http://blog.csdn.net/xzzppp/article/details/52317863#

<span style="font-size:24px;">#功能描述：生成多尺度、多宽高比的anchors。
# 尺度为：128,256,512; 宽高比为：1:2,1:1,2:1

import numpy as np  #提供矩阵运算功能的库  
  
#生成anchors总函数：ratios为一个列表，表示宽高比为：1:2,1:1,2:1  
#2**x表示:2^x，scales:[2^3 2^4 2^5],即:[8 16 32]  
def generate_anchors(base_size=16, ratios=[0.5, 1, 2],  
                     scales=2**np.arange(3, 6)):  
    """ 
    Generate anchor (reference) windows by enumerating aspect ratios X 
    scales wrt a reference (0, 0, 15, 15) window. 
    """  
    base_anchor = np.array([1, 1, base_size, base_size]) - 1  #新建一个数组：base_anchor:[0 0 15 15]  
    ratio_anchors = _ratio_enum(base_anchor, ratios)  #枚举各种宽高比  
    anchors = np.vstack([_scale_enum(ratio_anchors[i, :], scales)  #枚举各种尺度，vstack:竖向合并数组  
                         for i in xrange(ratio_anchors.shape[0])]) #shape[0]:读取矩阵第一维长度，其值为3  
    return anchors  
  
#用于返回width,height,(x,y)中心坐标(对于一个anchor窗口)  
def _whctrs(anchor):  
    """ 
    Return width, height, x center, and y center for an anchor (window). 
    """  
    #anchor:存储了窗口左上角，右下角的坐标  
    w = anchor[2] - anchor[0] + 1  
    h = anchor[3] - anchor[1] + 1  
    x_ctr = anchor[0] + 0.5 * (w - 1)  #anchor中心点坐标  
    y_ctr = anchor[1] + 0.5 * (h - 1)  
    return w, h, x_ctr, y_ctr  
  
#给定一组宽高向量，输出各个anchor，即预测窗口，**输出anchor的面积相等，只是宽高比不同**  
def _mkanchors(ws, hs, x_ctr, y_ctr):  
    #ws:[23 16 11]，hs:[12 16 22],ws和hs一一对应。  
    """ 
    Given a vector of widths (ws) and heights (hs) around a center 
    (x_ctr, y_ctr), output a set of anchors (windows). 
    """  
    ws = ws[:, np.newaxis]  #newaxis:将数组转置  
    hs = hs[:, np.newaxis]  
    anchors = np.hstack((x_ctr - 0.5 * (ws - 1),    #hstack、vstack:合并数组  
                         y_ctr - 0.5 * (hs - 1),    #anchor：[[-3.5 2 18.5 13]  
                         x_ctr + 0.5 * (ws - 1),     #        [0  0  15  15]  
                         y_ctr + 0.5 * (hs - 1)))     #       [2.5 -3 12.5 18]]  
    return anchors  
  
#枚举一个anchor的各种宽高比，anchor[0 0 15 15],ratios[0.5,1,2]  
def _ratio_enum(anchor, ratios):  
    """   列举关于一个anchor的三种宽高比 1:2,1:1,2:1 
    Enumerate a set of anchors for each aspect ratio wrt an anchor. 
    """  
  
    w, h, x_ctr, y_ctr = _whctrs(anchor)  #返回宽高和中心坐标，w:16,h:16,x_ctr:7.5,y_ctr:7.5  
    size = w * h   #size:16*16=256  
    size_ratios = size / ratios  #256/ratios[0.5,1,2]=[512,256,128]  
    #round()方法返回x的四舍五入的数字，sqrt()方法返回数字x的平方根  
    ws = np.round(np.sqrt(size_ratios)) #ws:[23 16 11]  
    hs = np.round(ws * ratios)    #hs:[12 16 22],ws和hs一一对应。as:23&12  
    anchors = _mkanchors(ws, hs, x_ctr, y_ctr)  #给定一组宽高向量，输出各个预测窗口  
    return anchors  
  
#枚举一个anchor的各种尺度，以anchor[0 0 15 15]为例,scales[8 16 32]  
def _scale_enum(anchor, scales):  
    """   列举关于一个anchor的三种尺度 128*128,256*256,512*512 
    Enumerate a set of anchors for each scale wrt an anchor. 
    """  
    w, h, x_ctr, y_ctr = _whctrs(anchor) #返回宽高和中心坐标，w:16,h:16,x_ctr:7.5,y_ctr:7.5  
    ws = w * scales   #[128 256 512]  
    hs = h * scales   #[128 256 512]  
    anchors = _mkanchors(ws, hs, x_ctr, y_ctr) #[[-56 -56 71 71] [-120 -120 135 135] [-248 -248 263 263]]  
    return anchors  
  
if __name__ == '__main__':  #主函数  
    import time  
    t = time.time()  
    a = generate_anchors()  #生成anchor（窗口）  
    print time.time() - t   #显示时间  
    print a  
    from IPython import embed; embed()  
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