将具有AVMutableVideoComposition的文字叠加到特定的时间范围

Question

我发现了几个示例，说明如何在视频上添加文字叠加层。

雷的教程 - http://www.raywenderlich.com/30200/avfoundation-tutorial-adding-overlays-and-animations-to-videos

这SO回答 - How can I add a watermark in a captured video on iOS

有报道说，我引用以及其他几个人。我的代码看起来几乎与该答案相同，我一直在试图调整它以使文本叠加在视频的开头只显示一两秒钟。任何帮助我如何才能做到这一点？

这是我的代码。这样做的目的是在整个持续时间内显示带有覆盖层的视频。

if(vA) { 
    videoCompositionTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid]; 
    [videoCompositionTrack insertTimeRange:videoTimerange ofTrack:[[vA tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:kCMTimeZero error:&error]; 
    [videoCompositionTrack setPreferredTransform:[[[vA tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] preferredTransform]]; 
    if(error) 
     NSLog(@"%@", error); 
} 

if(aA) { 
    audioCompositionTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid]; 
    [audioCompositionTrack insertTimeRange:audioTimerange ofTrack:[[aA tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0] atTime:kCMTimeZero error:&error]; 
    if(error) 
     NSLog(@"%@", error); 
} 

CGSize videoSize = [videoCompositionTrack naturalSize]; 

UIImage *myImage = [UIImage imageNamed:@"cover.png"]; 
CALayer *aLayer = [CALayer layer]; 
aLayer.contents = (id)myImage.CGImage; 
aLayer.frame = CGRectMake(5, 25, 100, 56); 
aLayer.opacity = 0.7; 

CATextLayer *titleLayer = [CATextLayer layer]; 
titleLayer.string = titleText; 
titleLayer.fontSize = 18; 
titleLayer.foregroundColor = titleColor.CGColor; 
titleLayer.alignmentMode = kCAAlignmentCenter; 
titleLayer.frame = CGRectMake(20, 10, videoSize.width - 40, 20); 
[titleLayer displayIfNeeded]; 

CALayer *parentLayer = [CALayer layer]; 
CALayer *videoLayer = [CALayer layer]; 
parentLayer.frame = CGRectMake(0, 0, videoSize.width, videoSize.height); 
videoLayer.frame = CGRectMake(0, 0, videoSize.width, videoSize.height); 
[parentLayer addSublayer:videoLayer]; 
[parentLayer addSublayer:aLayer]; 
if(titleText && [titleText length] > 0) { 
    [parentLayer addSublayer:titleLayer]; 
} 

AVMutableVideoComposition *videoComp = [AVMutableVideoComposition videoComposition]; 
videoComp.renderSize = videoSize; 
videoComp.frameDuration = CMTimeMake(1, 30); 
videoComp.animationTool = [AVVideoCompositionCoreAnimationTool videoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer inLayer:parentLayer]; 

AVAssetTrack *videoTrack = [[mixComposition tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0]; 
AVMutableVideoCompositionLayerInstruction *layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoTrack]; 

AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction]; 
[instruction setTimeRange:CMTimeRangeMake(kCMTimeZero, [mixComposition duration])]; 
instruction.layerInstructions = [NSArray arrayWithObject:layerInstruction]; 
videoComp.instructions = [NSArray arrayWithObject:instruction]; 

AVAssetExportSession *_assetExport = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetMediumQuality]; 
_assetExport.videoComposition = videoComp; 
_assetExport.outputFileType = AVFileTypeMPEG4; 
_assetExport.outputURL = outputFileUrl; 

[_assetExport exportAsynchronouslyWithCompletionHandler:^{ 
    AVAssetExportSessionStatus status = [_assetExport status]; 
    switch (status) { 
     case AVAssetExportSessionStatusFailed: 
      NSLog(@"Export Failed"); 
      NSLog(@"Export Error: %@", [_assetExport.error localizedDescription]); 
      NSLog(@"Export Error Reason: %@", [_assetExport.error localizedFailureReason]); 
      break; 
     case AVAssetExportSessionStatusCompleted: 
      NSLog(@"Export Completed"); 
      [self performSelectorOnMainThread:@selector(updateProgressIndicator:) withObject:[NSNumber numberWithFloat:2] waitUntilDone:YES]; 
      break; 
     case AVAssetExportSessionStatusUnknown: 
      NSLog(@"Export Unknown"); 
      break; 
     case AVAssetExportSessionStatusExporting: 
      NSLog(@"Export Exporting"); 
      break; 
     case AVAssetExportSessionStatusWaiting: 
      NSLog(@"Export Waiting"); 
      break; 
    } 
}]; 

Answer 1

我想通了什么，我需要做的。这真的没什么特别的。它只是需要更好地理解什么是可能的。

基本上我所要做的只是添加一个基本的不透明度动画到文本中的图层。

// My original code for creating the text layer 
CATextLayer *titleLayer = [CATextLayer layer]; 
. 
. 
. 
[titleLayer displayIfNeeded]; 

// the code for the opacity animation which then removes the text 
CABasicAnimation *animation = [CABasicAnimation animationWithKeyPath:@"opacity"]; 
[animation setDuration:0]; 
[animation setFromValue:[NSNumber numberWithFloat:1.0]]; 
[animation setToValue:[NSNumber numberWithFloat:0.0]]; 
[animation setBeginTime:1]; 
[animation setRemovedOnCompletion:NO]; 
[animation setFillMode:kCAFillModeForwards]; 
[titleLayer addAnimation:animation forKey:@"animateOpacity"];