网上有很多代码，但是都没说清楚，所以来整一套简单粗暴又完整的视频添加水印的方法，传入视频的本地路径，读取视频的音、视频轨道，编辑视频部分，添加水印，然后再重新合成视频，存入相册。水印的位置大小根据需求调整
直接上代码：

/**
 视频添加水印并保存到相册

 @param path 视频本地路径
 */
+ (void)addWaterPicWithVideoPath:(NSString*)path
{
    //1 创建AVAsset实例
    AVURLAsset*videoAsset = [AVURLAsset assetWithURL:[NSURL fileURLWithPath:path]];
    
    AVMutableComposition *mixComposition = [[AVMutableComposition alloc] init];

    
    //3 视频通道
    AVMutableCompositionTrack *videoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo
                                                                        preferredTrackID:kCMPersistentTrackID_Invalid];
    [videoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration)
                        ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeVideo] firstObject]
                         atTime:kCMTimeZero error:nil];
    
    
    //2 音频通道
    AVMutableCompositionTrack *audioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio
                                                                        preferredTrackID:kCMPersistentTrackID_Invalid];
    [audioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration)
                        ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeAudio] firstObject]
                         atTime:kCMTimeZero error:nil];
    
    //3.1 AVMutableVideoCompositionInstruction 视频轨道中的一个视频，可以缩放、旋转等
    AVMutableVideoCompositionInstruction *mainInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
    mainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, videoAsset.duration);
    
    // 3.2 AVMutableVideoCompositionLayerInstruction 一个视频轨道，包含了这个轨道上的所有视频素材
    AVMutableVideoCompositionLayerInstruction *videolayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoTrack];

    [videolayerInstruction setOpacity:0.0 atTime:videoAsset.duration];
    
    // 3.3 - Add instructions
    mainInstruction.layerInstructions = [NSArray arrayWithObjects:videolayerInstruction,nil];
    
    //AVMutableVideoComposition：管理所有视频轨道，水印添加就在这上面
    AVMutableVideoComposition *mainCompositionInst = [AVMutableVideoComposition videoComposition];
    
    AVAssetTrack *videoAssetTrack = [[videoAsset tracksWithMediaType:AVMediaTypeVideo] firstObject];
    CGSize naturalSize = videoAssetTrack.naturalSize;
    
    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 applyVideoEffectsToComposition:mainCompositionInst size:naturalSize];
    
    //    // 4 - 输出路径
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *myPathDocs =  [documentsDirectory stringByAppendingPathComponent:
                             [NSString stringWithFormat:@"FinalVideo-%d.mp4",arc4random() % 1000]];
    NSURL* videoUrl = [NSURL fileURLWithPath:myPathDocs];
    
    // 5 - 视频文件输出
    AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition
                                                                      presetName:AVAssetExportPresetHighestQuality];
    exporter.outputURL = videoUrl;
    exporter.outputFileType = AVFileTypeMPEG4;
    exporter.shouldOptimizeForNetworkUse = YES;
    exporter.videoComposition = mainCompositionInst;
    [exporter exportAsynchronouslyWithCompletionHandler:^{
        dispatch_async(dispatch_get_main_queue(), ^{
            
            if( exporter.status == AVAssetExportSessionStatusCompleted ){
               
                UISaveVideoAtPathToSavedPhotosAlbum(myPathDocs, nil, nil, nil);
  
            }else if( exporter.status == AVAssetExportSessionStatusFailed )
            {
                NSLog(@"failed");
            }
            
        });
    }];
}


/**
 设置水印及其对应视频的位置

 @param composition 视频的结构
 @param size 视频的尺寸
 */
+ (void)applyVideoEffectsToComposition:(AVMutableVideoComposition *)composition size:(CGSize)size
{
    // 文字
//    CATextLayer *subtitle1Text = [[CATextLayer alloc] init];
//    //    [subtitle1Text setFont:@"Helvetica-Bold"];
//    [subtitle1Text setFontSize:36];
//    [subtitle1Text setFrame:CGRectMake(10, size.height-10-100, size.width, 100)];
//    [subtitle1Text setString:@"ZHIMABAOBAO"];
//    //    [subtitle1Text setAlignmentMode:kCAAlignmentCenter];
//    [subtitle1Text setForegroundColor:[[UIColor whiteColor] CGColor]];
    
    //图片
    CALayer*picLayer = [CALayer layer];
    picLayer.contents = (id)[UIImage imageNamed:@"videoWater2"].CGImage;
    picLayer.frame = CGRectMake(size.width-15-87, 15, 87, 26);
    
    // 2 - The usual overlay
    CALayer *overlayLayer = [CALayer layer];
    [overlayLayer addSublayer:picLayer];
    overlayLayer.frame = CGRectMake(0, 0, size.width, size.height);
    [overlayLayer setMasksToBounds:YES];
    
    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];
    
    composition.animationTool = [AVVideoCompositionCoreAnimationTool
                                 videoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer inLayer:parentLayer];
    
}

设置layer的contentsScale属性可改变水印的清晰度：

///高清水印
subtitle1Text.contentsScale = [UIScreen mainScreen].scale;