今天下午，一個csdn上的朋友加我QQ，問我如何在指定時間段內給視訊新增水印，很小眾的需求，但是確實有一些應用場景。我研究了一下，在之前加水印的基礎上，加了一個動畫，效果還可以，故分享給大家。

- (void)applyVideoEffectsToComposition:(AVMutableVideoComposition *)composition size:(CGSize)size{
    // 1 - set up the overlay
    CALayer *overlayLayer = [CALayer layer];
    UIImage *overlayImage  = [UIImage
 
 imageNamed:@"waterMark"];

    [overlayLayer setContents:(id)[overlayImage CGImage]];
    overlayLayer.frame = CGRectMake(size.width - 146, 24, 135, 18);
    [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];

    //*********** For A Special Time
    CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:@"opacity"
 
];
    [animation setDuration:0];
    [animation setFromValue:[NSNumber numberWithFloat:1.0]];
    [animation setToValue:[NSNumber numberWithFloat:0.0]];
    [animation setBeginTime:5];
    [animation setRemovedOnCompletion:NO];
    [animation setFillMode:kCAFillModeForwards];
    [overlayLayer addAnimation:animation forKey:@"animateOpacity"];

    // 3 - apply magic
    composition.animationTool = [AVVideoCompositionCoreAnimationTool
                                 videoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer inLayer:parentLayer];
}

其中，//* For A Special Time 程式碼段的內容，表示在第5秒的時候，將水印隱藏，從而達到了水印只顯示前5秒的效果。如果想實現其他效果，只要對layer的 opacity 屬性做修改即可。