VOCDevkit 中 Mask 标签的生成函数

嗯，这是一个非常精巧的函数

Matlab 代码是这样的：

function cmap = labelcolormap(N)
cmap = zeros(N,3);
for i=1:N
    id = i-1; r=0;g=0;b=0;
    for j=0:7
        r = bitor(r, bitshift(bitget(id,1),7 - j));
        g = bitor(g, bitshift(bitget(id,2),7 - j));
        b = bitor(b, bitshift(bitget(id,3),7 - j));
        id = bitshift(id,-3);
    end
    cmap(i,1)=r; cmap(i,2)=g; cmap(i,3)=b;
end

转成 Python 就变成了这样的：

def labelcolormap(n):
    ans = []
    for i in range(n):
        x = i
        r, g, b = 0, 0, 0
        for j in range(7):
            r = r ^ ((x & 1) >> 0 << (7 - j))
            g = g ^ ((x & 2) >> 1 << (7 - j))
            b = b ^ ((x & 4) >> 2 << (7 - j))
            x = x >> 3
        ans.append([r, g, b])
    return ans

比如，给 10 个物体打 mask 标签，标签值分别为

     0     0     0
   128     0     0
     0   128     0
   128   128     0
     0     0   128
   128     0   128
     0   128   128
   128   128   128
    64     0     0
   192     0     0