CIFaceFeature *feture = [[CIFaceFeature alloc]init];
这个CIFaceFeature的所有属性都是只读的，请问自己创建一个实例，怎样初始化它的值啊？谢谢！
CIFaceFeature的定义如下（苹果的官方定义）：
/** A face feature found by a CIDetector.
All positions are relative to the original image. */
CORE_IMAGE_CLASS_EXPORT
@interface CIFaceFeature : CIFeature
{
CGRect bounds;
BOOL hasLeftEyePosition;
CGPoint leftEyePosition;
BOOL hasRightEyePosition;
CGPoint rightEyePosition;
BOOL hasMouthPosition;
CGPoint mouthPosition;
BOOL hasTrackingID;
int trackingID;
BOOL hasTrackingFrameCount;
int trackingFrameCount;
BOOL hasFaceAngle;
float faceAngle;
BOOL hasSmile;
BOOL leftEyeClosed;
BOOL rightEyeClosed;
}
/** coordinates of various cardinal points within a face.
Note that the left eye is the eye on the left side of the face
from the observer's perspective. It is not the left eye from
the subject's perspective. */
@property (readonly, assign) CGRect bounds;
@property (readonly, assign) BOOL hasLeftEyePosition;
@property (readonly, assign) CGPoint leftEyePosition;
@property (readonly, assign) BOOL hasRightEyePosition;
@property (readonly, assign) CGPoint rightEyePosition;
@property (readonly, assign) BOOL hasMouthPosition;
@property (readonly, assign) CGPoint mouthPosition;
@property (readonly, assign) BOOL hasTrackingID;
@property (readonly, assign) int trackingID;
@property (readonly, assign) BOOL hasTrackingFrameCount;
@property (readonly, assign) int trackingFrameCount;
@property (readonly, assign) BOOL hasFaceAngle;
@property (readonly, assign) float faceAngle;
@property (readonly, assign) BOOL hasSmile;
@property (readonly, assign) BOOL leftEyeClosed;
@property (readonly, assign) BOOL rightEyeClosed;
@end
