我正在使用 AVCaptureMovieFileOutput 来录制一些视频。我使用 AVLayerVideoGravityResizeAspectFill 显示预览层，它会稍微放大。我遇到的问题是最终视频更大，包含在预览期间不适合屏幕的额外图像。

这是预览和生成的视频

有没有一种方法可以使用 AVAssetExportSession 指定要从视频中剪切的 CGRect？

编辑----

当我将 CGAffineTransformScale 应用于 AVAssetTrack 时，它会放大视频，并使用 AVMutableVideoComposition renderSize设置为 view.bounds 它会裁掉两端。太好了，只剩下 1 个问题了。视频的宽度没有拉伸(stretch)到正确的宽度，只是被黑色填充。

编辑 2 ---- 建议的问题/答案不完整..

我的一些代码:

在我的 - (void)captureOutput:(AVCaptureFileOutput *)captureOutput didFinishRecordingToOutputFileAtURL:(NSURL *)outputFileURL fromConnections:(NSArray *)connections error:(NSError *)error 方法中我有这个要裁剪并调整视频大小。

- (void)flipAndSave:(NSURL *)videoURL withCompletionBlock:(void(^)(NSURL *returnURL))completionBlock
{
    AVURLAsset *firstAsset = [AVURLAsset assetWithURL:videoURL];

    // 1 - Create AVMutableComposition object. This object will hold your AVMutableCompositionTrack instances.
    AVMutableComposition *mixComposition = [[AVMutableComposition alloc] init];
    // 2 - Video track
    AVMutableCompositionTrack *firstTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo
                                                                        preferredTrackID:kCMPersistentTrackID_Invalid];
    [firstTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, firstAsset.duration)
                        ofTrack:[[firstAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:kCMTimeZero error:nil];

    // 2.1 - Create AVMutableVideoCompositionInstruction
    AVMutableVideoCompositionInstruction *mainInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
    mainInstruction.timeRange = CMTimeRangeMake(CMTimeMakeWithSeconds(0, 600), firstAsset.duration);

    // 2.2 - Create an AVMutableVideoCompositionLayerInstruction for the first track
    AVMutableVideoCompositionLayerInstruction *firstlayerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:firstTrack];
    AVAssetTrack *firstAssetTrack = [[firstAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
    UIImageOrientation firstAssetOrientation_  = UIImageOrientationUp;
    BOOL isFirstAssetPortrait_  = NO;
    CGAffineTransform firstTransform = firstAssetTrack.preferredTransform;
    if (firstTransform.a == 0 && firstTransform.b == 1.0 && firstTransform.c == -1.0 && firstTransform.d == 0) {
        firstAssetOrientation_ = UIImageOrientationRight;
        isFirstAssetPortrait_ = YES;
    }
    if (firstTransform.a == 0 && firstTransform.b == -1.0 && firstTransform.c == 1.0 && firstTransform.d == 0) {
        firstAssetOrientation_ =  UIImageOrientationLeft;
        isFirstAssetPortrait_ = YES;
    }
    if (firstTransform.a == 1.0 && firstTransform.b == 0 && firstTransform.c == 0 && firstTransform.d == 1.0) {
        firstAssetOrientation_ =  UIImageOrientationUp;

    }
    if (firstTransform.a == -1.0 && firstTransform.b == 0 && firstTransform.c == 0 && firstTransform.d == -1.0) {
        firstAssetOrientation_ = UIImageOrientationDown;
    }
//    [firstlayerInstruction setTransform:firstAssetTrack.preferredTransform atTime:kCMTimeZero];

//    [firstlayerInstruction setCropRectangle:self.view.bounds atTime:kCMTimeZero];





    CGFloat scale = [self getScaleFromAsset:firstAssetTrack];

    firstTransform = CGAffineTransformScale(firstTransform, scale, scale);

    [firstlayerInstruction setTransform:firstTransform atTime:kCMTimeZero];

    // 2.4 - Add instructions
    mainInstruction.layerInstructions = [NSArray arrayWithObjects:firstlayerInstruction,nil];
    AVMutableVideoComposition *mainCompositionInst = [AVMutableVideoComposition videoComposition];
    mainCompositionInst.instructions = [NSArray arrayWithObject:mainInstruction];
    mainCompositionInst.frameDuration = CMTimeMake(1, 30);

//    CGSize videoSize = firstAssetTrack.naturalSize;
    CGSize videoSize = self.view.bounds.size;
    BOOL isPortrait_ = [self isVideoPortrait:firstAsset];
    if(isPortrait_) {
        videoSize = CGSizeMake(videoSize.height, videoSize.width);
    }
    NSLog(@"%@", NSStringFromCGSize(videoSize));
    mainCompositionInst.renderSize = videoSize;




