在做iOS视频相关的开发的时候，遇到了一点点坑。

iOS视频添加水印，你首先会想到使用 animationTool , 苹果官方是这样写的

A video composition describes, for any time in the aggregate time range of its instructions, the number and IDs of video tracks that are to be used in order to produce a composed video frame corresponding to that time. When AV Foundation’s built-in video compositor is used, the instructions an AVVideoComposition comprises can specify a spatial transformation, an opacity value, and a cropping rectangle for each video source, and these can vary over time via simple linear ramping functions.

然后就按照步骤实现就可以了

// 1 - set up the overlay

CALayer *overlayLayer = [CALayer layer];

[overlayLayer setContents:(id)[image CGImage]];

overlayLayer.frame = CGRectMake(0, 0, size.width, size.height);

[overlayLayer setMasksToBounds:YES];

// 2 - set up the parent layer

CALayer *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];

[parentLayer addSublayer:overlayLayer];

// 3 - apply magic

composition.animationTool = [AVVideoCompositionCoreAnimationTool videoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer

但是在iOS 10下发现，有时候这个方法导出的视频不能立即播放，要等一会儿视频才能播放，于是就使用了下面的这个接口

CIFilter *watermarkFilter = [CIFilter filterWithName:@"CISourceOverCompositing"];

mainVideoComposition= [AVMutableVideoComposition videoCompositionWithAsset:mixComposition applyingCIFiltersWithHandler:^(AVAsynchronousCIImageFilteringRequest * _Nonnull request) {

CIImage *watermarkImage  =  [[CIImage alloc] initWithCGImage:image.CGImage];

CIImage *source = request.sourceImage;

[watermarkFilter setValue:source forKey:kCIInputBackgroundImageKey];

[watermarkFilter setValue:[watermarkImage imageByApplyingTransform:CGAffineTransformMakeScale(source.extent.size.width/watermarkImage.extent.size.width, source.extent.size.height/watermarkImage.extent.size.height)] forKey:kCIInputImageKey];

[request finishWithImage:watermarkFilter.outputImage context:nil];

}];