ios - 使用 AVMutableVideoCompositionLayerInstruction 修改 Track，尺寸和位置偏差

Question

设置Transform后，我使用AVMutableVideoCompositionLayerInstruction修改大小和位置。 'iPhone 5'可以正常，'iPhone6'显示有偏差。经过测试，总偏移量大约在30左右。如上图，应该显示在有绿色边框的矩形中。下面是我的代码，请问这是怎么回事？

我的英文很烂，可能是我没有把问题描述清楚，抱歉。谢谢。

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做准备工作，初始化composition和videoComposition:

_composition = [[AVMutableComposition composition] init];
_audioMix = [[AVMutableAudioMix alloc] init];
_videoComposition = [[AVMutableVideoComposition alloc] init];
_videoComposition.frameDuration = CMTimeMake(1, 30);
_videoComposition.renderSize = CGSizeMake(320, 320);
_videoComposition.renderScale = 1.0;

创建一个视频轨道，用它来合成新的视频:

－(void)initializationMutableTrackWithComposition:(AVMutableComposition *)composition {
self.mutableTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];

CMTimeRange timeRange = CMTimeRangeMake(CMTimeMake(1, 1), self.asset.duration);
AVAssetTrack * resourceTrack = [[self.asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];

BOOL  insertedResult =  [self.mutableTrack insertTimeRange:timeRange
                                                    ofTrack:resourceTrack
                                                     atTime:kCMTimeZero
                                                      error:nil];
self.mutableTrack.preferredTransform = CGAffineTransformIdentity;
NSLog(@"插入结果是------>%@",insertedResult ? @"成功" : @"失败");
}

创建AVMutableVideoCompositionLayerInstruction，比例为naturalSize 首先跟踪 Assets 和绿色矩形计算大小，移动x和y的绿色矩形。

－(void)initializationVideoLayer {
    AVMutableVideoCompositionLayerInstruction * layer = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:self.mutableTrack];

    CGAffineTransform preferredTransform = CGAffineTransformConcat(self.elementAttribute.scale, self.elementAttribute.move);

    [layer setTransform:preferredTransform atTime:kCMTimeZero];

    //设置 Video 透明度
    //[layer setOpacity:0.1f atTime:kCMTimeZero];

    self.videoLayer = layer;
}

下一步是分配 layerInstruction AVMutableVideoCompositionInstruction 并创建 AVPlayerItem 并播放。

-(void)refreshVideoConpositiom:(NSNotification *) notify{
    _compositionInstruction.timeRange = CMTimeRangeMake(kCMTimeZero,CMTimeMake(16, 1));

    NSMutableArray * layerInstructions = [NSMutableArray array];
    NSMutableArray * audioParameters = [NSMutableArray array];

    for (CompositionLayerInstruction *layer in NSArray){
        [layerInstructions addObject:layer];
    }

    self.compositionInstruction.layerInstructions = [layerInstructions copy];

    for (AVMutableAudioMixInputParameters * audioParameter in NSArray){
        [audioParameter addObject:audioParameter];
    }

    self.audioMix.inputParameters = audioParameters;
    [self makePlayerItem];
}

-(void)makePlayerItem{
    self.videoComposition.instructions = @[[self.compositionInstruction]];

    //Create playerItem.
    AVPlayerItem * playerItem =  [AVPlayerItem playerItemWithAsset:self.composition];
    playerItem.videoComposition = self.videoComposition;
    playerItem.audioMix = self.audioMix;

    //The result callback to display layer.
    if ([NSThread isMainThread])
        [self callBackPlayerItemToDelegateWithPlayerItem:playerItem];
    else
        [self performSelector:@selector(callBackPlayerItemToDelegateWithPlayerItem:)
                         onThread:[NSThread mainThread]
                       withObject:playerItem
                    waitUntilDone:NO];
}  

Answer 1

之所以我已经找到问题，是因为renderSize属性的大小和计算VideoCompostion的大小缩放和移动不一致，导致应用中出现Transform偏差。我猜想，是使用AVMutableVideoCompositionLayerInstruction VideoCompostion的默认大小renderSize，所以如果renderSize不一致，计算出来的大小和比例，就会有偏差。

感谢上帝。 ^~^