iOS：AVCaptureStillImageOutput提供错误的图像输出

Question

我使用AVCaptureVideoPreviewLayer以编程方式启动摄像头，然后使用以下代码自动拍照。图片的输出分辨率不是很高，比我们通常使用原生相机拍照还要亮。这有什么问题，有人可以帮忙吗？

   -(void) capturePicture
{
    // Get all cameras in the application and find the frontal camera.
    AVCaptureDevice *backCamera;
    NSArray *allCameras = [AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];

    // Find the back camera.
    for ( int i = 0; i < allCameras.count; i++ ) {
        AVCaptureDevice *camera = [allCameras objectAtIndex:i];

        if ( camera.position == AVCaptureDevicePositionBack ) {
            backCamera = camera;
        }
    }

    // If we did not find the camera then do not take picture.
    if ( backCamera != nil ) {
        // Start the process of getting a picture.
        AVCaptureSession *session = [[AVCaptureSession alloc] init];

        // Setup instance of input with back camera and add to session.
        NSError *error;
        AVCaptureDeviceInput *input =
        [AVCaptureDeviceInput deviceInputWithDevice:backCamera error:&error];

        if ( !error && [session canAddInput:input] ) {
            // Add frontal camera to this session.
            [session addInput:input];

            // We need to capture still image.
            AVCaptureStillImageOutput *output = [[AVCaptureStillImageOutput alloc] init];

            // Captured image. settings.
            [output setOutputSettings:
             [[NSDictionary alloc] initWithObjectsAndKeys:AVVideoCodecJPEG,AVVideoCodecKey,nil]];

            if ( [session canAddOutput:output] ) 
            {

                [session addOutput:output];

                AVCaptureConnection *videoConnection = nil;
                for (AVCaptureConnection *connection in output.connections) {
                    for (AVCaptureInputPort *port in [connection inputPorts]) {
                        if ( [[port mediaType] isEqual:AVMediaTypeVideo] ) 
                        {
                            videoConnection = connection;
                            break;
                        }
                    }
                    if (videoConnection) 
                    { 
                        break; 
                    }
                }

                // Finally take the picture
                if ( videoConnection ) 
                {
                    [session startRunning];

                    [output captureStillImageAsynchronouslyFromConnection:videoConnection completionHandler:^(CMSampleBufferRef imageDataSampleBuffer, NSError *error) 
                     {
                         if (imageDataSampleBuffer != NULL) 
                         {
                             NSData *imageData = [AVCaptureStillImageOutput 
                                                  jpegStillImageNSDataRepresentation:imageDataSampleBuffer];
                             UIImage *photo = [[UIImage alloc] initWithData:imageData];

                             UIImageWriteToSavedPhotosAlbum(photo, nil, nil, nil);

                             [session stopRunning];

                             NSInvocation *myInvocation = [NSInvocation invocationWithMethodSignature:[self methodSignatureForSelector:@selector(endScreen)]];
                             [myInvocation setSelector:@selector(endScreen)];
                             [myInvocation setTarget:self];

                             appDelegate.bImageTaken = YES;

                             UIImageWriteToSavedPhotosAlbum(photo, nil, nil, nil);

                             [NSTimer scheduledTimerWithTimeInterval:0.5 invocation:myInvocation repeats:NO]; 
                         }

                     }];

                }
            }
        }
    }

}