如何在低分辨率预览配置文件中使用AVCaptureVideoDataOutput并以高分辨率拍摄照片(预览时)

Question

我想用它AVFoundation Famework来预览和拍摄照片.我创建了一个AVCaptureSession和添加AVCaptureVideoDataOutput,AVCaptureStillImageOutput本届会议.我将预设设置为AVCaptureSessionPresetLow.

现在我想以全分辨率拍摄照片.但在captureStillImageAsynchronouslyFromConnection分辨率内与我的预览代表相同.

这是我的代码:

AVCaptureSession* cameraSession = [[AVCaptureSession alloc] init];
cameraSession.sessionPreset = AVCaptureSessionPresetLow;

AVCaptureVideoDataOutput* output = [[AVCaptureVideoDataOutput alloc] init];
[cameraSession addOutput:output];

AVCaptureStillImageOutput* cameraStillImage = [[AVCaptureStillImageOutput alloc] init];
[cameraSession addOutput:cameraStillImage];

// delegation
dispatch_queue_t queue = dispatch_queue_create("MyQueue", NULL);
[output setSampleBufferDelegate:self queue:queue];
dispatch_release(queue);

[cameraSession startRunning];

拍照:

//[cameraSession beginConfiguration];
//[cameraSession setSessionPreset:AVCaptureSessionPresetPhoto];  <-- slow
//[cameraSession commitConfiguration];
[cameraStillImage captureStillImageAsynchronouslyFromConnection:photoConnection completionHandler:^(CMSampleBufferRef imageDataSampleBuffer, NSError *error)
{
    ...
}];

我通过将预设更改为照片来尝试拍摄图像.但这非常慢(更改预设需要2-3秒).我不想有这么大的延迟.

我怎样才能做到这一点？谢谢.