    // 3 - Audio track
    AVMutableCompositionTrack *AudioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio
                                                                        preferredTrackID:kCMPersistentTrackID_Invalid];
    [AudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, firstAsset.duration)
                        ofTrack:[[firstAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0] atTime:kCMTimeZero error:nil];

    // 4 - Get path
    NSString *outputPath = [[NSString alloc] initWithFormat:@"%@%@", NSTemporaryDirectory(), @"cutoutput.mov"];
    NSURL *outputURL = [[NSURL alloc] initFileURLWithPath:outputPath];
    NSFileManager *manager = [[NSFileManager alloc] init];
    if ([manager fileExistsAtPath:outputPath])
    {
        [manager removeItemAtPath:outputPath error:nil];
    }
    // 5 - Create exporter
    AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition
                                                                      presetName:AVAssetExportPresetHighestQuality];
    exporter.outputURL=outputURL;
    exporter.outputFileType = AVFileTypeQuickTimeMovie;
    exporter.shouldOptimizeForNetworkUse = YES;
    exporter.videoComposition = mainCompositionInst;
    [exporter exportAsynchronouslyWithCompletionHandler:^{
        switch ([exporter status])
        {
            case AVAssetExportSessionStatusFailed:
                NSLog(@"Export failed: %@ : %@", [[exporter error] localizedDescription], [exporter error]);
                completionBlock(nil);

                break;
            case AVAssetExportSessionStatusCancelled:

                NSLog(@"Export canceled");
                completionBlock(nil);

                break;
            default: {
                NSURL *outputURL = exporter.outputURL;
                dispatch_async(dispatch_get_main_queue(), ^{
                    completionBlock(outputURL);
                });

                break;
            }
        }
    }];
}

最佳答案

以下是我对您问题的解释:您正在屏幕比例为 4:3 的设备上捕获视频，因此您的 AVCaptureVideoPreviewLayer 为 4:3，但视频输入设备以 4:3 捕获视频16:9，因此生成的视频比预览中看到的“更大”。

如果您只是想裁剪预览未捕获的额外像素，请查看此 http://www.netwalk.be/article/record-square-video-ios .本文介绍如何将视频裁剪成正方形。但是，您只需要进行一些修改即可裁剪为 4:3。我已经去测试了这个，这里是我所做的更改:

获得视频的 AVAssetTrack 后，您需要计算新的高度。

// we convert the captured height i.e. 1080 to a 4:3 screen ratio and get the new height
CGFloat newHeight = clipVideoTrack.naturalSize.height/3*4;

然后修改这两行，使用newHeight。

videoComposition.renderSize = CGSizeMake(clipVideoTrack.naturalSize.height, newHeight);

CGAffineTransform t1 = CGAffineTransformMakeTranslation(clipVideoTrack.naturalSize.height, -(clipVideoTrack.naturalSize.width - newHeight)/2 );

所以我们在这里所做的是将 renderSize 设置为 4:3 的比例 - 确切的尺寸基于输入设备。然后我们使用 CGAffineTransform 来转换视频位置，这样我们在 AVCaptureVideoPreviewLayer 中看到的就是呈现到我们文件中的内容。

编辑:如果您想将所有内容放在一起并根据设备的屏幕比例(3:2、4:3、16:9)裁剪视频并考虑视频方向我们需要添加一些东西。

首先是修改后的示例代码，其中有一些重要的改动:

// output file
NSString* docFolder = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) lastObject];
NSString* outputPath = [docFolder stringByAppendingPathComponent:@"output2.mov"];
if ([[NSFileManager defaultManager] fileExistsAtPath:outputPath])
    [[NSFileManager defaultManager] removeItemAtPath:outputPath error:nil];

// input file
AVAsset* asset = [AVAsset assetWithURL:outputFileURL];

AVMutableComposition *composition = [AVMutableComposition composition];
[composition  addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];

// input clip
AVAssetTrack *videoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];

// crop clip to screen ratio
UIInterfaceOrientation orientation = [self orientationForTrack:asset];
BOOL isPortrait = (orientation == UIInterfaceOrientationPortrait || orientation == UIInterfaceOrientationPortraitUpsideDown) ? YES: NO;
CGFloat complimentSize = [self getComplimentSize:videoTrack.naturalSize.height];
CGSize videoSize;

if(isPortrait) {
    videoSize = CGSizeMake(videoTrack.naturalSize.height, complimentSize);
} else {
    videoSize = CGSizeMake(complimentSize, videoTrack.naturalSize.height);
}

