Python 用条件减去2个Numpy数组

我有两个Numpy阵列，如下所示：

a = [[ [1,2,3], [4,5,6] ], 
    [  [7,8,9], [10,11,12] ]]

b = [[ [1,1,1], [0,0,0] ], 
     [ [3,3,3], [4,4,4] ]]

c = [[ [255,255,255], [4,5,6] ], 
     [ [255,255,255], [255,255,255] ]]

我想执行

c=a-b

，条件是

c=255，如果b>0，否则a

所以c应该是这样的：

a = [[ [1,2,3], [4,5,6] ], 
    [  [7,8,9], [10,11,12] ]]

b = [[ [1,1,1], [0,0,0] ], 
     [ [3,3,3], [4,4,4] ]]

c = [[ [255,255,255], [4,5,6] ], 
     [ [255,255,255], [255,255,255] ]]

---

如何在没有任何循环的情况下有效地执行此操作？

使用

np.where

>>> c = np.where(np.array(b)>0, 255, a)
>>> c
array([[[255, 255, 255],
        [  4,   5,   6]],

       [[255, 255, 255],
        [255, 255, 255]]])

顺便说一句，这里没有减法；也许可以改变你问题的标题