我正在尝试在我的iOS设备上上传视频之前旋转视频，因为其他平台（如Android）不能正确地解释iOS录制的视频中的旋转信息，导致播放时显示为不正确的方向。
我查看了以下堆栈帖子，但没有成功将它们应用到我的情况中：
我复制了Apple AVSimpleEditor 项目示例，但不幸的是，在创建AVAssetExportSession并调用exportAsynchronouslyWithCompletionHandler后，不会执行任何旋转操作，并且更糟糕的是，旋转元数据被剥离出导出的文件。
这里是运行导出代码：
AVAssetExportSession *exportSession = [[AVAssetExportSession alloc] initWithAsset:[_mutableComposition copy] presetName:AVAssetExportPresetPassthrough];
exportSession.outputURL = outputURL;
exportSession.outputFileType = AVFileType3GPP;
exportSession.shouldOptimizeForNetworkUse = YES;
exportSession.videoComposition = _mutableVideoComposition;
[exportSession exportAsynchronouslyWithCompletionHandler:^(void)
{
NSLog(@"Status is %d %@", exportSession.status, exportSession.error);
handler(exportSession);
[exportSession release];
}];
这个方法在此处初始化了可变组合值_mutableComposition和可变视频组合值_mutableVideoComposition：
- (void) getVideoComposition:(AVAsset*)asset
{
AVMutableComposition *mutableComposition = nil;
AVMutableVideoComposition *mutableVideoComposition = nil;
AVMutableVideoCompositionInstruction *instruction = nil;
AVMutableVideoCompositionLayerInstruction *layerInstruction = nil;
CGAffineTransform t1;
CGAffineTransform t2;
AVAssetTrack *assetVideoTrack = nil;
AVAssetTrack *assetAudioTrack = nil;
// Check if the asset contains video and audio tracks
if ([[asset tracksWithMediaType:AVMediaTypeVideo] count] != 0) {
assetVideoTrack = [asset tracksWithMediaType:AVMediaTypeVideo][0];
}
if ([[asset tracksWithMediaType:AVMediaTypeAudio] count] != 0) {
assetAudioTrack = [asset tracksWithMediaType:AVMediaTypeAudio][0];
}
CMTime insertionPoint = kCMTimeZero;
NSError *error = nil;
// Step 1
// Create a composition with the given asset and insert audio and video tracks into it from the asset
// Check whether a composition has already been created, i.e, some other tool has already been applied
// Create a new composition
mutableComposition = [AVMutableComposition composition];
// Insert the video and audio tracks from AVAsset
if (assetVideoTrack != nil) {
AVMutableCompositionTrack *compositionVideoTrack = [mutableComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
[compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, [asset duration]) ofTrack:assetVideoTrack atTime:insertionPoint error:&error];
}
if (assetAudioTrack != nil) {
AVMutableCompositionTrack *compositionAudioTrack = [mutableComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
[compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, [asset duration]) ofTrack:assetAudioTrack atTime:insertionPoint error:&error];
}
// Step 2
// Translate the composition to compensate the movement caused by rotation (since rotation would cause it to move out of frame)
t1 = CGAffineTransformMakeTranslation(assetVideoTrack.naturalSize.height, 0.0);
// Rotate transformation
t2 = CGAffineTransformRotate(t1, degreesToRadians(90.0));
// Step 3
// Set the appropriate render sizes and rotational transforms
// Create a new video composition
mutableVideoComposition = [AVMutableVideoComposition videoComposition];
mutableVideoComposition.renderSize = CGSizeMake(assetVideoTrack.naturalSize.height,assetVideoTrack.naturalSize.width);
mutableVideoComposition.frameDuration = CMTimeMake(1, 30);
// The rotate transform is set on a layer instruction
instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
instruction.timeRange = CMTimeRangeMake(kCMTimeZero, [mutableComposition duration]);
layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:(mutableComposition.tracks)[0]];
[layerInstruction setTransform:t2 atTime:kCMTimeZero];
// Step 4
// Add the transform instructions to the video composition
instruction.layerInstructions = @[layerInstruction];
mutableVideoComposition.instructions = @[instruction];
TT_RELEASE_SAFELY(_mutableComposition);
_mutableComposition = [mutableComposition retain];
TT_RELEASE_SAFELY(_mutableVideoComposition);
_mutableVideoComposition = [mutableVideoComposition retain];
}
我从 AVSERotateCommand 这个链接中提取了该方法。有人能否建议为什么这个方法不能成功地将我的视频旋转90度？