[算法]iOS 视频添加水印，合成视频两种方案（整体渲染和分割渲染）

现手机里有一段视频，通过APP给他添加一个水印。iOS提供了在视频上添加layer的接口，添加一个水印还是很方便的(添加水印)。添加完水印有一个渲染过程，在手机这种设备上还是比较慢的，比如：对1分钟的高清视频(960x540)进行渲染需要20秒左右。如何在现有API基础上提高渲染速度，提升用户体验，成了问题。笔者发现两种渲染方法：
先看图，这有一个6秒的视频，我抓了四张关键帧。只在第2，3两张关键帧上添加字幕（一个关键帧代表1.5秒。所以，两个关键帧就代表是3秒时长）

第一种方案：视频分割 + 逐段渲染 + 合并
将视频分割为3端，即第1、第2，3、第4。有水印的是一组，没水印也是一组。对第2，3渲染。最后将三段视频合并到一起。
第二种方案：整体渲染
水印虽然不是从一开始出现，但是，我们可以对layer添加动画（参考下面OC代码）。
举个例子，假如水印少的情况。比如只对第一帧添加水印。分割渲染方案是3.0秒，整体渲染方案是3.5秒。假如水印多的情况，第1，2，3帧都有水印。分割渲染方案是4.1秒，整体渲染方案还是3.5秒。通过结果得出一点结论：整体渲染，无论水印多还是少，耗时是一样的。分割渲染怎么会有这么个差别呢？
分割渲染：由于涉及三步：1.分割，2.渲染视频，3.合并视频。每一步都需要时间。根据测试经验，第一步分割视频的时间很少，可以忽略。那么就剩下渲染和合并时间了。就拿刚才6秒的视频来说：一张水印和三张水印。合并视频时间是一样的。都是两段（一段1.5秒，另外一段4.5秒）。但是水印多了渲染时间就长了。
最后得出结论：水印少的时候，使用分割渲染方法。水印多的情况使用整体渲染。

