Ios 使用AVFoundation裁剪AVAsset视频

我正在使用

AVCaptureMovieFileOutput

录制一些视频。我使用略微放大的

AVLayerVideoGravityResizeAspectFill

显示预览层。我的问题是最终的视频更大，包含的额外图像在预览时不适合屏幕

这是预览和生成的视频

是否有一种方法可以指定要使用

AVAssetExportSession

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，并将

AVMutableVideoComposition

renderSize

设置为

view.bounds

时，它会从末端裁剪。太好了，只剩下一个问题了。视频的宽度没有延伸到正确的宽度，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未完成根据OutputFileAttribute:（NSURL*）outputFileURL fromConnections:（NSArray*）connections error:（NSError*）error

方法中，我使用此方法来裁剪和调整视频大小

- (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;
            }
        }
    }];
}

---

以下是我对您问题的解释：您在屏幕比例为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16:9捕获视频，因此生成的视频比预览中看到的视频“大”

如果您只是希望裁剪预览未捕获的额外像素，请查看此项。本文展示了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到4:3。我已经进行了测试，以下是我所做的更改：

一旦你有了视频的

AVAssetTrack

，你就需要计算一个新的高度

// 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;

然后使用newHeight修改这两行

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

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

因此，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

cGraffeTransform

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，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，我们需要添加一些内容

首先，这里是经过修改的示例代码，其中有一些关键的改动：

// 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) {

             }
         }];
    }
}];

我们在这里添加的是一个调用，根据视频尺寸与屏幕比例的裁剪来获取视频的新渲染大小。一旦我们缩小尺寸，我们需要转换位置以重新居中视频。所以我们抓住它的方向，让它朝正确的方向移动。这将解决我们在

UIInterfaceOrientation和scapeLeft中看到的偏心问题。最后，
CGAffineTransform t2、t3
水平镜像视频

以下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;
}

这些都很直截了当。唯一需要注意的是，在
getComplaidSize:
方法中，我们必须手动调整16:9的比率，因为iPhone5+的分辨率在数学上低于真正的16:9。
AVCaptureVideoDataOutput是AVCaptureOutput的一个具体子类，您可以使用它来处理所捕获视频中未压缩的帧，或访问压缩帧

AVCaptureVideoDataOutput的一个实例生成可以使用其他媒体API处理的视频帧。您可以使用
captureOutput:didOutputSampleBuffer:fromConnection:
delegate方法访问帧

配置会话
您可以在会话中使用预设来指定所需的图像质量和分辨率。预设是一个常数，用于标识多个可能配置中的一个；在某些情况下，实际配置是特定于设备的：


这些预设值代表各种设备的实际值，请参阅“”和“捕获静止图像”

如果要设置特定于大小的配置，应先检查是否支持该配置，然后再进行设置：

if ([session canSetSessionPreset:AVCaptureSessionPreset1280x720]) {
    session.sessionPreset = AVCaptureSessionPreset1280x720;
}
else {
    // Handle the failure.
}

可能与我之前看过的代码重复。就连答案评论都说它不完整。好吧，很抱歉。您可能希望显示实际代码…添加了代码。我想知道我是否应该改用
avcapturedevideodataoutput
，并在帧进入时进行裁剪。默认的iOS摄像头应用程序只记录屏幕上的内容，所以它必须是可能的。我觉得我错过了一些简单的事情。善用赏金。我正打算提出这个建议。哇，谢谢你。这让我非常接近我所需要的。你的代码有两个问题。1，这在iPad上的效果非常好，但在iPhone5上，屏幕不是4/3，所以视频更小。我们能用屏幕大小代替比例吗？2、在横向录制时，视频会旋转，这意味着它总是横向显示。@Darren我扩展了我的原始答案，以包括交替的屏幕比例和方向。添加时请不要忘记更新此内容：
[transformer setTransform:t2 atTime:kCMTimeZero]不幸的是，我对转换也不够熟悉，无法提供快速解决方案。我尝试了一些方法，但没有解决问题。我的猜测是，当我们应用CGAffineTransformTranslate并旋转视频时，以下位置和比例更改会受到新定位点/矩阵的影响。试着胡扯一下，或者问一个原创的问题。我相信有人能更好地理解转换——我的猜测有点不靠谱。更新了我的答案。压扁了您在
ui界面方向和scapeleft中看到的偏离中心的问题。并添加了几个变换以水平镜像。应该这样做：）