cocoa - Cocoa QTKit 解析问题

Question

根据很多网上的参考资料，改变分辨率的方法是使用下面的代码。

NSDictionary * pixelBufferAttr = [NSDictionary dictionaryWithObjectsAndKeys:
     [NSNumber numberWithInt:320], kCVPixelBufferWidthKey,
     [NSNumber numberWithInt:240], kCVPixelBufferHeightKey,
     [NSNumbernumberWithBool:YES], 
                  (id)kCVPixelBufferOpenGLCompatibilityKey,
     [NSNumber 
        numberWithUnsignedInt:kCVPixelFormatType_422YpCbCr8],
                  (id)kCVPixelBufferPixelFormatTypeKey,
                                        nil];

NSArray *outputsConnected = [mCaptureSession outputs];

QTCaptureVideoPreviewOutput * previewOutput = [outputsConnected objectAtIndex:1];

[previewOutput setDelegate:self];

[previewOutput setPixelBufferAttributes:pixelBufferAttr];

但是改分辨率后captureView可以看到不一样，但是用代码检查分辨率不正确。

有谁知道如何更改设备分辨率？

谢谢大家！