAsset;
AVURLAsset *secondAsset;
AVMutableVideoComposition *mainComposition;
AVMutableComposition *mixComposition;
NSMutableArray * audioMixParams;
NSURL * audioUrl;
算了，最后贴上github地址，里面包含了我自己测试用的音视频合成以及即时播放的代码，不过都是临时使用的，尤其是播放，并不一定会生效。
https://github.com/Thetiso/MergeVideo
