说明：  算法描述 SIFT特征不只具有尺度不变性，即使改变旋转角度，图像亮度或拍摄视角，仍然能够得到好的检测效果。整个算法分为以下几个部分： （1）构造尺度空间：DoG尺度空间 （2）检测DoG尺度空间极值点 （3）去除不好的特征点 （4）给特征点赋值一个128维的方向参数。每个关键点都包含三个信息：位置、尺度和方向。 （5）关键点描述子的生成： 首先将坐标轴旋转为关键点的方向，以确保旋转不变性。以关键点为中心取8×8的窗口。 （6）最后进行特征匹配。
(Algorithm description SIFT features not only have scale invariance, but also can obtain good detection results even if the rotation angle, image brightness or shooting angle are detected. The whole algorithm is divided into the following parts: (1) tectonic scale space: DoG scale space (2) detecting the spatial extreme points of DoG scale (3) remove bad feature points (4) assign a 128 dimensional direction parameter to the feature point. Each key point contains three information: location, scale and direction. (5) generation of key point descriptors: Firstly, the coordinate axis is rotated as the direction of the key point to ensure the rotation invariance. Take the key point as the center and take the window of 8 x 8. (6) feature matching is performed at last.)