imhist分析灰度图阈值分界点
bwlabel分析连通区域
SIFT
Scale Invariant：尺度不变性
DoG: Difference of Gaussian, calculated by multiple Laplace of Gaussian. 计算高斯核函数差
Laplacian Scale Space, depiction of high contrast value.如何生成？
Rotation Invariant：旋转不变性
Compute Image Gradient 计算图像梯度
descriptor: histogram of gradient orientation