整体渲染代码：

#define kEffectVideoFileName_Animation @"tmpMov-effect.mov"- (void)renderWholeVideo:(AVAsset *)asset{// 1 - Early exit if there's no video file selectedif (!asset) {UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Error" message:@"Please Load a Video Asset First"delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];[alert show];return;}// 2 - Create AVMutableComposition object. This object will hold your AVMutableCompositionTrack instances.AVMutableComposition *mixComposition = [[AVMutableComposition alloc] init];// 3 - Video trackAVMutableCompositionTrack *videoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideopreferredTrackID:kCMPersistentTrackID_Invalid];[videoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, asset.duration)ofTrack:[[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0]atTime:kCMTimeZero error:nil];AVMutableCompositionTrack *audioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];[audioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, asset.duration) ofTrack:[asset tracksWithMediaType:AVMediaTypeAudio][0] atTime:kCMTimeZero error:nil];// 3.1 - Create AVMutableVideoCompositionInstructionAVMutableVideoCompositionInstruction *mainInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];mainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, asset.duration);// 3.2 - Create an AVMutableVideoCompositionLayerInstruction for the video track and fix the orientation.__block     CGSize naturalSize;AVMutableVideoCompositionLayerInstruction *videolayerInstruction = [self transformVideo:asset track:videoTrack isVideoAssetPortrait:^(CGSize finalSize) {naturalSize = finalSize;}];[videolayerInstruction setOpacity:0.0 atTime:asset.duration];// 3.3 - Add instructionsmainInstruction.layerInstructions = [NSArray arrayWithObjects:videolayerInstruction,nil];AVMutableVideoComposition *mainCompositionInst = [AVMutableVideoComposition videoComposition];float renderWidth, renderHeight;renderWidth = naturalSize.width;renderHeight = naturalSize.height;mainCompositionInst.renderSize = CGSizeMake(renderWidth, renderHeight);mainCompositionInst.instructions = [NSArray arrayWithObject:mainInstruction];mainCompositionInst.frameDuration = CMTimeMake(1, 30);[self applyVideoEffectsWithAnimation:mainCompositionInst size:naturalSize];NSString *myPathDocs = [NSTemporaryDirectory() stringByAppendingPathComponent:kEffectVideoFileName_Animation];NSURL *url = [NSURL fileURLWithPath:myPathDocs];/*先移除旧文件*/[PublicUIMethod removeFile:url];// 5 - Create exporterAVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixCompositionpresetName:AVAssetExportPresetHighestQuality];[self.exportSessions addObject:exporter];exporter.outputURL=url;exporter.outputFileType = AVFileTypeQuickTimeMovie;exporter.shouldOptimizeForNetworkUse = YES;exporter.videoComposition = mainCompositionInst;weakifyself;[exporter exportAsynchronouslyWithCompletionHandler:^{strongifyself;dispatch_async(dispatch_get_main_queue(), ^{switch ([exporter status]) {case AVAssetExportSessionStatusFailed:DDLogWarn(@"render Export failed: %@ and order : %d", [exporter error], 0);break;case AVAssetExportSessionStatusCancelled:NSLog(@"render Export canceled order : %d", 0);break;default:{NSLog(@"'%@' render finish",[myPathDocs lastPathComponent]);[self pushToPreviePage:myPathDocs];}break;}});}];[self monitorSingleExporter:exporter];
}- (void)applyVideoEffectsWithAnimation:(AVMutableVideoComposition *)composition size:(CGSize)size
{// Set up layerCALayer *parentLayer = [CALayer layer];CALayer *videoLayer = [CALayer layer];parentLayer.frame = CGRectMake(0, 0, size.width, size.height);videoLayer.frame = CGRectMake(0, 0, size.width, size.height);[parentLayer addSublayer:videoLayer];/**/CMTime timeFrame = [self frameTime];CGFloat granularity = CMTimeGetSeconds(timeFrame);/*caption layer*/for (int j=0; j<self.effectsArray.count; j++) {NSArray* effectSeries = (NSArray *)self.effectsArray[j];FSVideoCaptionDescriptionModel *description = [[effectSeries firstObject] as:FSVideoCaptionDescriptionModel.class];NSArray *captions = [description reOrder];if (!captions || captions.count == 0) {//没有字幕就别瞎搞了continue;}FSCaptionModel *captionModel = captions.firstObject;UIImage *image = captionModel.image;/*将水印生成图片，采用图片方法添加水印*/CGFloat scaleY = captionModel.scaleY;CGFloat scaleHeight = captionModel.scaleHeight;CALayer *layer = [CALayer layer];layer.frame = CGRectMake(0, size.height * scaleY, size.width, size.height * scaleHeight);layer.contents = (__bridge id)image.CGImage;/*字幕动画由两个组成:1. 显示所有字幕<动画开始前保持，初始状态。动画时间是0.结束后不移除动画>2. 隐藏字幕，到最后。*/CGFloat showStartTime = description.startIndex * granularity;CGFloat hiddenAginStartTime = showStartTime + effectSeries.count*granularity;CABasicAnimation *animation = nil;if (showStartTime > 0) {animation = [CABasicAnimation animationWithKeyPath:@"opacity"];animation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionDefault];[animation setFromValue:[NSNumber numberWithFloat:0.0]];[animation setToValue:[NSNumber numberWithFloat:1.0]];[animation setBeginTime:showStartTime];[animation setFillMode:kCAFillModeBackwards];/*must be backwards*/[animation setRemovedOnCompletion:NO];/*must be no*/[layer addAnimation:animation forKey:@"animateOpacityShow"];}/*最后一个字幕片段不是整的1.5s或者5秒。就不隐藏动画了*/if (j != self.effectsArray.count-1) {animation = [CABasicAnimation animationWithKeyPath:@"opacity"];animation.timingFunction = [CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionDefault];[animation setFromValue:[NSNumber numberWithFloat:1.0]];[animation setToValue:[NSNumber numberWithFloat:0.0]];[animation setBeginTime:hiddenAginStartTime];[animation setRemovedOnCompletion:NO];/*must be no*/[animation setFillMode:kCAFillModeForwards];[layer addAnimation:animation forKey:@"animateOpacityHiddenAgin"];}[parentLayer addSublayer:layer];}parentLayer.geometryFlipped = YES;composition.animationTool = [AVVideoCompositionCoreAnimationToolvideoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer inLayer:parentLayer];
}- (void)monitorSingleExporter:(AVAssetExportSession *)exporter{double delay = 1.0;int64_t delta = (int64_t)delay * NSEC_PER_SEC;dispatch_time_t poptime = dispatch_time(DISPATCH_TIME_NOW, delta);dispatch_after(poptime, dispatch_get_main_queue(), ^{if (exporter.status == AVAssetExportSessionStatusExporting) {NSLog(@"whole progress is %f",  exporter.progress);[self monitorSingleExporter:exporter];}});
}
-(AVMutableVideoCompositionLayerInstruction *) transformVideo:(AVAsset *)asset track:(AVMutableCompositionTrack *)firstTrack isVideoAssetPortrait:(void(^)(CGSize size))block{AVMutableVideoCompositionLayerInstruction *videolayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:firstTrack];AVAssetTrack *videoAssetTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];UIImageOrientation videoAssetOrientation_  = UIImageOrientationUp;BOOL isVideoAssetPortrait_  = NO;CGAffineTransform videoTransform = videoAssetTrack.preferredTransform;if (videoTransform.a == 0 && videoTransform.b == 1.0 && videoTransform.c == -1.0 && videoTransform.d == 0) {videoAssetOrientation_ = UIImageOrientationRight;videoTransform = CGAffineTransformMakeRotation(M_PI_2);videoTransform = CGAffineTransformTranslate(videoTransform, 0, -videoAssetTrack.naturalSize.height);isVideoAssetPortrait_ = YES;}if (videoTransform.a == 0 && videoTransform.b == -1.0 && videoTransform.c == 1.0 && videoTransform.d == 0) {videoAssetOrientation_ =  UIImageOrientationLeft;//这个地方很恶心，涉及到reveal看不到的坐标系videoTransform = CGAffineTransformMakeRotation(-M_PI_2);videoTransform = CGAffineTransformTranslate(videoTransform, - videoAssetTrack.naturalSize.width, 0);isVideoAssetPortrait_ = YES;}if (videoTransform.a == 1.0 && videoTransform.b == 0 && videoTransform.c == 0 && videoTransform.d == 1.0) {videoAssetOrientation_ =  UIImageOrientationUp;}if (videoTransform.a == -1.0 && videoTransform.b == 0 && videoTransform.c == 0 && videoTransform.d == -1.0) {videoTransform = CGAffineTransformMakeRotation(-M_PI);videoTransform = CGAffineTransformTranslate(videoTransform, -videoAssetTrack.naturalSize.width, -videoAssetTrack.naturalSize.height);
//        videoTransform = CGAffineTransformRotate(videoTransform, M_PI/180*45);videoAssetOrientation_ = UIImageOrientationDown;}[videolayerInstruction setTransform:videoTransform atTime:kCMTimeZero];CGSize naturalSize;if(isVideoAssetPortrait_){naturalSize = CGSizeMake(videoAssetTrack.naturalSize.height, videoAssetTrack.naturalSize.width);} else {naturalSize = videoAssetTrack.naturalSize;}if(block){block(naturalSize);}return videolayerInstruction;
}

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[算法]iOS 视频添加水印，合成视频两种方案（整体渲染和分割渲染）

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