AVMutableVideoComposition* videoComposition = [AVMutableVideoComposition videoComposition];
videoComposition.renderSize = videoSize;
videoComposition.frameDuration = CMTimeMake(1, 30);

AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
instruction.timeRange = CMTimeRangeMake(kCMTimeZero, CMTimeMakeWithSeconds(60, 30) );

// rotate and position video
AVMutableVideoCompositionLayerInstruction* transformer = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoTrack];

CGFloat tx = (videoTrack.naturalSize.width-complimentSize)/2;
if (orientation == UIInterfaceOrientationPortrait || orientation == UIInterfaceOrientationLandscapeRight) {
    // invert translation
    tx *= -1;
}

// t1: rotate and position video since it may have been cropped to screen ratio
CGAffineTransform t1 = CGAffineTransformTranslate(videoTrack.preferredTransform, tx, 0);
// t2/t3: mirror video horizontally
CGAffineTransform t2 = CGAffineTransformTranslate(t1, isPortrait?0:videoTrack.naturalSize.width, isPortrait?videoTrack.naturalSize.height:0);
CGAffineTransform t3 = CGAffineTransformScale(t2, isPortrait?1:-1, isPortrait?-1:1);

[transformer setTransform:t3 atTime:kCMTimeZero];
instruction.layerInstructions = [NSArray arrayWithObject: transformer];
videoComposition.instructions = [NSArray arrayWithObject: instruction];

// export
exporter = [[AVAssetExportSession alloc] initWithAsset:asset presetName:AVAssetExportPresetHighestQuality] ;
exporter.videoComposition = videoComposition;
exporter.outputURL=[NSURL fileURLWithPath:outputPath];
exporter.outputFileType=AVFileTypeQuickTimeMovie;

[exporter exportAsynchronouslyWithCompletionHandler:^(void){
    NSLog(@"Exporting done!");

    // added export to library for testing
    ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
    if ([library videoAtPathIsCompatibleWithSavedPhotosAlbum:[NSURL fileURLWithPath:outputPath]]) {
        [library writeVideoAtPathToSavedPhotosAlbum:[NSURL fileURLWithPath:outputPath]
                                    completionBlock:^(NSURL *assetURL, NSError *error) {
             NSLog(@"Saved to album");
             if (error) {

             }
         }];
    }
}];

我们在此处添加的是根据根据屏幕比例裁剪视频的尺寸来获取视频的新渲染大小的调用。缩小尺寸后，我们需要平移位置以使视频重新居中。所以我们捕获它的方向以将它移动到正确的方向。这将解决我们在 UIInterfaceOrientationLandscapeLeft 中看到的偏离中心的问题。最后 CGAffineTransform t2, t3 水平镜像视频。

下面是实现这一目标的两种新方法:

- (CGFloat)getComplimentSize:(CGFloat)size {
    CGRect screenRect = [[UIScreen mainScreen] bounds];
    CGFloat ratio = screenRect.size.height / screenRect.size.width;

    // we have to adjust the ratio for 16:9 screens
    if (ratio == 1.775) ratio = 1.77777777777778;

    return size * ratio;
}

- (UIInterfaceOrientation)orientationForTrack:(AVAsset *)asset {
    UIInterfaceOrientation orientation = UIInterfaceOrientationPortrait;
    NSArray *tracks = [asset tracksWithMediaType:AVMediaTypeVideo];

    if([tracks count] > 0) {
        AVAssetTrack *videoTrack = [tracks objectAtIndex:0];
        CGAffineTransform t = videoTrack.preferredTransform;

        // Portrait
        if(t.a == 0 && t.b == 1.0 && t.c == -1.0 && t.d == 0) {
            orientation = UIInterfaceOrientationPortrait;
        }
        // PortraitUpsideDown
        if(t.a == 0 && t.b == -1.0 && t.c == 1.0 && t.d == 0) {
            orientation = UIInterfaceOrientationPortraitUpsideDown;
        }
        // LandscapeRight
        if(t.a == 1.0 && t.b == 0 && t.c == 0 && t.d == 1.0) {
            orientation = UIInterfaceOrientationLandscapeRight;
        }
        // LandscapeLeft
        if(t.a == -1.0 && t.b == 0 && t.c == 0 && t.d == -1.0) {
            orientation = UIInterfaceOrientationLandscapeLeft;
        }
    }
    return orientation;
}

这些都非常简单。唯一需要注意的是，在 getComplimentSize: 方法中，我们必须手动调整 16:9 的比例，因为 iPhone5+ 的分辨率在数学上低于真正的 16:9。

关于ios - 使用 AVFoundation 裁剪 AVAsset 视频，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/21077240/