目標檢測------錨框-------anchor box

錨框：以每個像素點爲中心生成多個大小和寬高比不一樣的邊界框

圖像 高h , 寬w

大小爲s∈(0,1]  且   寬高比爲r>0

錨框的高 h1 = hs/√r

錨框的寬 w1 = ws√r

s1，...，sn 與 r1，... ,rm  這樣一張圖的錨框有 whnm 個

但對於一點來講，咱們感興趣的框有：(s1,r1),(s1,r2)......(s1,rm),(s2,r1),......(sn,r1)

假如  w=416  h=416  s=[0.75,0.5,0.25]  r=[1,2,,0.5]   在 （200，200 ）處生成5個框：

[44, 44, 356, 356], [96, 96, 304, 304], [148, 148, 252, 252], [0, 90, 416, 310], [90, 0, 310, 416]

accard係數（Jaccard index）能夠衡量兩個集合的類似度。給定集合A和B，它們的Jaccard係數即兩者交集大小除以兩者並集大小：

J(A,B)=|A∩B|/|A∪B|

import math
import numpy as np

w = 416
h = 416
s = [0.75, 0.5, 0.25]
r = [1, 2, 0.5]


def create_box(x, y,imgw, imgh, sizes, ratios):
    # nn = int(len(sizes) + len(ratios) - 1)
    size_1 = sizes[0]
    ratio_1 = ratios[0]
    ratio_2 = ratios[1:]
    x = x / imgw
    y = y / imgh
    list_1 = [[
        x - i * math.sqrt(ratio_1) / 2,
        y - i / (2 * math.sqrt(ratio_1)),
        x + i * math.sqrt(ratio_1) / 2,
        y + i / (2 * math.sqrt(ratio_1))
    ] for i in sizes]
    list_2 = [[
        x - size_1 * math.sqrt(i) / 2,
        y - size_1 / (2 * math.sqrt(i)),
        x + size_1 * math.sqrt(i) / 2,
        y + size_1 / (2 * math.sqrt(i))
    ] for i in ratio_2]
    list_1.extend(list_2)
    list_all = []
    for l in list_1:
        if l[0] < 0:
            l[0] = 0
        else:
            l[0] = round(l[0] * imgw)

        if l[1] < 0:
            l[1] = 0
        else:
            l[1] = round(l[1] * imgh)

        if l[2] > 1:
            l[2] = imgw
        else:
            l[2] = round(l[2] * imgw)

        if l[3] > 1:
            l[3] = imgh
        else:
            l[3] = round(l[3] * imgh)
        list_all.append(l)
    return list_all
if __name__ == '__main__':
    print(create_box(200, 200, w, h, s, r))